Deutsches Zentrum fUr Entwicklungstechnologien - GATE
Deutsches Zentrum fUr Entwicklungstechnologien - GATE - stands for German Appropriate Technology Exchange. It was founded in 1978 as a special division of the Deutsche
Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH. GATE is· a centre for the dissemination and promotion of appropriate technologies for developing countries. GATE
defines "Appropriate technologies" as those which are suitable and acceptable in the light of
economic, social and cultural criteria. They should contribute to socio-economic development whilst ensuring optimal utilization of resources and minimal detriment to the environment. Depending on the case at hand a traditional, intermediate or highly-developed can be
the "appropriate" one. GATE focusses its work on three key areas:
- Dissemination of Appropriate Technologies: Collecting, processing and disseminating
information on technologies appropriate to the needs of the developing countries; ascertaining
the technological requirements of Third World countries; support in the form of personnel,
material and equipment to promote the development and adaptation of technologies for
developing countries.
- Research and Development: Conducting and/or promoting research and development
work in appropriate technologies.
- Environmental Protection: The growing importance of ecology and environmental protection require better coordination and harmonization of projects. In order to tackle these tasks
more effectively, a coordination center was set up within GATE in 1985.
GATE has entered into cooperation agreements with a number of technology centres in
Third World countries.
GATE offers a free information service on appropriate technologies for all public and private
development institutions in developing countries, dealing with the development, adaptation,
introduction and application of technologies.
Deutsche Gesellschaft fUr Technische Zusammenarbeit (GTZ) GmbH
The government-owned GTZ operates in the field of Technical Cooperation. 2200. German
experts are working together with partners from about 100 countries of Africa, Asia and
Latin America in projects covering practically every sector of agriculture, forestry, economic
development, social services and institutional and material infrastructure. - The GTZ is
commissioned to do this work both by the Government of the Federal Republic of Germany
and by other government or semi-government authorities.
The GTZ activities encompass:
- appraisal, technical planning, control and supervision of technical cooperation projects
commissioned by the Government of the Federal Republic or by other authorities
- providing an advisory service to other agencies also working on development projects
- the recruitment, selection, briefing, assignment, administration of expert personnel and
their welfare and technical backstopping during their period of assignment
- provision of materials and equipment for projects, planning work,. selection, purchasing
and shipment to the developing countries
- management of all financial obligations to the partner-country.
Deutsches Zentrum fUr Entwicklungstechnologien - GATE
in: Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH
P. O. Box 5180
0-6236 Eschborn
Federal Republic of Germany
Telephon: (06196) 79-0
Telex: 41523-0 gtz d
Fax: (06196) 794820
Henrik Norsker, James Danisch
Forming Techniques for the Self-Reliant Potter
A Publication of Deutsches Zentrum fur Entwicklungstechnologien - GATE
A division of the Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH
II
Springer Fachmedien Wiesbaden GmbH
The authors:
Henrik Norsker has been making pottery since 1970. He left his pottery workshop in Oenmark
in 1976 to establish a pottery school in a village in Tanzania. Since then he has continued
working in developing countries with promotion of small scale ceramics industries. Besides
Tansania he has been employed in ceramic projects in Burma, Bangladesh and Nepal.
James Oanisch has been making, selling and experimenting with ceramics since 1963. He has
taught college level ceramics in Scotland and California, and has conducted workshops in the
US, South America and Canada. From 1984 to 1992, he has been working with small scale and
rural ceramics development in Nepal. His articles on ceramics have been published in several
magazines, and he has studied traditional and modem techniques in Europe, Nepal, India,
Thailand, Burma, South America and Mexico.
Books by the same author include:
The Self-Reliant Potter: Refractories and Kilns
GATE / Vieweg, 1987
Clay Materials - for the Self-Reliant Potter
GATE / Vieweg, 1990
Die Deutsche Bibliothek - CIP-Einheitsaufnahme
Norsker, Henrik:
Forming techniques - for the self-reliant potter : a publication
of Deutsches Zentrum fUr Entwicklungstechnologien - GATE
in: Deutsche Gesellschaft fUr Technische Zusammenarbeit
(GTZ) GmbH / Henrik Norsker ; James Danisch. Braunschweig : Vieweg, 1991
ISBN 978-3-528-02061-3
ISBN 978-3-663-06867-9 (eBook)
DOI 10.1007/978-3-663-06867-9
NE: Danisch, James:
AII rights reserved.
© Springer Fachmedien Wiesbaden, 1991
UrsprUnglich erschienen bei Deutsche Gesellschaft fUr Technische Zusammenarbeit (GTZ) GmbH, Eschbom 1991
Published by Friedr. Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig
ISBN 978-3-528-02061-3
CONTENTS
1. INTRODUCTION AND SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2. PRODUCTS AND OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1. MARKET SECTORS AND FORMING CONSIDERATIONS . . . . . . . .
2.1.1. THE EXPORT MARKET . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2. THE LOCAL MARKET . . . . . . . . . . . . . . . . . . . . . . . . .
2.2. INVESTMENT DECISIONS . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. CHOOSING YOUR PRODUCT LINE ... . . . . . . . . . . . . . . . . . .
2.3.1. MACHINERY CONSIDERATIONS . . . . . . . . . . . . . . . . . .
8
8
8
9
3. MAIN PRODUCT LINES AND FORMING OPTIONS
.
.
.
.
.
.
.
.............. .
3.1. HOUSEHOLD WARE, CROCKERY, DECORATIVE AND
NOVELTY ITEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1. HAND METHODS
3.1.2. POTTER'S WHEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3. PRESS MOULD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.4. SLIP CASTING
3.1.5. nGGER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.6. OTHER FORMING METHODS . . . . . . . . . . . . . . . . . . . . . .
3.2. TILES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1. SCOPE FOR THE SMALL PRODUCER . . . . . . . . . . . . . . . . .
3.2.2. HANDMADE TILES . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3. PLASTIC-PRESSED TILES . . . . . . . . . . . . . . . . . . . . . . . .
3.2.4. SEMIDRY PRESSED TILES . . . . . . . . . . . . . . . . . . . . . . .
3.2.5. GLAZED AND UNGLAZED TILE CONSIDERATIONS . . . . . . . .
3.3. REFRACTORIES AND MISCELLANEOUS INDUSTRY SUPPORT
PRODUCTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1. FORMING METHODS
3.3.2. KILN FURNITURE . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. PIPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1. KINDS OF CERAMIC PIPE . . . . . . . . . . . . . . . . . . . . . . ..
3.5. ELECTRICAL PRODUCTS . . . . . . . . . . . . . . . . . . . . . . . . . . ..
3.5.1. SEMIDRYPRESSING . . . . . . . . . . . . . . . . . . . . . . . . . ..
3.6. MISCELLANEOUS PRODUCTS . . . . . . . . . . . . . . . . . . . . . . . ..
3.6.1. WATER FILTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6.2. AGRICULTURAL ITEMS . . . . . . . . . . . . . . . . . . . . . . . ..
3.6.3. SANITARY WARE . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
11
12
13
13
14
14
14
15
15
16
16
16
16
17
17
17
18
18
18
19
19
20
21
21
21
22
22
3
4. PLASTIC CLAY FORMING . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
4.1. PLASTIC CLAY PREPARATION . . . . . . . . . . . . . . . . . . . . . . ..
4.1.1. DRY PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2. WET PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. HAND-FORMING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1. TOOLS FOR HAND-FORMING . . . . . . . . . . . . . . . . . . . . .
4.2.2. THE BASICS: HOW TO MAKE COILS AND SLABS . . . . . . . . .
4.2.3. ASSEMBLING COILS AND SLABS
4.2.4. USEFUL HINTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
4.3. POTTER'S WHEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1. CLAY REQUIREMENTS FOR THE POTTER'S WHEEL
......
4.3.2. TYPES OF POTTER'S WHEELS . . . . . . . . . . . . . . . . . . . .
4.3.3. FORMING ON THE POTTER'S WHEEL . . . . . . . . . . . . . . . .
4.3.4. TOOLS FOR THE POTTER'S WHEEL . . . . . . . . . . . . . . . . .
4.3.5. FINISHING ON THE POTTER'S WHEEL . . . . . . . . . . . . . . .
4.3.6. DRYING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4. JOINING TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1. HANDLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.2. JOINING COMPLICATED FORMS . . . . . . . . . . . . . . . . . . .
4.4.3. SPECIAL JOINING TECHNIQUES . . . . . . . . . . . . . . . . . . .
4.5. JIGGER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.1. APPROPRIATE USE OF THE JIGGER . . . . . . . . . . . . . . . . .
4.5.2. AVERAGE PRODUCTION QUANTITIES . . . . . . . . . . . . . . ..
4.5.3. JIGGER WORK FLOW . . . • . . . . . . . . . . . . . . . . . . . . . .
4.5.4. JIGGER MACHINE PRINCIPLES AND CONSTRUCTION . . . . . .
4.5.5. MOULD MAKING FOR THE JIGGER . . . . . . . . . . . . . . . . ..
4.5.6. FORMING PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.7. FINISHING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
4.5.8. DRYING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.9. JIGGER PROBLEMS AND SOLUTIONS . . . . . . . . . . . . . . ..
4.6. EXTRUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.1. HAND EXTRUDERS . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.2. MOTORIZED EXTRUDERS . . . . . . . . . . . . . . . . . . . . . . .
4.6.3. TYPICAL EXTRUDED PRODUCTS
23
23
23
25
29
29
29
30
33
34
35
35
41
53
54
58
59
60
64
65
66
67
67
67
70
72
73
75
77
77
78
78
79
84
5. SEMIDRY FORMING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1. ADVANTAGES AND DISADVANTAGES . . . . . . . . . . . . . . . . . . .
5.2. PRESS MACHINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1. DIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2. HAND-OPERATED SCREW PRESS
5.2.3. FRICTION PRESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4. TOGGLE PRESS
5.2.5. ECCENTRIC PRESS . . . . . . . . . . . . . . . . . . . . . . . . . . ..
95
96
97
97
99
100
101
101
4
5.2.6. SIMPLE LEVER PRESS . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.7. HYDRAULIC PRESSES . . . . . . . . . . . . . . . . . . . . . . . . ..
5.3.SEMIDRYPRESSINGWITHLEATHERHARDCLAY . . . . . . . . . . . .
5.4. PRESSING WITH POWDER CLAY . . . . . . . . . . . . . . . . . . . . . ..
5.4.1. SEMIDRY POWDER PRESSING WORK FLOW . . . . . . . . . . . .
5.4.2. HINTS FOR SUCCESSFUL PRESSING . . . . . . . . . . . . . . . . .
5.4.3. SAFETY SAFETY SAFETY! . . . . . . . . . . . . . . . . . . . . . . .
5.4.4. PRESSING PROBLEMS AND SOLUTIONS . . . . . . . . . . . . . ..
5.4.5. REFRACTORIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
101
102
102
105
105
108
110
111
112
6. MOULD MAKING AND PLASTER OF PARIS . . . . . . . . . . . . . . . . . . .
6.1.STAMPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
6.2. MOULDS FOR PLASTIC CLAY PRESSING, BISCUIT CLAY MOULDS ...
6.2.1. ONE-PIECE MOULDS . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2. PIECE MOULDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3. PLASTER SUBSTITUTES . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4. PLASTER OF PARIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.1. GYPSUM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.2. PLASTER OF PARIS PRODUCTION . . . . . . . . . . . . . . . . . .
6.4.3. SHIPPING AND STORAGE . . . . . . . . . . . . . . . . . . . . . . ..
6.4.4. MIXING PLASTER OF PARIS . . . . . . . . . . . . . . . . . . . . . .
6.5. PLASTER MOULD MAKING . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5.1. MODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
6.5.2. MAKING MOULDS FOR PRODUCTION . . . . . . . . . . . . . . . .
6.5.3. MOULD PROBLEMS AND SOLUTIONS . . . . . . . . . . . . . . . .
114
114
116
116
118
120
121
121
122
125
126
129
131
135
146
7. SLIP CASTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1. CASTING BODY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.1. GUIDELINES FOR DEVELOPING A CASTING BODY . . . . . . . .
7.1.2. CASTING SLIP PREPARATION . . . . . . . . . . . . . . . . . . . . .
7.1.3. CASTING PROBLEMS, CAUSES AND CURES . . . . . . . . . . . .
7.2. SLIP BUILDING AND FIBERSLIP . . . . . . . . . . . . . . . . . . . . . . .
148
149
152
155
156
158
8. FINISHING AND QUALITY CONTROL . . . . . . . . . . . . . . . . . . . . . ..
8.1. IMPORTANCE OF FINISHING . . . . . . . . . . . . . . . . . . . . . . . . .
8.2. STEPS IN FINISIDNG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3. METHODS OF FINISIDNG . . . . . . . . . . . . . . . . . . . . . . . . . . ..
8.4. QUALITY CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
161
161
162
163
163
9. DRYING OF WARE AND MOULDS . . . . . . . . . . . . . . . . . . . . . . . . .
9.1. DRYING SYSTEMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
9.2. DRYING OF PLASTER MOULDS . . . . . . . . . . . . . . . . . . . . . . .
166
169
172
10. WORKFLOW AND FACTORY LAYOUT . . . . . . . . . . . . . . . . . . . . .
10.1 WORK FLOW EXAMPLE . . . . . . . . . . . . . . . . . . . . . . . . . . ..
174
177
5
APPENDIX
181
GLOSSARY
181
..................... .
184
SIEVE MESH SIZES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
185
DENSITY, TWADDELL SCALE, DRY CONTENT OF LIQUID, PINT WEIGHTS . . .
186
BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
188
INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189
TABLES OF WEIGHTS AND MEASURES
6
1. INTRODUCTION AND SCOPE
This book is intended as a practical guide
for cottage and small-scale ceramics production, primarily ~n developing countries,
using locally-based technology and available
raw materials, where machinery usually
needs to be made locally in order to be practical. However, we think it may also be of
use in developed countries, as many "studio" potters are shifting from one-person
production to a group approach, creating interest in the revival of jiggers, handmade
tiles, etc.
It is assumed that the reader is already producing ceramics, or is looking into the possibility of starting ceramics as a business.
Much of the material is based on the direct
experience of the authors in developing
countries, in particular Nepal, Burma, Tanzania, India, Bangladesh and Thailand,
where we both have been involved for several
years in ceramics development projects, and
have looked closely at the various problems
and solutions, past and present. Information
about forming systems and machinery designs is for the most part not available in
books. Nor has there been much documentation of successful systems that small factories use in developing countries. We hope
that this book will help to fill the gap.
7
2. PRODUCTS AND OPTIONS
This chapter presents a summary of various
articles that the small producer can make.
2.1. MARKET SECTORS AND
FORMING CONSIDERATIONS
In developing countries, there are two main
market sectors: the export market and the
local market. They require different kinds of
products in most cases.
2.1.1. THE EXPORT MARKET
CAUTION!
Exportation from many countries is very
difficult, due to customs regulations. Before
dreaming about huge profits from exports, it
is important to find out about rules and
costs, and to be sure that you have a reliable
buyer.
profitability
Because of high labor costs in industrialized
countries, it has become profitable in developing countries to produce ceramics for export. There is now a very large market for
floor and wall tiles, tableware, and decorative or novelty items, which are very competitive in the world market in spite of high
shipping costs. Any product which is laborintensive (for example, requiring hand-painting or large amounts of handling) has a
good scope for export, because it takes advantage of low labor costs.
8
success in Thailand
For example, factories in northern Thailand
producing cheap novelty items for export
have orders for the coming two years, and
are finding it difficult to increase their capacity enough to supply the demand. Many of
these factories are small-scale and use simple forming techniques, such as slip casting
or jiggering. The products are often designed by the customer, and most of these
products do not require special finishing and
quality control. For example, 1,000,000 "Elvis Presley" figurines were ordered from
Thailand for a U.S. promotion campaign.
They were produced by slip casting with
something like a 500% profit margin.
traditional products
Another market sector is "ethnic" arts, for
example ceramic dolls dressed in the traditional costumes of their country. Although
this is not a large volume market, it can be
profitable for a small producer. In Nepal,
one small pottery makes most of its profit
sending hand-painted dolls to Japan. Similarly, decorative pottery (like candlestands,
ashtrays, etc.) that is done with traditional
shapes and decoration has its own market
for export.
labor-intensive products
Anything handmade has an export market for example, special designs of handmade
tile can fetch a high price in the West. Similarly, there are some businesses which
import high-quality tableware from Europe,
hand-paint it with overglaze, and send it
back to the country of origin for a high profit.
joint venture factories
Another export sector which is growing rapidly is joint-venture floor and wall tile production. This is usually done on a large scale,
using high capital-investment foreign machinery, and as such is outside the scope of
this book.
punctual delivery
Products for export need to be selected very
carefully, and problems of quality control
and punctual delivery must be thought about.
Western buyers expect to get their orders on
time, and will not accept excuses for late delivery. This means that the producer must
have a realistic idea about whether he has
the capacity to produce an order, and he has
to make an accurate estimate of the time it
will take. It may be tempting to accept a big
order, but if it requires ten times your normal capacity, it would be foolish to do so.
Be sure of your ability to produce
Traditional designs already being made in
your country, but adjusted for export, are
easier to control for quality, because they
are familiar to the workers. Orders for designs that have not been tried before should
never be accepted - the rule is, do not try to
fool the customer, because it will only backfire on you in the end.
However, needs for new products are changing fast, in line with modem times. It is easy
to see that in every country the introduction
of plastic and aluminum has revolutionized
the way people live. In the same way, a
ceramics producer who keeps his eyes open
to how things are changing will find a good
market for new products.
example: Nepal
Nepal had no glazed ceramics production
until very recently. However, the market
had developed based on household ware imported from India, which is mostly low-quality white stoneware. Because there are no
suitable local raw materials for stoneware,
Ceramics Promotion Project developed low
temperature glazed red clay technology.
Now, small producers are finding a good
market. The new industry has found customers ranging from tourist hotels to local
housewives. Because the products are lowcost, they can be sold even though the quality still has to be improved. This is a good
example of introducing a new variation into
an existing market.
local market possibilities
There are at least three main possibilities for
the local market:
Import substitution: where feasible, local
production of goods that are usually imported.
Improving existing products.
2.1.2. THE LOCAL MARKET
Introducing new products.
The local market usually already has a demand for glazed ceramics, except in remote
areas where they may still be unknown.
Import substitution is only feasible if locallymade products can be made more cheaply
than the imports, or if import duties keep the
9
FIGURE 2.1.2-A Stoneware crockery for restaurants and households.
cost high. Wherever there is a tourist trade,
obvious customers are hotels and restaurants. Hotels often like to have locally-made
ceramics as decoration and advertisement,
such as ashtrays and flower vases. A small
producer who can capture the orders of even
one large hotel has a guaranteed business, as
smart hotels encourage customers to take
ashtrays home as souvenirs - it is good advertising for them.
Where there is already local production, a
clever producer may find ways to produce
the same quality at a lower cost, or to improve the product in a way (better glaze,
more attractive decoration) that attracts customers.
10
New products are often risky for the producer, since the market is unknown. However,
part of being a successful producer is being
able to see a need that nobody else has seen.
For example, everybody needs containers
for drinking water, and replacing traditional
jars with new designs fitted with a tap appeals to customers wanting to be modem.
Likewise, there is a lot of money in rural sanitation projects, and these projects generally
are interested in promoting local production
efforts.
hotels and restaurants
Within the local market, there are also several different sectors. Mainly, there is the
profitable hotel and restaurant trade, which
is low in volume but has a high profit margin.
In the beginning, producers will try to capture that until the market is saturated. In the
long run, most producers will be supported
by the local market for cheap household
items, which is large in volume but has a
much lower profit margin.
2.2. INVESTMENT DECISIONS
Because risk is high in starting any new industry, it usually is best to start small and
expand gradually.
A profitable ceramics business can be run in
the corner of a room, and requires only a table, clay, glaze and a small electric kiln. The
total investment may be less than US$l00,
and the products would be small, handmade
decorative and novelty items (like animals,
dolls, etc.).
At the other extreme, perhaps the most costly "small" -scale investment would be a
sewage pipe factory, where equipment and
kilns could total US$ 750,000. This is also a
profitable business, but requires large volume production in order to pay back the machinery.
questions to ask
Before deciding to invest in ceramics, you
should answer the following questions,
which are standard questions for any new
business:
1. Is there a market for the product? If
so, what is the size of the market? If the product is being made locally, how much of the
market sector can you expect to capture? If
the product is imported, can you equal the
quality or make it significantly cheaper?
2. Do you know enough about the business? Can you get machinery? Technical assistance?
3. Is there enough skilled labor available?
4. Is the amount of investment required
too much? Are you able to get a loan? If so,
what is the payback period - and are you
able to get the business running smoothly in
time?
2.3. CHOOSING YOUR PRODUCT
LINE
For new producers of ceramics, there is one
main rule to follow: KEEP IT SIMPLE.
Choose only one product line, and make
that succe-ssful before trying to diversify.
Avoid trying too many kinds of forming at
once (don't start jiggering, slip casting and
semidry pressing at the same time). As with
most businesses, if there is one successful
unit, it will soon be copied. If the market
sector is large enough, this is a safe way to
go. On the other hand, the really successful
producers are usually the ones who start a
new business before anyone else does.
There is more risk in this, but by defmition
being a businessman means being willing to
take greater risks than most people.
expanding your product line
For existing producers wanting to expand
their product line, it is important to think
about the total system. In other words, if
you are already producing cups by jigger, it
is easy to make new moulds for producing
soup bowls, with little additional investment. On the other hand, this may mean
purchasing additional jigger machines, getting additional workers, and expanding kiln
11
capacity. If you want to produce glazed tiles
by semidry pressing, this requires a greater
investment, since the clay processing, production line and even the kilns will have to
be different.
2.3.1. MACHINERY
CONSIDERATIONS
Depending on the size of your industry,
there are several options for machinery.
Ready-made machinery
Most countries have suppliers for ceramics
machinery that is appropriate for local conditions. In big countries such as India, there
are many suppliers producing different
qualities. Try to get the right machine for
the work to be done. Before choosing an expensive machine, you should get as much
advice as possible regarding what capacity
and quality you need. For example, nondeairing pug mills can be very cheap, and are
often satisfactory if you do not run the machine all the time and do not mind doing periodic maintenance and repairs (they usually
are made from low quality steel and have
cheap bearings). However, if you expect to
be running your pug mill at full capacity for
2 shifts per day, it probably is a cheaper
long-term investment to get a more expensive, better quality pug mill.
Talk to other producers using machinery to
get a better idea. Never believe a machinery
salesman until you have talked to his other
customers.
Used machinery
It is often possible to buy already-used machinery at low cost from factories that are
upgrading their machinery or that have gone
12
out of business. Many small producers have
started out this way.
If you are starting a small-scale industry
which requires heavier machinery, it is often
possible to get used machines from Europe
at a low cost. Their condition is generally
certified, and many are almost new. European factories have to upgrade their equipment quite frequently in order to compete,
and many have gone out of business because of competition from imports! So there
are some good bargains available. Catalogs
and price lists are available from advertisements in "Interceram" and "Tile & Brick International" magazines (see Appendix).
Making your own machinery
It is not recommended to make your own
machinery unless you have another machine
to copy, or a lot of money to spend. Even
simple machines like hand extruders need
some attention to detail. We have made a lot
of prototype machinery in Nepal (potter's
wheels, pug mills, vibrating screens, etc.,
many of which are shown in this book), and
have rarely had a successful prototype the
first time. On the other hand, an inventive
mechanic could probably follow plans and
have a good chance of success - but he will
probably have to try the machine out and
make some changes before it works correctly.
Make it strong
In any case, it is most important to get the
details right: for example, use good bearings
if available, and always make things stronger than you think is necessary. Even simple
machines like hand extruders can have problems: the metal piece that holds the die in
place can break if the steel is too thin, the
handle can bend if too thin, etc.
3. MAIN PRODUCT LINES AND
FORMING OPTIONS
This chapter focuses on a catalog of products, and what is required for forming them.
The specific forming systems are discussed
in detail in later chapters.
3.1. HOUSEHOLD WARE,
CROCKERY, DECORATIVE
AND NOVELTY ITEMS
Household ware and crockery mean approximately the same thing. These are products such as cups, plates, storage jars, teapots, bowls, lamp bases, etc. Decorative
and novelty items are such things as flower
vases, figurines, wall hangings and souvenirs. These products are capable of being
produced in one and the same factory, as
long as the forming methods are the same.
Household items are usually the easiest to
produce without introducing complicated
forming techniques. In most countries, there
is already a tradition of ceramic household
items, although they may not be glazed.
From the point of view of forming, this does
not present a major change in technology.
The following is a list of forming methods,
and some typical products that can be successfully made.
FIGURE 3.1.1-A Novelty items from Thailand.
13
3.1.1. HAND METHODS
3.1.2. POTTER'S WHEEL
Hand methods refer to using slabs, coils and
pieces of clay to assemble various forms,
using only the hands and simple tools.
The potter's wheel still remains a very important production tool. It can be used for
unique items, or for series of identical products. It depends on having highly-skilled
"throwers" to produce quality items. Although handmade items are no longer so attractive in the local market, industrialized
countries have a big demand for handmade
products, and there is growing local prestige market and a tourist market for handcrafted ceramics.
novelty items
There are few highly marketable items that
can be made by these methods, but there are
some successful microbusinesses that produce very small novelty items by hand, such
as animals, flowers, etc. The process for animals is to make the body from a piece of
clay, and then to add feet, ears, tail, etc., all
by hand. The main point is to use [me detail,
which makes the product attractive to the
customer. These items are made at very low
cost (mainly labor) and have a high profit
margin. They are easy to export, being
small.
Typical products are cups, containers,
bowls, candlestands, lamp bases, flowerpots
and novelties, as well as hundreds of other
household and decorative items.
FIGURE 3.1.2-A Potter at potter's wheel, Tanzania.
3.1.3. PRESS MOULD
Press moulding is the process of forming
plastic clay by forcing it by hand into plaster, cement or wooden moulds. It is used to
produce items with detailed relief decoration, or large forms that cannot be made any
other way. Like all moulded products, it has
the advantage of producing large numbers
of identical items with low investment. In
pressmoulding, several shapes are often
pressed separately and then assembled into
complicated sculptural forms.
Typical products are tiles with relief designs, sculpture (animals, gods), large flowerpots with relief designs, water containers,
pipes, etc.
14
FIGURE 3.1.3-A Plaster mould (right) and a pressed tile (left).
FIGURE 3.1.4-A Slip casting of flower vases. Moulds
are held together by rubber bands made from inner
tubes.
3.1.4. SLIP CASTING
Slip casting is the process of pouring liquid
clay slip into plaster of paris moulds. It has
the advantage of producing large quantities
of identical forms, and is especially good for
products that are not round (square pots, irregular shapes, etc.).
Typical products are flower vases, lamp
bases, relief design tableware, figurines, etc.
3.1.5. JIGGER
This is a machine that is really a semiautomatic potter's wheel, which uses plaster
moulds to produce identical items. It is most
useful for hollow ware, which means open
forms (cups, bowls, plates) that have a big
mouth, and is best for round shapes. This
machine is frequently called a "jigger-jol15
3.2.1. SCOPE FOR THE SMALL
PRODUCER
Nowadays, most standard glazed tiles are
produced by very large-scale, automated
factories, which are able to make their profit
through large volume and low amount of
handling. Because the industry is so large
and aggressive, it is impossible for the small
producer to compete directly in terms of
quality and cost.
FIGURE 3.1.5-A Jiggering rice bowls in Thailand.
ly", but for simplicity' s, in this book it will
always be called "jigger".
The jigger is the standard method for making dinner plates, soup bowls, teacups, and
similar forms.
3.1.6. OTHER FORMING METHODS
Other methods usually are combinations of
the above. For example, cups produced by
jigger may have handles made by slip casting.
3.2. TILES
Tiles refer to flat plates of clay that can be
set together to cover walls and floors, or
specially shaped tiles for roofing. There are
many kinds of tiles, glazed and unglazed,
with relief designs and plain. Tiles are popular all over the world for use in and on
buildings.
16
competing with big industries
However, there is also a good market for
special decorative tiles, which may have relief designs or painted glaze designs. These
are in great demand in industrialized countries, where they have a high profit margin.
Another product is unglazed red clay tiles,
which are very popular for floor and wall
covering. These are best for the local market, since they normally are not transported
for long distances (which increases their
cost too much). They have a low profit margin, but like the brick industry, they are profitable because of large volume production.
Similarly, unglazed roofing tiles are also
marketed in the local area.
3.2.2. HANDMADE TILES
Handmade tiles are a good business in many
countries. There are two types:
- unglazed red clay tiles, which can range
from 30 cm x 30 cm x 3 cm for floors, down
to about 10 cm x 10 cm x 1.5 cm.
- glazed relief design tiles, which usually
range from 15 cm x 15 cm x 2 cm down to
small sizes.
These tiles are made in frames or plaster of
paris moulds, and require only a small investment in machinery.
3.2.3. PLASTIC-PRESSED TILES
These tiles are usually plain-surfaced, but
are more uniform in size and finishing compared to handmade tiles. They are made
with a press machine which uses metal dies,
and the machine may be hand-operated or
motorized. They use clay in the plastic state.
They are relatively easy to produce, but require quite a large amount of transporting
and hand finishing.
The process is most suitable for unglazed
floor and wall tiles, but can also be used for
glazed tiles and is the normal system for
making roofing tiles.
3.2.4. SEMIDRY PRESSED TILES
These are tiles made with clay powder having a low moisture content of up to 10%,
or using leather-hard clay (a variation of plastic pressing). They are usually plain-surfaced, and are faster to produce compared to
plastic-pressed tiles. However, they require
more control in clay preparation, and may
need more investment in machinery. They
use the same type of press machine as above
(with higher pressure), but have the advantage of being almost dry after they are pressed. This reduces drying problems and minimizes the amount of handling and transport
required.
FIGURE 3.2.5-A Roofing tile making, Nepal.
3.2.5. GLAZED AND UNGLAZED
TILE CONSIDERATIONS
As mentioned above, the small producer is
not advised to compete directly with standard glazed tile manufacturers. It is much easier to enter into unglazed tile production,
which will give better results with fewer
problems. There is always a good scope for
unglazed floor tile, facing brick, etc. There
are many successful businesses in Thailand
making unglazed relief tile for the outside of
buildings. Likewise, there is good scope for
unglazed roofing tile - in countries like
Nepal and Burma, there is not enough good
quality roofing tile to meet the demand.
FIGURE 3.2.5-8 Samples of relief tiles.
The process is commonly used for glazed
and unglazed wall and floor tiles.
17
Glazed relief tile has a good scope for the
small producer, and requires only investment in a few plaster of paris moulds. Even
small orders can be profitable, since there
can be a good profit margin added. A producer who is already making household
items can consider making relief tile, without
having to change any of his equipment.
factories need special items like porcelain
balls and lining bricks for ball mills, and
porcelain jars for pot mills. Where a substantial ceramics industry already exists,
there is often enough business to support a
refractories factory. If you have a fairly
large ceramics industry and are purchasing
expensive imported refractories, it is worth
thinking about producing certain items
yourself as a sideline, and possibly selling
them to nearby factories as well.
3.3. REFRACTORIES AND
MISCELLANEOUS
INDUSTRY SUPPORT
PRODUCTS
Miscellaneous industry support products are
pot mill jars, balls and brick linings for ball
mills, mortars and pestles, chemical storage
containers, etc.
relief tile
Refractories are all of the products that are
necessary for firing ceramics. "Refractory"
means "capable of withstanding high temperature", and the refractories industry produces firebricks, insulating bricks, kiln shelves
(setter slabs), etc. Additionally, ceramics
3.3.1. FORMING METHODS
Refractories can be made by simple methods,
including hand-extrusion, semi dry
pressing, hand-moulding and casting.
FIGURE 3.3-A Examples of ceramic products: a mortar
and pestle, a jar mill and lining bricks and pebbles for
ball mills.
3.3.2. KILN FURNITURE
Kiln furniture refers to setter slabs (kiln
shelves), saggars and various stands, specialized setters for plates, tiles, etc. These
are generally formed as follows:
- Extrusion: A hand extruder can be used
for producing stands to support kiln shelves.
- Semidry pressing: Kiln shelves are made
with semidry (about 10 % water) clay.
They cannot be made in standard screw
press machines, which do not have sufficient pressure. The classical and still best
way to make them is in a metal frame, handbeating the clay to make it uniform and
18
aligns the clay particles better). Setter slabs
(shelves) are also sometimes cast.
For more infonnation, see Refractories and
Kiln£, in this series of books.
3.4. PIPE
3.4.1. KINDS OF CERAMIC PIPE
Ceramic pipe can be glazed or unglazed, depending on its use and required strength.
There are several uses for pipe:
FIGURE 3.4.1-A A pipe is formed by wrapping a clay
slab around a wooden mould.
FIGURE 3.3.2-A Setter slabs for saucers made by slip
casting and spacers made by extrusion.
dense. Other specialized shapes, such as
"thimbles" for tile setting, are made in presses.
Firebricks are made either by pressing in
screw presses, or, for better quality, beating
clay by hand into metal moulds.
- Hand-moulding: Setters for plates can be
moulded by hand on the potter's wheel.
Porcelain balls for ball mills are often made
simply by rolling clay in the hands. Saggars
are usually made by wrapping clay slabs
around wooden fonns and beating them into
shape. Insulating bricks are usually "slopmoulded" in wooden fonns (as is done
everywhere for common bricks), and are
sometimes pressed in a screw press when
they have stiffened to give them a more
unifonn size.
- Slip casting: Ball mill jars are sometimes
cast, or simply thrown on the potter's wheel
(which makes a stronger jar because it
19
- Simple, low-pressure water pipe: This is
- Sewage pipe: This is glazed and fired to
usually unglazed, is used only for short distances, and can be made on the potter's
wheel or in a two-piece mould. Another
system involves wrapping plastic clay slabs
around a wooden form. It requires almost no
investment, and is suitable for traditional
(no kiln) firing systems.
higher temperatures, and is best made in
quantity by an extruder machine. Sewage
pipe is also suitable for carrying irrigation
water, and is capable of withstanding relatively high pressure. It is suitable for smallscale industry, and requires high capital investment.
- Perforated drainage pipe: This is unglazed, has holes along one side, and is used
for collecting excess water under roads, or
in soak pits (leach fields) for septic tanks. It
can be made on the potter's wheel, extruded,
or in a two-piece mould. It has the same requirements as water pipe.
- Chimney pipe for stoves: This is unglazed, and also is made on the potter's
wheel or in a two-piece mould. It is the
same as making water pipe.
- Semicircular roofing tiles: These are
thrown as small pipe and later cut in half.
This is the usual way to make village roofing tiles, which is slowly disappearing because the roof is very heavy and requires too
much wood for construction.
3.5. ELECTRICAL PRODUCTS
Electrical products suitable for small producers are low-tension insulators and fuse
holders. These are relatively easy to produce
FIGURE 3.S.I-A Pressing fuse holders, Khurja, India.
20
and do not require too much quality control.
These products are formed by:
3.6. MISCELLANEOUS
PRODUCTS
3.6.1. WATER FILTERS
3.5.1. SEMIDRY PRESSING
- Fuse holders are made in small toggle
presses or screw presses. These hand-operated presses require low investment. They
often are made as a cottage industry, where
there is one central clay supplier and firing
center. The fuse holders are pressed in
homes, usually by one person using one
toggle press. Each morning, these workers
come to the center with their finished goods
and collect enough clay for the day's production. They are paid by the piece.
- Low-tension insulators are formed in
presses. They can be made according to the
same system as fuse holders. They sometimes are extruded and then shaped on a
potter's wheel.
With more and more education on hygiene
and the need for clean drinking water,
water filter units are becoming popular. The
water containers can be made cheaply, on
the potter's wheel or by using plaster moulds
and hand-pressing. The filter candles are
more complicated, because they require a
special clay body, and have to be fired to an
accurate temperature. They can be formed
by casting, or by pressing in a metal mould.
FIGURE 3.6.l-A Water container and filter. The filters
are fitted in the top container.
FIGURE 3.5.1-B Extruded insulators are turned on a
wheel.
21
3.6.2. AGRICULTURAL ITEMS
Agricultural items include animal feeding
containers and irrigation pipes (as mentioned above). These are usually made by
hand, on the potter's wheel, or by using
simple moulds.
3.6.3. SANITARY WARE
Although the small producer should not expect to make high quality vitrified china
sanitary ware, there is good scope for simple
products intended for the low-cost market.
These can be formed with a low investment,
but, in the case of toilet pans, require a large
kiln. Products include:
- simple toilet pans, footrests and traps,
using red or white clay, which are formed
by hand in press moulds;
- large unglazed red clay rings used for
lining pit toilets, which are formed in hand
press moulds.
22
FIGURE 3.6.3-A Borax-glazed toilet pan, trap and
footrests made by press moulding, Badravati, India.
4. PLASTIC CLAY FORMING
4.1. PLASTIC CLAY
PREPARATION
This is a brief review of plastic clay preparation. A complete explanation of clay mining and processing is found in "Clay Materials - for The Self-Reliant Potter", also in
this series.
definition of plastic clay
Plastic clay means clay that has about 30 %
water content. It can be manipulated into
any shape, and is the standard consistency
for most forming processes. Nowadays, the
word "plastic" is understood to mean the
range of modem materials used for familiar
products such as plastic bags, buckets, etc.
Originally, "plastic" meant a material that
did not have any form of its own, but could
easily be formed into any shape.
NOTES ON PLASTIC CLAY:
- For most traditional pottery, the (dry)
clay lumps are simply soaked with water, allowed to set, then kneaded by hand and
FIGURE 4.1.I-A Work flow for dry process clay making.
A) Dry clay storage. Store enough dry clay to last
throughout the rainy season.
There are two methods of preparing plastic
clay, depending on the kind of product and
quality of the raw materials. These are:
4.1.1. DRY PROCESS
This process is used mainly for red clay products, which do not require extremely fine
clay. It is cheaper to set up than the wet process (described below). It also is satisfactory
for white clay products, where already
washed raw materials can be obtained. The
work flow is shown in Fig. 4.1.1-A.
-:-.
~
:.
,
:
- "
.
~.
...
._. ~ ",' ...
,.
.
.'
":
""
~
I~
B) Drying in the sun directly on the ground,or, better
still, on raised brick platforms which can be covered by
plastic in case of rain.
23
Figure 4.1.1-A continued
C) Making clay powder by hand, with a rice huller or a
E) Mixing with other materials (clays, feldspar, talc,
hammer mill.
quartz) manually or in a drum mixer.
F) Soaking is necessary to allow all clay particles to absorb water (for a minimum of 24 hours).
D) Screening to remove coarse particles (16-1 ()() mesh
depending on required fineness) by hand sieve or mo-
G) Kneading can be done manually (or with the feet) or
torized vibrating screen.
in a pug mill.
24
Figure 4.1.1-A continued
foot. Large lumps, rocks and roots are removed during the hand kneading (wedging)
process. Only for more refined glazed products may screening be necessary.
- Many industries making whiteware do
H) Maturing under a plastic cover for a minimum of 3
days, or, better still, several weeks.
not do their own clay processing. If washed
and ground raw materials can be easily obtained from a supplier, this is usually cheaper than doing your own clay processing.
4.1.2. WET PROCESS
I) The plastic clay will need a little kneading before
being used for forming.
The wet process is used to make finer clay,
and is necessary for white bodies where it is
necessary to remove iron particles. It is easy
FIGURE 4.1.2-A Clay is mixed with water in brick tanks (I) and transferred to a dewatering pond lined with jute bags (2).
25
and cheap when done by hand for cottage
industries, but is complicated and expensive
when it is done on an industrial scale.
To do it BY HAND requires only two tanks
of brick lined with cement. The work flow
is shown in Fig. 4.1.2-B.
C) Soak clay and water in a tank, until it makes a fluid
slip. NEVER STIR CLAY BEFORE IT IS COMPLETEL Y SOAKED.
Clay preparation BY MACHINE (Fig.
4.1.2-C) follows the same principle as by
hand. The machines make the initial costs
much higher, but the two methods could be
combined. The filter press is the most costly
part and it could be replaced by dewatering
trays.
FIGURE 4.1.2-B Work flow of manual wet clay proD) The clay slip is screened and transferred to another
cess.
tank.
E) The clay is left in the settling tank.
A) Clay storage shed .
...
I
F) When the clay has settled, clear water is siphoned
B) Clay that is completely dried is easier to soak, but
off and the thick clay slip is transferred to a dewatering
drying is not necessary.
tray.
26
Figure 4.1.2-B continued
G) Dewatering tanks can be common bricks, or cloth
placed in a depression in the ground.
H) Kneading by hand or foot.
J) Clay is matured in order to increase its plasticity.
FIGURE 4.1.2-C Work flow of wet process by machine.
A) Clay storage open or covered.
B) Electric-powered blunger turns the clay into slip.
27
Figure 4.1.2-C continued
C 1) From the blunger
2) From an underground tank
3) In the filter press water runs off and the clay is re-
the slip runs through a
with a stirrer (3) the clay slip is
tained by filter cloth.
vibrating sieve (I) and
pumped (4) under high pressure
a strong magnet (2)
into a filter press.
which removes iron
particles.
D) The filter press clay "cakes" are removed from the
E) Pugging is often done in a de-airing pug mill.
press and taken to the pugmill.
F) Pugged clay is transferred to clay storage.
28
G) Clay is stored under moist conditions.
4.2. HAND-FORMING
Hand-forming methods use only the hands
and a few simple tools. They are the oldest
methods of making ceramics, but still are
useful in modem times. They require a very
small investment.
Typical products made by hand-forming
are:
- Small items, such as animal figurines and
small sculpture
- Large items, such as storage jars for grain
and water, special flowerpots, basins for
washing clothes, and big sculpture.
4.2.2. TIlE BASICS: HOW TO MAKE
COILS AND SLABS
Coils are long cylinders or "snakes" of plastic clay. Depending on how they are to be
used they can be as small as a matchstick or
as large as a man's arm. There are three
techniques which need to be mastered:
Coilmaking
Coils are easily made by rolling plastic clay
on a smooth surface. The difficult part is
keeping the coil even in diameter and round.
This requires some practice. It is helpful to
remember to roll the coil from the center to
the outside, and to roll it as far as possible
each time (from the tips of the fingers to the
base of the hand) - this will help to keep it
round.
A method requiring more expertise is to
4.2.1. TOOLS FOR HAND-FORMING
The basic operations in hand-forming are
cutting, scraping, joining, smoothing and
slab making. Tools are easy to make by
yourself. Useful tools are:
start with an elongated ball of clay between
the hands, and by rolling the hands back and
forth, to gradually cause a coil to move
downwards. This is a very useful method
when working on large pots, and, when mastered, is very fast.
wooden tools;
Coils can also be made by using a hand extruder (see page 78).
- for cutting, smoothing, scraping: metal
Slabmaking
tools, which can be made from old hacksaw
blades and heavy wire. Various shapes
made from old plastic buckets, etc.
There are several ways of making slabs, all
of which require a flat surface to work on:
- for smoothing and cutting: carved
- for shaping: wooden paddles for beating
clay and flattening slabs;
- for joining, finishing: sponges for wetting clay.
Small slabs can be made by simply pounding a piece of clay with the hand and a
wooden paddle. Or, when the slab is partly
flattened, it can be rolled with a wooden cylinder (such as is used for rolling pastry
dough).
29
FIGURE 4.2.2-B Cutting slabs witb wooden stick guides.
FIGURE 4.2.2-A Excess clay is cut from a slab made
by beating plastic clay into a wooden frame.
Large slabs can be started by slamming a
ball of clay down on the floor, turning it
over, and repeating the process until it is
sufficiently thin. With practice, slabs almost
1 meter by 1 meter can be made quickly
with this method.
Large, thick slabs for big flowerpots, etc.,
are best made on a flat floor - it is helpful to
sprinkle dry clay powder to keep the slab
from sticking. Partly-flattened lumps of clay
are joined by walking and jumping on them.
They then are thinned as much as required
by further jumping and beating with a wooden paddle.
Another way to make slabs is the "cutting
method". This is often used for making
rough slabs for tiles. A large rectangular
lump of clay is prepared and set on a flat
surface. Then a series of long wooden
30
sticks, cut the thickness of the desired slab,
is piled up on either side of the clay. These
are used as guides for a cutting wire. After
each slab is cut, one stick is removed from
each pile, and the next slab is cut. This is a
cheap and fast method.
Slabs are sometimes made by pouring deflocculated slip. This process is described in
the section on slip casting.
4.2.3. ASSEMBLING COILS AND
SLABS
No matter how you make slabs, they are
used as basic building elements for a large
variety of shapes. Because slabs are most
appropriate for flat surfaces, they are most
frequently used for the bottoms of large
pots, and for constructing rectangular
shapes.
rectangular forms
Rectangular shapes are made by first letting
the slabs dry to the leather-hard (half-dry)
stage. Pieces of the appropriate size are cut
and assembled.
joining parts
Joining leather-hard slabs must be done
carefully, so that the joints do not separate
while drying. The safest method is shown in
Fig.4.2.3-B.
coils
Forming pots by coil is a standard method,
which is still used in many countries for
making traditional pottery as well as more
modem items. Usually the coil method is
combined with beating and scraping, and is
often used in combination with the potter's
wheel (see below).
FIGURE 4.2.3-A A model of a traditional house made
from slabs and coils. Chiang Mai, Thailand.
The coil method is best used for large round
or curved shapes, such as flowerpots, that
are too big to make on the potter's wheel.
FIGURE 4.2.3-B Slab joining step by step.
A) Apply a coat of slip to the surfaces to be joined.
B) The slip then is worked into the clay by scratching
the surface - this is most easily done with a tool such as
an ordinary fork, or a piece of sheet metal with a serrated edge.
31
Figure 4.2.3-B continued
C) The pieces are then pressed together while the slip is
0) After joining the pieces, gently beat all the seams
still wet. REMEMBER TO FIRST APPLY THE SLIP,
with a wooden paddle. This helps the clay particles to
THEN TO SCRATCH IT - this works much better than
join very securely. When the joining slip is no longer
doing it the other way around.
wet, the seams can be finished with wooden and metal
tools.
E) Joins of large pieces can be reinforced with a clay
coil on the inside.
The basic method is:
- Make a base for the pot from a slab.
This usually is placed on a wooden plank or
a plaster plate (bat), and it is easiest to put
this on top of a turntable of some kind (although not essential).
- A supply of coils is then made and kept
near the working area to be used immediately (coils should not be stored, as they are
likely to dry out). Otherwise, many potters
make coils as they go along. The diameter
of the coils depends on the size of the pot,
but usually will be between 2 and 5 cm.
32
- Join the first coil to the bottom slab very
carefully - this is best done with clay slip
and scratching as for slab pots (see above).
For large forms, the coil is pinched between
the fingers to make it slightly thinner and to
raise it up.
- Then build up the walls of the pot by adding a coil all around. It works best to add
the coil to the inside of the wall, and then to
pinch the two coils together. This makes a
strong joint, since it is overlapped, and, because the wall tends to move out when
pinching it, it makes it easier to control the
shape.
FIGURE 4.2.3-D The final shape of the pot is produced
by beating it thinner with a pallet. Deokhuri, Nepal.
FIGURE 4.2.3-C A coil is added all around while one
hand pinches the inside. Once the coil is on, pinching
and smoothing are done on the outside.
- Repeat the process until the pot is tall
enough. Large pots can be built up over a
period of several days to allow the clay
walls to harden in order to carry the additional weight.
remember
Usually coil pots are made narrower and
thicker than their final shape. As they start
to dry, the final shape is given by beating
the walls of the pot, which makes the clay
thinner and expands the form. This is done
by holding a rounded piece of fired clay inside and beating the pot with a flat wooden
pallet from the outside (Fig 4.2.3-C). The
surface is then smoothed by sponging and
scraping.
It is difficult to make coil pots round and
symmetrical - this comes with practice, and
an expertly-made coil pot is difficult to distinguish from a wheel-thrown pot.
4.2.4. USE FUL HINTS
- Clay for coiling and slab making should
be less plastic than clay for throwing.
33
FIGURE 4.2.3-E An owl made by coiling technique is covered with a plastic bag for slow drying.
- If coils are added too rapidly the lower
part may sag. When making large pots, allow the walls to dry while keeping the rim
moist.
- When joining different parts make sure
they have the same moisture content otherwise the pot may crack when drying.
- Allow large pieces to dry slowly by covering them with plastic. Parts of the pot tending to dry faster, like handles, can be covered with wet cloth or plastic so they will
dry at the same rate as the rest of the pot.
34
4.3. POTTER'S WHEEL
The potter's wheel was invented in order to
make pots faster than is possible by hand
methods. It is a very old tool, and has many
variations from country to country. All potter's wheels work on the same principle,
which is that a rotating plate (wheelhead)
spins a lump of clay around, which then can
be pushed and shaped by the hands of the
potter. There are many local variations in
size, method of propulsion, height, etc.
Shapes that can be made on the wheel are
necessarily round, but they may be altered
after forming by beating or stretching to
make oval or square shapes.
The process of forming clay on the wheel is
called "throwing" in English. Nobody
knows why this word is used - its meaning
has nothing to do with "throwing" a rock.
Usually the forming process is referred to as
"throwing a pot" or "turning a pot", and the
skilled worker who does so is called a "thrower" or "turner".
4.3.1. CLAY REQUIREMENTS FOR
THE POTTER'S WHEEL
Almost any kind of plastic clay can be used
on the wheel, and sometimes clay is used
that is very difficult to shape (such as porcelain). Ideally, though, most potters prefer
clay that is quite plastic, free from rocks and
roots, and strong enough to hold any shape.
These requirements are met by clay that has
been carefully prepared, and then matured
for several weeks or months (even years in
some cases) to develop plasticity.
clay for small and big pots
For small pots, the clay can be fairly soft,
but not so soft that it loses its sllape when
removed from the wheel. Potters who make
large items often prefer clay that has up to
30 % grog or sand to reduce shrinkage, and
prepared as stiff as possible in order to prevent it from collapsing. Grog can be very
fine (even dust) if smooth finishing is necessary, but often is up to 20 mesh in size. It is
always best to have a variety of grog sizes for example from 60 to 30 mesh.
4.3.2. TYPES OF POTTER'S WHEELS
There are many types of potter's wheels,
and generally they are chosen to suit work
habits of the particular culture. There are
two main categories: unmotorized and
motorized. All potter's wheels have the
following main parts:
Flywheel: a heavy circular disc that provides momentum. It sometimes is also used
as the working surface. Flywheels can be
made from wood, cement, steel, clay/cow
manure, car tires or combinations of these.
Wheelhead: a circular disc that is used as
the working surface to which the clay is attached. Wheelheads are made from steel,
aluminum, wood, plaster of paris, cement,
etc.
Axle: the shaft to which the flywheel and
wheelhead are attached. Axles are usually
made of wood or steel.
Bearing(s): the device in which the axle rotates. Bearings may be metal bushing types,
ball bearings, wood, leather, or stone.
Power source: this is either the foot, the
hand or a motor.
UNMOTORIZED WHEELS
Unmotorized wheels usually depend on a
heavy flywheel to keep them moving once
started. The main types are:
35
Lightweight "turntables": These are used
Asian low wheel (single bearing): This type
for making small pots, and are nothing more
than a round wooden plate about 20 cm in
diameter fastened at the center to a round
stick that turns in a hole in the ground. The
potter attaches a small lump of clay, which
is shaped with one hand while he rotates the
wheel with the other hand. Often, the "coil
and throw" method is used (see below).
of wheel has a combined flywheel and
wheelhead, which may be from 60 cm to
100 em in diameter. It can weigh up to 50 kg.
The axle is usually wooden and very short,
and has a pointed end which rotates in a single stone bearing. Sometimes the axle is
fixed to the flywheel and the bearing is set
into the ground, and sometimes it is done
the opposite way. The wheel is usually rotated
with a stick that is placed into a socket on
the edge of the wheel. After it is started, the
weight of the wheel keeps it going level,
based on the gyroscopic principle (like a toy
top). It is suitable for small products such
as simple cups which are made very quickly
entirely by throwing; larger products are
partly thrown on the wheel and finished by
beating and stretching.
Advantages: very cheap; often one potter
will have a dozen or more wheels which he
works on in rotation.
Disadvantages: is only suitable for fairly
small products.
FIGURE 4.3.2-A Turntable used for "coil and throw"
method, Thailand.
These wheels are traditionally made from
wood, or from a wood and bamboo frame
which is covered with a heavy layer of clay
mixed with straw, cow's dung, hair and
sugar.
Nowadays, the wheel is often made from a
cast cement disc, or from a truck tire,
mounted on a wooden crosspiece which is
attached to the bearing.
Advantages: easy to make with low technology and local materials.
Disadvantages: Because of only the one
bearing, the wheel does not stay level. This
makes finishing on the wheel very difficult.
Large pots cannot be made by throwing
alone.
36
FIGURE 4.3.2-B One-bearing Asian wheel. Nepal.
Asian low wheel (two-bearing type): This
wheel also sits near the ground, and also
uses its flywheel as the wheelhead. It has a
slightly longer axle set in the ground, and
FIGURE 4.3.2-C Two-bearing Asian wheel made from
a truck tire.
the flywheel is fitted with two bearings
(which may be wooden, or preferably ball
bearings). As above, the wheel is rotated
with a stick.
Advantages: Because the wheel is stable, it
permits throwing larger forms quite thin,
and accurate finishing of the bottom is possible. Because the potter works in a squatting
position, there is no strain on his back (this
is also true for the single-bearing type
above).
Disadvantages: The large diameter fly
wheel keeps the potter from getting close to
his work. This is not really a problem
for a skilled potter.
37
Treadle wheel: This is in principle like a
kick wheel, but the axle has an eccentric
which is driven by an attached foot treadle,
and the flywheel is much lighter in weight
(10-20 kg). The potter can work either standing or sitting, and unlike with other wheels
keeps his foot working the treadle continuously.
Advantages: The body is not excessively
FIGURE 4.3.2-D European-type kick wheels. Tanzania.
European kick wheel: This type of wheel
has a long axle, and a separate flywheel and
wheelhead. The flywheel may weigh from
30 to 150 kg. The potter works in a sitting
position, and can rotate the wheel with his
feet while working on the clay, which
makes the speed easy to vary as required.
Kick wheels may be constructed with
wooden or metal frames, and the flywheel
may be made of wood, cement, or steel. The
main requirement is an extremely sturdy frame that will not vibrate when the flywheel
is kicked.
Advantages: The potter can get closer to
his work because of the small wheelhead,
which makes it easy to brace the arms for
stability. It is also a convenient wheel for fmishing.
Disadvantages: Puts strain and stress on the
potters back because of the awkward sitting
position, makes it difficult to see the shape
of the pot because the eyes are above it,
causes uneven strain on the leg muscles because one leg works harder than the other. It
is a very heavy machine and is difficult to
move to another location.
38
strained because of the standing position,
and the wheel speed can be easily varied
with the foot treadle.
Disadvantages: mechanically more complicated than the above wheels, but not difficult to construct with simple machine shop
facilities.
FIGURE 4.3.2-E The treadle wheel is operated by
pushing the foot treadle (1) back and forth. The foot
treadle can be placed either on the left or the right.
Two-person wheels: These wheels use one
person to turn the wheel, while the potter
works the clay. This can be done with the
Asian two-bearing wheel, where one person
rotates the wheel with his foot; or sometimes
a special wheel with a flywheel and separate
wheelhead is equipped with a hand-driven
crank which drives a cycle chain and gear
attached to the axle. A third version is
shown in Fig. 4.3.2-F
Advantages: This system is often used for
making very large pots by the coil and
throw system, when a single thrower cannot
manage to turn the wheel and throw at the
same time. It also is used where labor is
very cheap: otherwise it is a very costly
method.
MOTORIZED WHEELS
There are many vanatlOns on motorized
wheels. The two main types use fixed speed
motors or variable speed motors.
fixed speed motors
These wheels all use standard AC electric
motors. Usually, the smallest size motor for
continuous production is 0.5 HP. If large
pots are being made (up to 25 kg), it is better to increase to 1.0 HP or for very large
pots (25 kg and more) 2.0 HP.
Sometimes motors are directly coupled to
the wheel, which allows only one speed to
be used. This means the wheel is either run-
Disadvantages: Requires too much manpower; if electricity is available, it is usually
cheaper in the long run to invest in a motorized wheel.
FIGURE 4.3.2-G Kick wheel powered by an electric
motor, Nepal.
FIGURE 4.3.2-F Two-person wheel, Sinde in Burma.
The girl pushes the bar back and forth which turns the
wheel through an eccentric.
39
ning full speed or stationary. (IMPOR-TANT:
do NOT try to make an AC motor work
with a variable resistor (rheostat) - this will
bum up your motor at low speeds). Fixed
speed wheels are suitable for producing the
same product repeatedly (for example, factories making only large flowerpots sometimes use this system). In fact, some factories
have one large motor connected to several
potter's wheels with a shaft and belt system.
The advantages of a one-speed wheel are
the low cost and mechanical simplicity. The
disadvantage is lack of versatility.
FIGURE 4.3.2-H Friction drive wheel with combined
flywheel and wheelhead, Nepal.
changing the speed
Potter's wheels that have to produce a variety of sizes use several different systems to
convert fixed speed motors into variable
speeds:
Friction drive: This simplest system is also
called the "power-assisted kick wheel". It is
simply a kick wheel which has a motor
added. The motor is provided with a rubber
wheel on its shaft, and is mounted in such a
way that the wheel can be pressed against
the flywheel by use of a pedal. Essentially,
this replaces kicking with the foot, which
can be done if there is a power failure. The
speed of the wheel is controlled by engaging
and disengaging the motor. This is a simple
system to install, and is very reliable - it
only requires periodic replacing of the rubber drive wheel.
A variation of this wheel was developed in
Nepal. It has the advantage of low cost and
light weight, because the flywheel and wheelhead are combined. The maximum speed of
the wheel is about 200 r.p.m., which has
proven to be suitable for all sizes of product.
40
Cone drive: Another old system that is
often used in factories is the "cone drive"
system. Between the motor and wheelhead
is a pair of cones that rotate against each
other, and move up and down to vary the
speed. This is a fairly complicated system to
manufacture, as well as being expensive,
and has mainly been replaced by electronically-controlled DC motors.
Ring cone system: A variation on the cone
drive system is called the "ring cone" system
- the motor has a metal cone mounted on its
shaft, and the whole motor can be moved in
and out, so that the cone drives a rubber ring
connected to the wheelhead at variable
speeds. This type of wheel usually is made
to run from almost 0 r.p.m. up to a maximum of 300 r.p.m.
If you have to construct your own wheel
(with the help of a simple machine shop), it
is probably best to use the friction drive system, as this can be managed by a mechanic
who is unfamiliar with potter's wheels.
variable Speed Motors
There are two types of variable speed motors. The older type uses the principle of
movable brushes to provide variable speed
with constant torque. These motors are expensive, and are difficult to find nowadays.
They have been replaced by variable speed
electronically-controlled DC motors, which
are also quite expensive.
DC motor wheels
The advantage of a DC motor is that it provides variable speed with constant torque,
and can be controlled very accurately. The
potter's wheel is mechanically simple, because the motor is coupled with a V-belt to
the wheelhead. Unless you have a lot of
money to spend, this type of wheel is not recommended, especially in rural areas where
repairs are a problem. There is no doubt that
this is the system for the future, however.
OPEN FORMS
Open forms are those where the mouth diameter is larger than any other diameter of
the pot. This includes cups, bowls, flowerpots, plates, etc.
Here is a list of suggestions for successful
shaping:
general shaping techniques
When producing a large number of identical
shapes, weigh the clay lumps before starting
to work. A measuring tool is set according
FIGURE 4.3.3-A The various parts of a pot are named
after the human body, as shown in the drawing. From
the bottom up, they are I) the foot (if it is a ring, called
foot ring), 2) the belly, 3) the shoulder, 4) the neck, 5)
the mouth (or rim).
4.3.3. FORMING ON THE POTTER'S
WHEEL
It is not the intention of this book to give detailed instructions for using the potter's
wheel. There are many books which describe
the basic technique, and it is something that
can only be learned with the help of an experienced teacher. It should be remembered
that throwing is a highly-skilled technique,
and usually requires years of practice before
the potter deserves to be called a "master
potter".
Mouth
eck
There are a few advanced techniques, however, which are not often described in
books, and which can be easily understood
and learned by an experienced thrower:
some of these are described below.
41
to the size of mouth and depth of the pot.
Keep a record of the weight and measurements and a drawing of all your standard
products.
Open forms are likely to warp in drying and
fir-ing: a rim that is a little bit thicker than
the wall of the pot, and which is given a
convex curve, will help to hold the shape of
the pot. The rim should be rounded (without
sharp comers) to prevent chipping in use.
FIGURE 4.3.3-B A post gauge placed next to the
wheelhead is used when throwing many pots of the
Never use clear water for throwing. It will
"cut" the clay and cause the pot to collapse
more easily during throwing. Instead use a
fairly thick clay slip for throwing.
same size. The upper pointer measures the height and
width and a second pointer may be used for marking
the width of the belly.
FIGURE 4.3.3-C Steps in throwing a large pot
42
A) Center the clay in a half ball shape.
big bowls
CLOSED FORMS
Bowls of 10-25 kg require a special technique, and to be made in one piece a motorized wheel is best. The clay should be as
stiff as possible and addition of grog helps.
Closed forms are those where the mouth
diameter is smaller than the largest diameter
of the pot: this includes bottles, vases, etc.
A wooden or plaster bat should be fixed to
the wheelhead first, so that the pot can be
removed when it is finished without collapsing it. If you forget to do this, it is helpful
to fasten a piece of newspaper over the
mouth of the bowl, using a little water to
stick it on. There are a variety of "lifters"
which help to remove large pots from the
wheel: a large flat wooden scraper can be
pushed under the pot, or two flat pieces of
sheet metal can be used.
big bottles and jars
Big jars for storage, lamp bases, etc., of
10 to 25 kg can be thrown very thin in one
piece if the clay is stiff enough and a motorized wheel is used. It requires a special
technique in order to get enough height and
to keep the mouth small. For best results, a
plaster or wooden bat should be fastened to
the wheelhead, so the finished pot can be removed without collapsing.
B) Open up the center at the same time as you shape a curved bottom. It is very important to keep the rim thick, so
that there is enough clay to stretch out without collapsing.
43
C) The walls of the bowl are pulled up straight the first time. Then the rim is pulled out a few inches, and the walls
are curved out to meet the rim. This MUST BE a single smooth curve, and it NEVER should be bigger in diameter
than the rim, or you will lose control.
D) This same process is repeated as the shape develops, using a rib on the inside and a sponge on the outside. As the
bowl gets bigger and bigger, the wheel must go more and more slowly.
44
E) Apply pressure to the bottom of the pot before finishing, using fingers or a rib, working from the outside in to the
center. This compresses the clay and prevents cracks in the base.
F) The rim is given its final profile and the shape of the curve is finished. Sponge off excess slip, and trim the base
with a V-shaped tool.
45
FIGURE 4.3.3-D Steps in throwing closed forms
A) Opening is done with the fist. going down the center with a thumb to keep it on center. It is very important not to
let the mouth get wider than absolutely necessary.
B) Form the bottom to its final width. Apply pressure to the bottom to prevent drying cracks.
46
<]C) Now pull up the cylinder, keeping it wider at the
base than at the mouth. If this is done correctly, 10-15
kg of clay will come up the full length of the arm. The
trick to keep the cylinder narrow is always to keep the
outside hand UNDER the inside hand. Repeat this process to make the cylinder sufficiently thin. The mouth
of the pot should be kept thick, so that it can be shaped
successfully.
E) Finally, the extra clay at the mouth can be pulled up
and shaped into a rim. As before remove excess slip,
trim the base with a V -shaped tool and the pot is finished.
<]D) Now shaping is begun: the first pull shifts the outside hand ABOVE the inside hand, which automatically allows the cylinder to widen out. This should be
done twice at the maximum, or the clay will become
too soft and may collapse. Final shaping is done with a
sponge inside and a flexible metal or plastic rib on the
outside.
47
FIGURE 4.3.3-E
1) The potter adds coils to a pot while slowly turning
the wheel. By working on many pots in rotation there is
time for the pots to dry in between adding more coils.
2) Once the coiling is finished the pot is given its final
shape by the throwing technique.
3) The thrower is finishing the rim while his helper is
turning the wheel.
- If the clay is the correct stiffness, and the
throwing is well done, the result will be a jar
with a wall about 0.5 cm thick above the
belly, and about 1 cm thick below the belly.
The only thick area will be the bottom 5 or
10 cm, which requires some trimming.
- Another way to make big jars is by joining 2 or more sections, or using a combination of coiling, throwing and beating.
Fig. 4.3.3-E shows how potters in Swebo,
Burma, are doing this.
48
JOINED FORMS
Joined foons are assembled from separatelythrown sections: they include large bottles,
cups with stands, candlestands, and other
foons that cannot be made in one piece.
They also include foons with lids - although these are not physically joined, they
need to be made to the same degree of accuracy. All joined foons require careful measuring. It is also important to make all the
pieces at the same time, so that they shrink
at the same rate and will fit correctly. IT
NEVER WORKS TO MAKE LIDS AFTER THEIR POTS ARE ALREADY DRY!
lids
The main purpose of a lid is either to keep
things from getting into a pot, or to keep the
contents of the pot from getting out. For this
reason, lids should fit accurately.
interlocking systems
There are many different kinds of lids,
which differ mainly in the way they fit
together. This is called the interlocking system. Lids may fit inside the mouth, outside
the mouth, etc. Measuring tools should always be used for fitting lids correctly. A
skilled potter can make lids that fit tightly
without much trimming, but usually it is
best to make the lid slightly oversize and
then fit it accurately when trimming. Also,
if you plan to glaze both the mouth of the
pot and the lid, there needs to be enough
space for the thickness of the glaze.
knobs
The knob is a small round handle attached
to the lid, used for lifting it off the pot.
Knobs may be small and solid, or are made
hollow for larger sizes. Instead of a knob a
pulled handle can be placed on top of the
lid.
lids that are thrown right side up
Some lids are thrown in the same position as
they will sit on the pot. Often, the knob is
made at the same time. This type of lid only
needs a small amount of trimming.
lids that are thrown upside down
These lids are thrown like bowls. The rim
becomes the part that interlocks with the
mouth of the pot. This type requires more
trimming to finish the knob. There are many
ways to make the knob: A) it can be separately thrown and attached after trimming,
B) a small piece of plastic clay is centered
on the trimmed lid and shaped into the
knob, C) the knob is thrown as the base of
the lid (FigA.3.3-G (1) & (2)) and trimmed
to shape, D) a handle is pulled and attached
to the top ofthe lid (Fig. 4.3.3-G (3)).
lid and pot thrown in one piece
Skilled throwers can throw small containers
with lids in one piece. The lid is made as a
continuation of the container as shown in
Fig. 4.3.3-H (1). The thrown piece is left
until leather-hard and then cut and trimmed.
cups with stands (goblets)
Although these are often attempted in one
piece, they never are really satisfactory because it is difficult to remove clay from the
stand when finishing. They are best thrown
(at the same time) as a cup, and a stand
which is made upside down. When both are
leather-hard, it is easy to center the cup upside down on the wheel, join the stand, and
then remove excess clay while making a
smooth connection.
49
...-
~
2
3
FIGURE 4.3.3-F Lids that are thrown right side up. Their interlocking with the pot is shown in the right column.
50
1
2
3
......
.. "
-,",'1...,.. W
,
I
\
FIGURE 4.3.3-G Lids that are thrown upside down. The left column shows how the lids are thrown on the wheel. In
the middle is shown how they are trimmed and the right column shows the interlocking with the pots.
51
1
2
two-piece bottles
Unless you are a highly-skilled thrower, it is
difficult to make large bottles with small,
tall necks in one piece. It is much easier to
throw the body and close it in to the base of
the neck, and then separately throw the
neck. When the parts have stiffened enough
to be picked up without deforming, they are
joined - this is best done by finishing the
foot of the bottle, centering it on the wheel,
and then joining the neck while the wheel is
rotating.
FIGURE 4.3.3-H Lid and pot thrown in one piece (1). When leatherhard, the lid and pot are separated and trimmed to fit.
FIGURE 4.3.3-1 Making a goblet form.
A) The cup is thrown.
52
B) The stand for the goblet cup is thrown upside down.
C) The two leather-hard pieces are joined on the wheel.
4.3.4. TOOLS FOR THE POTTER'S
WHEEL
The tools that you use for the potter's wheel
are mainly a matter of personal preference.
The main categories of tools are:
wetting and smoothing: sponges of various
sorts (the best are natural sea sponges), a
piece of cloth, a piece of soft leather.
cutting: A needle tool is essential for cutting rims and checking wall thickness. This
FIGURE 4.3.4-A Tools for throwing: ruler (I), pointed
knife (2), V -shaped cutter for the base (3), calipers for
measuring both inside (4) and outside (5).
53
can be made from a big needle, a nail, a
piece of wire. A knife can be used, or a
sharpened piece of wood or bamboo.
shaping: For smooth surfaces, wooden,
plastic, rubber or metal "ribs" are used,
mainly on the outside but sometimes on the
inside of the pot.
finishing: A triangular piece of sheet metal
or a sharp wooden knife is useful for undercutting the bottom before removing the pot
from the wheel.
common pots
Cheap storage jars and other low-cost items
are simply cut off the wheel and dried. With
some clays, this results in cracks in the base,
which are caused by not compressing the
bottom of the pot enough when opening up
the clay. This happens frequently when making multiple small pots from a single large
lump of clay. If this is a problem, it usually
can be corrected while throwing in the opening-up stage: the bottom is opened out as
usual, then the fingers move from the outside in to the center of the bottom, while pressing downward. This compresses the clay
particles in the bottom.
measuring: A variety of wooden sticks,
calipers, etc., are used for accurate measuring of mouth size, lid size, depth of pot, etc.
4.3.5. FINISHING ON THE POTTER'S
WHEEL
Finishing requires just as much skill as
throwing, and profitable production depends
on efficient fmishing. By "finishing", we
mean the process of removing excess clay
from the bottom of the pot, and forming the
foot ring that the pot sits on. This is sometimes called "trimming". The amount of
finishing and the style of finishing depend
upon the quality of the product being made.
Common pottery normally is not finished
very much, if at all, but high quality pottery
also needs high quality finishing.
Different qualities of finishing are described
below, starting with the simplest (for common pottery) and going on to the most complicated. Quality finishing takes a lot of
time and is too costly for common pottery often, making a foot ring takes longer than
the making of the pot itself.
54
Another way to correct the problem is to
beat the bottom of the pot when it is leatherhard (half dry). This is done by placing the
pot on a plaster of paris bat, and using a
round wooden stick with a flat end to compress the bottom from the inside.
rolled foot
The foot of a pot can be rounded and finished when leather-hard by rolling it on a flat
surface. It is also a good idea to slightly
press in the bottom to make it sit better and
to help prevent drying cracks.
better quality pots
These require foot rings. The purpose of the
foot ring is to give the pot a stable base to
sit on, and especially to make setting in the
kiln easier. The foot ring is usually not
glazed (except for high biscuit, low glaze
firing tableware), and it makes a logical
stopping place for the glaze.
The process of making the foot ring involves removal of excess clay. This is called
"trimming" or "turning" (the same word is
used for lathe work. This is confusing, be-
1
FIGURE 4.3.5-A Trimming tools: wire loop tools (1)
and tools (2) made from metal strips.
cause the same word is sometimes used for
throwing!). Trimming needs to be done
when the clay is at just the right stage of
leather-hardness - otherwise it is difficult
and the result will not be satisfactory. There
are many different tools used, and choosing a
tool is mostly a matter of individual preference. The main types of tools are wooden
tools, metal tools, and wire loop tools - a
variety is shown here.
centering for trimming
The first step is to center the pot UPSIDE
DOWN on the wheelhead. This requires a
lot of practice before it can be done quickly
and easily.
- One method is to slowly rotate the pot,
and hold a pointed tool steadily near the
foot: it will make a line where the pot
is farthest from center - this line is pushed
toward the center of the wheel, and the process is repeated until the tool makes an even
line all around the foot. This is a slow pro-
cess, and normally is done only for special
shapes, or pots with fairly small mouths that
do not sit stably on the wheelhead. The pot
is then secured to the wheelhead with pieces
of plastic clay.
- For small and medium open forms, experienced throwers use the "tap centering"
method. A small amount of water is first
placed on the wheelhead in the area of the
pot's mouth. The pot is then placed upside
down as much on center as possible, and the
wheel is rotated at medium speed. While
slightly pressing the bottom of the pot with
one hand, the potter gently taps the pot with
his other hand until it is on center (this process is impossible to describe in writing, and
must be learned from an experienced potter). It needs to be done quickly and surely,
and if the pot is at the right of stage of dryness, the water will stick it securely to the
wheelhead.
chucks
When trimming a large number of identical
shapes, or shapes that will not sit on the
wheelhead (closed forms), the potter makes
use of a "chuck". This is a cylinder of
bisque-fired or dry clay, or sometimes
plaster of paris, which is used to hold closed
forms inside it, or open forms outside it. The
potter makes his own chucks according to
his products.
When using chucks, the main problem is to
hold the pot firmly without damaging it.
The chuck needs to be centered on the
wheel, and then a coil of plastic clay is fastened inside it, and also must be centered
accurately. A plastic clay chuck, on the other hand, is sometimes covered with a piece
of cloth or plastic to keep it from sticking to
the pot.
55
potter is skilled. Usually, the foot ring is located first, and then excess clay is removed
from the inside. A skilled potter knows
when the bottom is thin enough just by the
feel of his tool. If he is in doubt, he can tap
gently on the center of the bottom with a
finger, and the sound will tell him if the
thickness is correct. Then the excess clay is
removed from the outside of the foot ring,
and the ring is smoothed with a fingertip or
a damp sponge.
It is usually helpful to use a smooth wooden
tool to compress the bottom, working from
the outside to the center. With sensitive
clay, this also helps to prevent cracks in the
base.
Before removing the trimmed pot from the
wheel, it should be smoothed with a damp
sponge (although many potters like to leave
the marks of the trimming tool).
After the completed pot is removed from the
wheelhead, the rim should be smoothed
with a sponge, and is usually placed upside
down for drying.
FIGURE 4.3.5-B Chuck used for trimming a vase with
a narrow mouth (I). Another chuck (2) is used for
trimming saucers of identical shape.
In either case, the pot is placed on the
chuck, and then is centered by making sure
its bottom is perfectly level. If done correct1y' the entire pot will then be on center, and
can now be trimmed.
Trimming itself is done in many different
ways, all of which work satisfactorily if the
56
If very high foot rings are required, they are
either made separately as for goblets
(above), or another technique is to throw the
foot ring directly on the pot. After centering
the leather-hard pot upside down on the
wheelhead, either a ball of plastic clay is
joined to the bottom and thrown to the
necessary shape; or, for larger pots, a thick
ring may be thrown separately, joined to the
bottom, and then thrown to the final shape.
The foot ring should in any case be made
wide enough to give stability to the pot. A
bowl with a narrow foot ring easily tilts,
spilling its contents.
FIGURE 4.3.5-C A cup rim is smoothed with a piece of cloth while the cup (1) is sitting in a chuck (2). Note the
chuck is covered with a piece of cloth to avoid marks on the cup.
57
FIGURE 4.3.5-D A bowl's function and appearance
depend on its foot. Here three different types are
shown. (I) is made by turning away excess clay, (2) is
made by throwing on the pot and (3) is made by joining
individually-made leather-hard parts; bowl and foot
ring.
4.3.6. DRYING
The main problems occurring during drying
are warping and cracking. As with all clay
FIGURE 4.3.6-A Saucers stacked rim to rim for drying.
58
products, the best way to prevent these problems is by drying slowly and evenly. This
means avoiding wind and direct sun, and
covering products in dry weather.
helpful hints for drying
Open forms (cups, bowls, plates) of the
same size are best dried stacked rim to rim.
This helps to prevent warping.
Pots that have accidentally become too dry
for joining should usually be thrown in the
scrap bucket. However, they can sometimes
be wetted by dipping them in clean water.
- When drying pots to the leather-hard
stage for trimming, they need to be watched
very carefully to be sure that the rims do not
get too dry. When the rims are leather-hard,
it is a good idea to tum the pots upside
down until the foot is ready for trimming.
4.4. JOINING TECHNIQUES
- Large open pieces can have their mouths
covered with plastic to keep them from drying too fast.
- Covering pots with wet cloth and plastic
can keep them from drying indefinitely.
Many products require joining of several
parts. In general, when clay is joined it often
causes problems of cracking or separation
during drying. It needs to be done carefully
and skillfully in order to be successful. This
section discusses special problems in joining.
FIGURE 4.4. I-A Pulling handle on the pot
A) A short lump of clay is attached to the leather-hard jug.
59
Figure 4.4.1-A continued
B) While holding the jug with one hand the handle is
C) When the handle has the right thickness the lower
pulled with the other.
end is attached to the jug.
4.4.1. HANDLES
Handles are fonned in a variety of ways.
The small workshop usually makes handles
by hand or extrusion, and then joins them to
leather-hard (and already-trimmed) products:
coil
A coil is rolled to the desired diameter, and
then it is shaped by pressing and smoothing
with a damp sponge.
60
pulling
A piece of plastic clay is slightly elongated
and then is stretched into a handle by repeatedly pulling it with a wet hand (like milking a cow). The wetting should be done with
clay slip, not clean water. Some prefer first
to attach a lump of clay to the pot then do
the pulling while holding the pot with their
other hand. The handle can also be pulled
first and then joined.
Figure 4.4. I-A Pulling handles - Pulling separately
D) A lump of clay is held in one hand while the handle
E) The handle is cut off and left to harden.
is pulled with the other.
F) The handle is attached to the jug.
extruding
Handles are extruded from a hand extruder
(see below). This is a fast method for making large numbers of handles. Fig.4.4.I-B
shows a method for unskilled workers to
produce quality handles. A skilled potter
will not need templates to produce evensized handles.
61
FIGURE 4.4.I-B Extruded handles.
I) After extrusion handles are cut to same size and
2) The curved handles are cut to same size in a mould.
curved on a template.
3) The cup's joining point is scratched to increase strength of the handle.
62
Figure 4.4.1-B continued
4) Slip is applied to both the cup and the handle.
5) Excess slip is sponged off and at the same time possible rough spots on the cup are cleaned.
63
FIGURE 4.4.1-C Handle plaster press mould. A rolled or an extruded clay coil is placed in one mould and the two
moulds are then pressed together. A thin seam remains to be trimmed off.
pressing
Handles are pressed from plastic clay, using
a small plaster two-piece mould.
slip casting
This is a standard method for industries producing large volumes of crockery by casting
or jiggering. The handles are cast in moulds
that produce many handles with one pouring.
drying items with handles
Products to have handles added should be
turned upside down as soon as possible after
throwing. This allows them to dry slowly
and evenly. Many products with handles are
64
damaged by cracking that occurs when a
wet handle is joined to a dry product. Dry
products can be wetted with clean water before joining the handle, but this should only
be done in emergencies. After joining the
handles, tum the pot upside down for even
drying.
4.4.2. JOINING COMPLICATED
FORMS
The main point to remember about joining
separate parts is that the clay must have
reached the correct stage of dryness meaning leather-hard. In general, all parts
should be at about the same dryness.
FIGURE 4.4.1-D Making handles by slip casting in plaster moulds.
When joining, always:
- FIRST coat both surfaces to be joined
with clay slip.
- Scratch both wetted surfaces with a fork
or serrated-edge tool, which works the slip
into the clay.
- Press the two surfaces together firmly.
- If possible, beat the joined area lightly
with a wooden paddle, and work the seam
together with a rounded wooden tool.
Another important point is that drying
should be slower than usual, so that the different areas ofthe pot dry evenly.
Some potters do not need to take this much
care when joining different parts, because
they have a tolerant clay body. But if you
have cracking problems you should follow
all the steps mentioned above.
4.4.3. SPECIAL JOINING
TECHNIQUES
detlocculated slip
If a lot of joining has to be done on a regular
basis (for example, a factory that produces
rectangular flowerpots using the slab
method), it is worthwhile using deflocculated
casting slip as "glue", which is made from
the same body as the slabs. This permits
faster joining, because the surfaces require a
minimum of scratching before joining. It also has about the same water content as plastic clay, so even a thick layer of it will
shrink about the same amount in drying,
which helps to prevent cracks. (See slip
casting and fiberslip below).
pegged joining
When joining big handles or handles that
have to carry a heavy load, it is a good idea
to use an interlocked joint, similar to a carpenter's joint. A hole can be cut in the pot,
and the end of the handle shaped to fit into
65
it. The surfaces are slipped and scratched as
usual (this is a particularly good use for
casting slip), then fitted together, and the
end of the handle inside the pot is pressed
and finished like a metal rivet. This is not
usually needed, but if there are problems
with specialized products, it does give them
an added amount of strength.
4.5. JIGGERl
The jigger is nothing more than a potter's
wheel which has been modified to produce
uniform products semiautomatically or
automatically. It has been a standard forming tool in the ceramics industry for many
years, and is likely to be around for a long
time in the future, as it continues to develop
into a more and more sophisticated (even
computer-controlled) machine. Roller head
machines are now replacing the jigger machine for flatware production in larger industries.
Basically, the jigger machine consists of a
spinning wheelhead which is fitted to hold
plaster of paris moulds, and a movable arm
which holds a metal profile. Clay is placed
in or on the mould, and the operator pulls
1 In some countries jiggering means that the spinning
mould forms the inside (saucers) and jollying means
that the spinning mould forms the outside (cups).
Here we shall term the technique jiggering only.
FIGURE 4.5-A Roller head forming of plates. Clay is cut (1) in round slices and placed on plaster moulds (2). A
spinning head (3) forms the plate over the mould and the plates with moulds are then placed in a mangle dryer (4).
66
down the profile, which forces the clay into
the shape of the product. The forming
method is called jiggering.
The forerunner of the jigger is still used in
many countries, usually for making plates
with shallow relief designs. Plaster moulds
for the inside curve of the plate are centered
and fastened to a potter's wheel, a slab of
clay is placed on the mould, and the potter
smooths and shapes the back of the plate
and foot ring. Excess clay is trimmed from
the rim, and the product is set aside to dry.
The rim is finished when it is stiff enough to
hold its shape.
4.5.2. AVERAGE PRODUCTION
QUANTITIES
Average production quantities for the basic
jigger vary greatly according to the skill and
management of workers, as well as being
different for large and small products.
However, the following figures can serve as
a guide to setting production quotas:
ThailandlBurma: 1200 teacups per day
India: 2000 mugs per day
Nepal: 800 cups per day
4.5.1. APPROPRIATE USE OF THE
JIGGER
Europe: 1600 sugar bowls per day
2400 saucers per day
It is a necessary tool for industries which
produce tableware, such as cups, plates,
bowls, and other open shapes. It is mostly
used for products that are round and simple
in shape, but there even have been jiggers
developed for making oval shapes.
It is not suitable for small, one-man industries, as it is only cost-effective if its full
capacity for production is used. One jigger
can produce several hundred pieces per day,
and the factory needs enough kiln capacity
to manage these quantities.
Most forms made on the jigger can also be
produced by slip casting, but this is actually
slower and requires more finishing time.
The jigger requires a large number of
moulds, but because the moulds are relatively thin, they can be dried and reused faster
than slip casting moulds - they also do not
absorb as much water, since plastic clay is
used in the process.
Semiautomatic jiggers in Europe produce
from 8 to 14 articles per minute.
Mould life also varies greatly, depending on
the quality of the product, the quality of the
plaster of paris, and care taken in handling
the moulds. Average figures are several
hundred times for one mould - high quality
products require more frequent mould replacement.
4.5.3. JIGGER WORK FLOW
JIGGER WORK FLOW AND REQUIREMENTS
raw materials
Plastic clay for jigger work can be any
standard clay body. However, it is prepared
much softer than clay for wheel throwing.
This allows it to be easily shaped in the
mould. The clay does not need to be as plas67
tic as for throwing, although plastic clay
gets a smoother finish.
mould use
The same mould can be used several times
per day, depending on how fast it is dried.
Most small producers air-dry their moulds,
which usually limits use to twice per day,
depending on how dry the air is. Producers
using an artificial dryer can use moulds up
to 4 times in a single shift.
FIGURE 4.5.3-A Work flow of jiggering
C) The operator takes empty moulds from a trolley rack
next to the jigger and returns them when they are filled.
When all moulds are finished the rack is replaced with
anew one.
A) Clay, softer than used for hand throwing, is brought
to the jigger. One worker prepares even lumps of clay
ready for forming.
D) The trolley rack with full moulds is placed in a dryer.
B) A plaster mould is placed in the chuck and forming
is done by bringing down the inside template.
68
E) After the formed items have dried a little they are taken out of the moulds and placed on ware boards. The moulds
are returned to the jigger machine.
F) Finishing of the rim is done in a spinning chuck.
G) The leather-hard items are ready for next production
step: attaching handles or drying.
finishing
The method of fInishing depends on the properties of the clay body. Nonnally, the only
finishing necessary is smoothing the mouth.
In many factories, this is done at the bonedry stage, using sandpaper. This is not rec-
ommended, because the dust is a health hazard for the operator; it should only be done
if a good exhaust fan and dust mask are
used. Instead, fInishing should be done with
a wet sponge at the leather-hard stage (Fig.
4.3.5-C).
69
4.5.4. JIGGER MACHINE
PRINCIPLES AND
CONSTRUCTION
We describe here only the simplest types of
jigger machines, since the more sophisticated ones cannot be made by a simple machine shop. The machine shown from Nepal
was constructed using only a machine lathe
and an electric arc welder.
main parts of the jigger
Wheelhead: This is equipped with a chuck
for holding the plaster mould, as shown.
There are two options: 1) chucks made of
mild steel or cast iron, 2) a flat wheelhead
on which a plaster chuck is fitted. The
wheelhead is usually set to rotate at 250350 r.p.m. Small products like cups can be
rotated faster - some machines go up to
400 r.p.m. It is very important for the chuck
to run true.
FIGURE 4.5.3-B Finishing of a dry bowl in a booth
with exhaust (Royal Copenhagen).
FIGURE 4.SA-A Jigger sy tem for forming plates.
Main pans are: wheelhead with chuck (I), mould (2).
plate (3), profile (4). jigger arm (S). counter balance
(6). stop adjustment (7).
70
and the machines are connected to the shaft
with flat belts. In this case, a simple clutch
system is necessary for each machine, in
order to stop it when necessary.
Clutch: The clutch is optional. Many machines have wheelheads with no clutch. A
skilled operator can insert and remove
moulds without stopping the wheel. Clutches
are of two types: a spring clutch is used with
V-belt drives, whereas a sliding belt system
is used with flat-belt drives.
FIGURE 4.5.4-B Jigger machine with a simple drive
system without clutch.
Because plaster chucks wear out fast, causing the moulds to go off center, they are
sometimes reinforced on the inside rim with a
rubber lining, a metal ring or a lead collar the latter is T'1elted and poured in place.
Axle and bearings: The axle should be
2.5 cm in diameter, and good quality ball
bearings should be used.
Drive system: Machines which have individual motors use pulleys and V-belts to reduce the motor speed and provide mechanical advantage. In factories where several
machines are needed for larger production,
one large motor often drives a long shaft,
Variable speed drive: This is also optional.
Most producers make standard products,
and one speed is sufficient for products
ranging from plates to cups. In more sophisticated machines, a friction drive allows the
speed to be reduced for larger items. A
cheaper solution is to fit multiple size pulleys on motor and axle so speed can be adjusted by changing the V-belt.
Jigger arm: There are numerous jigger arm
shapes. The main requirement is that it be
very rigid (it must not vibrate in use), so
cast iron is the preferred material. The bearing where it rotates also must be very solid:
the usual system is a cone-type bushing,
where the cone tension can be adjusted with
lock nuts.
Profile or template: Normally, the profile
is made from mild steel, which is carefully
ground to the exact shape. The steel needs
to be at least 3 mm thick. Metal profiles
are sometimes backed up with wood to give
them better rigidity. The profile will wear
down with use, and it is necessary to regrind
it from time to time. Additionally, master
profiles should be kept for standard production items, so that new working profiles can
be made as necessary.
71
The profile has to be adjusted so it is exactly
on center and does not leave a "bump" or
spiral in the center of the article.
For limited production items, profiles are
sometimes made from acrylic plastic or
even from wood. These, of course, have a
much shorter working life than mild steel.
The profile is mounted with adjustable nuts
and bolts, which allow it to be accurately
positioned and shifted as necessary.
4.5.5. MOULD MAKING FOR THE
JIGGER
As with slip-casting moulds, jigger moulds
require a plaster model, master mould, and
case mould from which the working moulds
are made. (Refer to section 6. on mould
making).
limitations on shape
One-piece moulds: Usually, moulds are
only made in one piece. Because the product
needs to be easily removed from the mould
when leather-hard, shapes must be designed
for this purpose. Moulds must not have any
undercuts which might catch the clay. Also,
there should not be any sharp edges - foot
rings must be rounded and not very high.
Cylinder shapes are not very successful because the working moulds are very difficult
to separate from the case mould. Instead,
straight-sided shapes should be slightly tapering.
Two-piece moulds: Sometimes, two-piece
moulds are used for shapes that curve in
slightly at the top, or have a foot ring that
curves out slightly. Usually, it is more efficient to form this kind of shape by slip casting.
decorated moulds: Moulds are often made
with shallow relief designs. This is particularly effective with plates. Some factories
use this technique, and then finish the plate
by hand-carving when leather-hard. This
gives an effect that seems 100 % handmade,
and greatly increases the retail price of the
product.
FIGURE 4.5.5-A The left jigger mould has undercut at (1). This will make it impossible to release the pot from the
mould. The right jigger mould has no undercuts and its slightly tapering shape makes mould release easy.
:... '
......
'0 ' .
:i··.: .
72
4.5.6. FORMING PROCESS
adjusting profiles:
The profile has to be adjusted so that the jiggered item has the right shape and thickness.
This is done by cutting the jiggered item in
half with a knife. The thickness should
match that of the original sample. The profile may get out of position and its surface
will wear out, so the production supervisor
should cut control samples at least once a
day to make sure products are up to standard.
profile on the inside:
- Prepare balls of clay in advance. They
should be weighed for accuracy, but experienced operators will be able to judge the
correct quantity by taking lumps from clay
on the bench. If clay is prepared in a pug
mill the extruded clay wad can be cut in
slices of a length corresponding with the required weight. They are then placed in a
convenient location for the worker.
- Also prepare ware boards and moulds
in advance, which means they MUST BE
CLEAN, and set in a convenient location.
FIGURE 4.5.6-A
A) Jiggering a cup. Excess clay is pressed upward.
73
- Place a mould in the chuck. (Normally,
the machine is running and does not need to
be stopped to insert or remove working
moulds).
- Place or throw a ball of clay in the
mould. It is pressed into the bottom and
then opened up with one hand for large
items. With small shapes, the profile can be
brought directly down on the ball of clay.
- Bring the jigger arm down into the
mould, which will force the clay into shape.
Excess clay is removed from the top of the
mould and from the profile. Before remov-
ing the profile, the inside of the article
must be wet with water to smooth the surface.
- Remove the mould from the chuck and
set it on a ware board.
profile on the outside:
- Prepare slabs of clay in advance. This is
usually done by cutting slabs from a block
of clay using wooden sticks and a cutting
wire (see slabs, above). Slabs can also be
beaten out on a wet plaster bat with a wet
plaster beater. (If the beater is dry, the slabs
will stick - it is soaked in water every night
Figure 4.S.6-A continued
B) A little water is dripped on the inside to smooth its surface and the excess clay is trimmed off.
74
for the next day's work). Plates made from
such slabs need more finishing work. For
better quality plates slabs are prepared by
throwing on a potter's wheel or on a bat making jigger machine as in Fig. 4.5 .6-B.
letting water drip from a sponge to smooth
the surface.
- Remove excess clay.
- Remove the mould and plate and set the
- Put a mould in the chuck with the al-
latter on a ware board.
ready-prepared slab on it.
- Sponge and press the slab with the hand
while it rotates to stick it firmly to the
mould.
- Bring the jigger arm down to form the
back surface of the plate, at the same time
4.5.7. FINISHING
Finishing is a separate operation and is best
done by a separate worker if there is enough
production. This keeps forming and finishing going continuously.
FIGURE 4.5.6-B A set of three jigger machines. The operator in the middle makes slabs on a flat wheelhead with a
flat profile. The slabs are used by the two other operators for jiggering bowls with an outside profile.
75
FIGURE 4.S.6-C
A) Water is dripped on the back of the saucer being formed.
B) Excess clay is removed from the rim. With a correctly shaped profile only a little clay has to be removed.
76
All clay products are best finished when
they are leather-hard. In this case, the piece
is set in the chuck, the wheel is rotated, and
the outside is finished with a wet sponge.
Normally, the area most needing finishing is
the mouth or rim.
4.5.8. DRYING
FIGURE 4.S.6-D Slabs for flatware jiggering prepared
by wet plaster beater.
Finishing can be done on a jigger wheel, or
on a potter's wheel. The wheel is set up with
a chuck, which often is thrown from plastic
clay to fit the inside of the form. It is best,
however, to make a chuck from plaster
which exactly fits the product.
FIGURE 4.S.7-A Finishing the rim of a jigger bowl
with a wet sponge.
Drying of jigger products requires no special techniques, but the general recommendations for potter's wheel products should
be followed, such as setting pieces rim to
rim to prevent warping. Wet jigger moulds
will not release the jiggered item and in
order to increase production artificial drying
of moulds should be considered (see
chapter 9.2).
4.5.9. JIGGER PROBLEMS AND
SOLUTIONS
Problem: Clay sticks to the moulds.
Solution: This is sometimes a difficulty
with new moulds. They may be dusted with
a small amount of talc or fine grog powder
to prevent sticking. Usually, the problem
stops after a mould has been used a few
times.
Problem: Breakage of moulds.
Solution: Early breakage of moulds may be
caused by poor quality plaster of paris.
Sometimes, adding about 5 % cement to the
plaster mix will help the strength, although
it is better to look for another plaster supplier, if possible. Or moulds may be too
thin, especially in the top rim, and should be
made thicker. Moulds are often reinforced
by setting a wire ring inside the rim during
mould casting.
77
Problem: "Feathering" in the bottom of the
inside (cups, bowls), or on the bottom of
saucers - this looks like a spiral or "butterfly".
Solution: Correct adjustment of the profile.
Metal of profile too thin, which causes
"chattering". Jigger arm too thin, which
causes vibration. First make the profile thicker or place a wooden plate on its back. If
this is not enough try to reinforce the jigger
arm.
Problem: Rapid wearing out of plaster
chucks.
Solution: Metal rings can be cut accurately
from 2-mm mild steel, and set into the rim
of the chuck for long chuck life. A rim of
rubber inside the chuck is often used but it
is difficult to make it run true. An old method is to pour melted lead into the chuck rim.
Melted lead does not harm the plaster and it
is easy to trim.
Problem: Uneven thickness of products.
Solution: Wrong setting of the profile causes uneven thickness of a product's cutthrough profile. Profile is refixed or it may
need to be reground. A plaster mould running off center causes the jigger item to be
thick on one side and thin on the other. The
problem may be that the jigger head or plaster chuck is off center or that the clearance
between mould and head has become too
great. The plaster mould may from the beginning be off center due to faulty original
models or case moulds.
4.6. EXTRUSION
Extrusion means the process of forcing plastic clay through a shaped mouth, called an
extrusion die or nozzle. The verb is "to ex78
trude". The simplest extruders are hand-powered for forming handles, etc., and the most
complicated ones are very sophisticated
machines for producing large products like
pipe.
4.6.1. HAND EXTRUDERS
Hand extruders consist of a cylinder to hold
clay, and a piston to force it through a die
attached to one end. A medical syringe
(with the needle removed) can be used to
produce very small coils for decoration - so
this is actually a very simple extruder. Hand
extruders can be used for small solid extrusions (like handles), and can also extrude
hollow pipe up to about 15 cm in diameter.
There are two systems for providing the necessary pressure:
- Screw system: This uses a long screw attached to the piston to compress the clay.
This machine is sometimes called a "wad
box".
- Lever system: A lever arm is attached to
the piston, and pulling it down forces the
clay through the die. There are two ways to
drive the lever arm: by a "ratchet" system,
where the end of the lever is engaged in
slots on the support arm, or by the friction
system, where a metal ring holds the lever
arm in position. In either case, the principle
is the same as a "bumper jack" for automobiles, and this type of jack can actually be
modified to power an extruder.
Dies for the extruder can be made from mild
steel, acrylic plastic, or waterproof plywood.
The extruder cylinder is fitted with a screwon cap which holds the dies, so it is easy to
change them as required.
There are several different kinds of pug
mills that can be made with simple technology. The main parts are:
- Motor: ranging from 1 HP for very small
mills up to 30 HP and more for the largest
ones.
- Reduction gear system: Because speeds
range from 15 to 25 r.p.m., reduction is usually done with gears. This may be a combined V-belt, pulley and gear system, or direct
gear system. Frequently, either an automobile transmission or differential gearbox is
used.
FIGURE 4.6.l-A Hand
extruder used for forming
posts for kiln setting.
FIGURE 4.6.2-A Horizontal pug mill with main parts
named: gearbox (1), 3-HP motor (2), bearing house (3),
Clay for hand extruders needs to be fairly
soft, so that it can be pushed through the die
easily. If pipes are being extruded, they can
only be made relatively short, because the
soft clay is difficult to transport without collapsing. These "pipes" are normally used as
cylinders for making products like flowerpots.
feed hopper (4), plunger (5), mixing barrel (6).
4.6.2. MOTORIZED EXTRUDERS
Motorized extruders are simply pug mills
equipped with extruder dies according to
their purpose. Most pug mills are used for
the purpose of producing plastic clay body
for other forming methods, but almost any
pug mill can be used for extruding products
such as small hollow bricks. Specially-designed pug mills are used for extruding stoneware pipes and split tiles, which require higher pressure and more control.
79
De-airing chambers have a door which can
be opened for cleaning, and the better ones
have an easily removable screen, for the
same reason.
FIGURE 4.6.2-B Interior of pug mill barrel with feed
auger under hopper (I), mixing blades (2) and extrusion
cone mouth (3).
- Axle and blades: The main problem with
the axle design is that any gears or bearings
should be isolated from the clay. Blades can
be made simply from flat mild steel. When
thorough mixing is required the blades are
spaced widely at the beginning and more
closely at the outlet. For high output it is
made the other way around, but in both cases
at least two blades are spaced opposite each
other at the outlet, like a propeller, to pro.
. 1.
duce compreSSIOn
and proper extruSIOn
High quality units use stainless steel for
both shaft and blades to avoid contamination of white clay bodies with iron oxide.
- De-airing chamber: This is optional. Deairing treatment improves plasticity and reduces lamination problems, but proper maturing and brief kneading after pugging will
do the same. It consists of an input screen
which feeds clay "noodles" into the box,
and a vacuum pump which removes air
from the clay. It is fairly complicated to
make your own de-airing chamber, as it needs
to be absolutely airtight. Correct design is
also very important, since a common
problem is that clay does not exit from the
chamber fast enough, resulting in blockage.
1 "The Kenya Ceramic Jiko - A Stovemaker's Manual"
by Hugh Allen, IT Publications, London 1990.
80
- Extruding mouth: For clay production,
either a round or rectangular extruding
mouth is used. This is a cone section with a
cylindrical mouth. The purpose of the cone
is to compress the clay, which improves its
quality. The diameter of the outlet mouth is
normally about 2/3 of the barrel's diameter.
Pug mills can be fitted with a variety of special extrusion dies for directly making products. Common dies are for solid bricks,
hollow bricks (of various shapes and sizes),
split tiles, and pipes. NOTE: Many of these
products require very stiff clay and therefore
specialized pug mills.
Common types of pug mills
Vertical pug mill
- drum type, non de-airing, side extrusion.
This is a pug mill which uses a drum up to
60 cm in diameter for mixing clay. The earliest version of this mill used an animal to
turn it, and the mill often consisted of a
wooden box, which contained a wooden
shaft mounted with a series of wooden
blades (which sometimes had wires running
between them to cut the clay). More modern
versions are made from metal drums, and
have a metal axle and blades which are
powered by an electric motor. The speed of
the pug mill is quite slow - about 8-15 r.p.m.,
and the speed reduction can be obtained by
a combination of V -belts and pulleys, which
are connected to an old automobile differential gearbox. The clay is extruded from a
rectangular opening about 10 x 15 cm at the
FIGURE 4.6.2-C Drum pug mill for mixing brick clay.
Gearing is done with V-belts and an automobile differential gearbox.
bottom. The limitation of these pug mills is
that they can only extrude rather soft clay,
and cannot be used for direct product
extrusion, but the low cost and simplicity of
the machine make it very popular for small
producers.
The pug mill shown in Fig. 4.6.2-C is larger
than usual, and is used mainly for mixing
clay for insulating bricks (50 % by volume
rice husks and sawdust), which normally
needs to be quite soft. It can be filled with
dry clay body mixture and water, unlike ordinary pug mi1ls, which are usually fed with
already plastic clay. It is equipped with a
door on the extruder mouth, which is closed
while the clay is mixing, and then opened to
empty the mill. It has a 5-HP, 3-phase
motor.
FIGURE 4.6.2-D Vertical pug mill with top barrel
removed. The blades are welded onto individual pipe
sections for easy replacement.
Vertical pug mill
- single shaft, non de-airing, bottom extrusion.
This is also a simple kind of pug mill, and is
capable of being made by a small producer.
It consists of a cylinder and a series of
blades which mix the clay and force it out of
the end. It cannot practically be used for
forming, because compression is too low.
Horizontal pug mill
- single shaft, non de-airing. This is as
simple as the horizontal pug mill, but sits
horizontally. It can be fitted with simple
dies and a cutting table, for products like
hollow bricks.
81
FIGURE 4.6.2-E Single shaft pug mill used for extrusion of solid bricks. Two rollers feed the clay from the hopper
to the auger. Capacity: 1200 brickslhour. Henan, China.
FIGURE 4.6.2-F De-airing pug mill, double shaft mixing barrel with additional extrusion shaft. Made in Thailand.
82
Horizontal pug mill
- single shaft, de-airing: Has a vacuum box
which removes air from the clay. This requires a screen, which extrudes small "noodles" of clay into the vacuum box. After
the air is removed, the clay is recompressed
and extruded. This is used for producing
high quality clay body, and can also be used
for hollow bricks and split tiles. De-airing
pug mills are rather sophisticated to construct, and should not be attempted by the
inexperienced. Single shaft pug mills are a
bit difficult to feed, because the clay tends
to get stuck in the feeding mouth. Many of
them are fitted with a metal roller, which
helps force the clay into the blades, or with
a plunger like the one shown in Fig. 4.6.2-A.
Horizontal pug mill
- double shaft, de-airing: This is the same
as above, but in the first mixing chamber, it
uses two shafts fitted with screw blades that
tum toward each other. This makes feeding
it easier, and it also mixes the clay better.
The clay is compressed and extruded by a
third shaft.
- Combined vertical drum mixer and
horizontal extruder, non de-airing: This type
of machine is widely used in small-scale tile
and hollow brick industries. It uses a drum
to thoroughly mix (already plastic) clay, and
then feeds the clay into a horizontal extruder.
- Combined horizontal mixer and vertical pipe extruder, de-airing: This is a mixer
which feeds a vertical pug mill equipped
with a die for pipes. The machinery is heavy
duty, because the clay for pipes must be
quite stiff.
FIGURE 4.6.2-G Vertical mixerlhorizontal extruder used for forming tile slabs for leather-hard pressing. Main
parts: Vertical mixer (I), extruder barrel (2), differential gearbox (3) and gearbox (4) from a car.
83
4.6.3. TYPICAL EXTRUDED
PRODUCTS
Extrusion in recent years has gained more
and more popularity, as it becomes very sophisticated. It probably finds its main use in
the heavy clay industry, for production of
hollow construction bricks and blocks, and
for split tiles. While most of these products
are too complicated for the small producer,
there is good scope for the smaller and
simpler types of hollow bricks.
Clay for hand extruders should be prepared
as for wheel throwing; i.e. thoroughly
kneaded so there are no air bubbles. The
clay is then formed into a cylinder slightly
smaller than the extruder pipe, and placed in
the pipe (which previously has been cleaned
and fitted with the appropriate die). Clay is
then extruded and cut to the needed length.
Extruded products
kiln shelf stands
Most small producers can benefit from the
use of a simple hand extruder, which is a
small investment that can quickly pay for itself in increased production of handles or
decorative elements.
Also called "posts", these are easily made
with a hand extruder (Fig.4.6.3-A). Plans
are given here for typical cross sections, and
the construction of dies for hollow forms is
shown.
FIGURE 4.6.3-A Cross section of kiln shelf stand with hollow extrusion die.
3
84
FIGURE 4.6.3-8 Extrusion of red floor tiles.
A typical clay body for stands is fireclay
with about 30 % fine grog (60 mesh and below).
The extruding step is quite easy - usually
long pieces (30 to 45 cm) are made, but
stands require accurate cutting and finishing
so they are easy to use. Cutting is best done
when the clay is softly leather-hard. First,
the extrusion is straightened and trued by
gently rolling it on a flat surface. A wire
cutter can be used with a cutting box similar
to a standard carpenter's miter box. When
enough stands of the same size have been
made, they should be placed together vertically on a flat surface, and accurately sized
by placing a flat wooden plank on top of
them. This is beaten gently to make the
stands exactly the same size. They then are
ready for drying.
tiles
The most common type of tile made by extrusion is the split tile, which is made by
special pug mill extruders that are capable of
using very stiff clay. The tiles are extruded
in pairs which are cut in two after firing.
Particle orientation and lamination2 produced by the auger action cause the tile to
warp in drying or firing. This tendency can
be reduced by striking the right balance between stiffness and plasticity of the clay
body, auger speed and length of reducer and
extrusion die. Sometimes wires are set inside the barrel after the last extruder blades
2 For more information on particle orientation and
lamination see "Clay Materials - for The Self-reliant
Potter" also in this series.
85
in order to reduce lamination by cutting up
the clay.
It is nearly impossible for the inexperienced
potter to make his own tile extrusion dies. In
industrialized countries this job is left to
specialists who will tailor dies for the particular extruder and clay body used by the
customer. It is not recommended for the
small producer to try to make his own extrusion dies for split tile making.
However, for small decorative tiles that do
not need to be especially flat or exactly
equal in size, the hand extruder can be used.
The pug mill can be used for extruding clay
slabs to be used for leather hard pressing,
because the pressing breaks up the auger lamination by realigning the clay particles.
FIGURE 4.6.3-C Extrusion of hollow bricks. The clay
handles
One of the best uses of the hand extruder is
for making handles. Lengths of clay are extruded, and then cut to the desired size (see
chapter 4.4.1).
bricks
Bricks are made with a horizontal pug mill,
or with a combined vertical drum pug mill
with horizontal extruder. There are two systems - using soft clay and using stiff clay which require different machinery. De-airing is not normally used for common quality
bricks made by small factories, although it
certainly is necessary when special quality
bricks (glazed bricks, for example) are desired. In large brick industries, where large
and complicated bricks are made, very
heavy duty de-airing extruders are used.
column runs onto a cutting table where it is cut into
bricks. Chiang Mai, Thailand.
4.6.3-C. This has the advantage of reducing
the amount of clay used for each brick and
thereby also reducing fuel cost. The bricks
are still strong enough for normal construction and they are better heat insulators compared to solid bricks.
clay for hollow bricks
The clay used is fairly rough clay suitable
for common bricks, but may need slightly
higher plasticity. It normally is not washed,
and in fact may often contain some roots
and rocks. For this reason, if the pug mill extruder has a screen, it is generally removed
so that it does not become blocked.
hollow bricks - soft clay system
hollow bricks
When bricks are formed by extrusion they
can be extruded hollow as shown in Fig.
86
Machinery for the soft clay system does not
need to be heavy duty, so it is quite inexpensive. It produces a brick from clay the
FIGURE 4.6.3-D Extrusion die for small hollow bricks with a finished brick sample.
consistency of wheel-throwing clay, so it is
feasible to fit a die on a standard pug mill.
The main disadvantage is that the product is
difficult to handle, so it requires large numbers of boards on which the wet bricks
are placed for drying. The holes are normally made to run through the largest face
of a standard size brick (as in Fig. 4.6.3-C),
because this gives it the greatest strength
when used in construction. Extruders with a
small barrel can be made to extrude bricks
with the holes running through the end faces
(Fig.4.6.3-D). These will be less strong but
they are quite suitable for filling in walls in
reinforced concrete constructions and for
single storied houses.
hollow bricks - stiff clay system
The stiff clay system requires more expensive machinery, which must be strong enough
to extrude almost leather-hard clay. The advantage is that the bricks are strong enough
to handle immediately after extrusion, and
can even be stacked for drying. It also
permits easily making larger size bricks.
Extruding machinery can be powered either
by an electric motor, where a 3-phase line of
suitable capacity is available, or by a petrol
or diesel engine. For a typical soft clay extruder, a lO-HP motor is required, and for
the larger stiff clay extruder, up to a 30-HP
motor.
87
Hollow bricks production need not be done
in a factory, and the bricks are usually made
during the dry season in monsoon countries.
The only requirement is a simple shed roof
to cover the machinery and motor. Kilns
may be the periodic type, which mayor may
not be covered with a roof, or a continuous
type of kiln is used (usually a "bull-ring"
FIGURE 4.6.3-E Hollow brick work flow
A) Clay is stored in the open. If possible the brickwork
is put up next to the clay pit.
C) Clay is mixed in a U-mixer or a drum mixer. This
normally feeds directly into the pug mill.
B) Clay is soaked with water.
D) The premixed clay is extruded onto a cutting table
from where the cut bricks are
taken for drying.
E) Soft extruded bricks are initially dried on
boards and as soon as they can be handled they
are dried in normal stacks.
F) Hollow bricks are fired in ordinary brick kilns.
88
-
kiln). Standard brick kilns can be used for
hollow bricks with no conversion. Another
system for firing uses rice husks as fuel.
Bricks are stacked inside a semipermanent
kiln wall which is also made of hollow
bricks, and gaps between the bricks are
filled with rice husks. The "kiln" is left
open at the top, and the fire is started from
one side on the bottom. As the rice husks
bum and settle, more rice husks are filled
from the top. This type of firing may take
several days.
pipe
Clay pipe has been used since ancient times
for carrying both fresh water and sewage. It
also is used as stovepipe for cooking stoves,
and as perforated pipe for drainage and irrigation.
Ceramic pipe can be made with diameters
up to about 1 meter. It usually is produced
in lengths up to one meter, as longer pipes
are difficult to dry and fire without bending.
Additionally, a variety of special shapes are
made: bends (elbows), tees, siphons, etc.
parts of ceramic pipe
- Barrel: This means the main tube of the
pipe.
- Flange: This is the expanded end of the
pipe, sometimes called the "collar".
- Socket: the inside of the flange
- Nipple: the small end of the barrel, which
fits inside the flange.
pipe forming and appropriate products
wheel throwing
This is generally used for relatively thin
pipe for stove chimneys, or for short dis-
tance, low pressure water lines. Pipes made
this way are relatively rough, but quite suitable for these uses and can easily be made
by traditional potters as a new product.
(Smokeless stoves are very popular in development work, and provide new sources
of income for traditional potters). These
pipes are generally 10 to 15 cm in diameter,
and are easily made 60 cm long.
The process is to make the pipe in 2 pieces,
each about 30 em long, on the potter's wheel.
One piece is a straight pipe, and the other
one is thrown with a flange on the top. The
sections are joined when leather-hard.
These pipes are generally fired at low temperatures, using common clay and traditional firing systems. There is no reason,
however, why they cannot be made from
stoneware clay and glazed, where small
quantities are required.
Average daily production: from 10 to 30
pieces.
slab construction on wooden moulds
This technique uses a wooden form shaped
to the inside of the pipe. Slabs are prepared
using any standard method (see slab construction, above) and then are wrapped
around the form and joined. A special shaping device fits into the top of the form, and
by rotating it around the slab, the flange is
fmished.
These pipes can be used in the same way as
hand-thrown pipe. They have the advantage
of being more uniform than wheel-thrown
pipes.
Average daily production: about 30 pieces,
by 2 persons.
89
fittings are usually made by piecing together
sections of straight pipes that have been cut
to the appropriate contour.
Average daily production: about 30 pieces.
extrusion
FIGURE 4.6.3-F Making pipes by wrapping a slab
around wooden moulds, Karnataka, India.
slab construction in plaster moulds
This uses a two-piece mould made from plaster of paris, or clay/cement (see moulds
below). Rather stiff clay is made into slabs,
and cut into two long strips which are fitted
into the two halves of the mould. The edges
are trimmed so that they come slightly
above the edges of the mould. Then they are
coated with slip and scratched. The two
halves of the mould are placed together and
pressure is applied. Then the seam is
pressed together from the inside, using a
sponge and the hands. If the clay is stiff
enough, the mould can immediately be
placed upright so the pipe sits on its small
end (not on the flange). The mould is carefully removed and can immediately be used
again. This is quite a fast process for an experienced worker.
This process is suitable for straight pipes,
and also is commonly used for special fittings, such as bends.
More complicated shapes such as tees, Yconnectors, junction boxes and reduction
90
The standard industrial process for making
stoneware sewage pipe and water pipe is by
extrusion. This is done with rather heavy
machinery, and depends on fairly large production quantities to make it profitable (starting from a minimum of about 8 tonnes per
day). Therefore it is recommended only if
there are a sure market and sufficient supply
of raw materials. Additionally, sewage pipe
because of its low profit margin tends to be
produced for a fairly local market, since
transportation cost over long distances is not
competitive.
Because pipe manufacturing on this scale
requires a major investment and specialized
equipment, it is recommended that first the
process be studied in an existing factory.
Additionally, machinery manufacturers
should be consulted for their recommendations, bearing in mind that they have a
vested interest. Before deciding to order machinery, the wise producer will first visit
FIGURE 4.6.3-G Pipe extrusion work flow.
A) Clay is stored dried.
Figure 4.6.3-G continued
B) Clay and grog are mixed and crushed in a pan grinder and then screened.
C) The screened pipe body is soaked with water.
G) The leather-hard pipes are cut to size on a trimming
machine.
D) The soaked clay is premixed in a pug mill.
H) The pipes are left to dry in the open, in a dryer or on
top of the kiln.
I) Before firing the pipes are glazed with a slip glaze.
E) Combined horizontal pug mill with vertical extruder
forms pipes with flanges.
~1_1~L-~~
llllllllllllllll
J
'\
-
-
,
."
;
~
F) The extruded pipes are left to dry until leather-hard.
1) The pipes are fired to vitrification in special kilns.
91
other factories using their machinery and
discuss problems with the operator. It is also
important to find out about servicing the
machinery. If your factory is far from the
equipment supplier, you should make sure
that the machinery can be maintained and
repaired by local mechanics.
dry mixing if necessary. For softer clays, a
rotary hammer mill or pin mill of sufficient
capacity will do a good job, as well as being
cheaper.
There are variations in the work flow, depending on what kind of clay, ftring system,
etc., are used. For example:
- Screening: Since pipe clay is rather
coarse, a rotary screen is sufficient for removing particles bigger than 16 mesh
(standard insect screen can be used). This
can easily be fabricated locally, is cheap and
easy to maintain. Alternatively, a very simple
vibrating screen can be used.
- different clays require different grinding
machinery. A hard, nonplastic clay usually
requires a pan grinder to break it down. For
this kind of dry grinding, the type of pan
grinder with a perforated pan works well,
but a solid pan type may be a better choice,
because it also can be used for wet or semi-
- An alternative to soaking the clay and
then using a standard pug mill is to use a
double shaft mixer (U-mixer), which mixes
powdered clay body and water efficiently.
However, a standard pug mill can be used
for other purposes if desired - for example,
for producing hollow bricks.
pipe production hints
FIGURE 4.6.3-H Pan grinder with rotary screen.
92
FIGURE 4.6.3-1 Double shafts rotating towards each other in aU-mixer.
FIGURE 4.6.3-J Pipe being extruded.
- The pipe extruding machine is really the
only specialized machine required for this
process. It is essentially a vertical extruder
equipped with a die for the pipe barrel. It is
equipped with a special die that first moves
up under the extruder to form the pipe
flange under pressure, then moves
downward as the pipe barrel is extruded.
- A clutch system disengages the extruder
while the pipe is cut or even just pulled off
the machine. Then the flange die moves
back into position under the extruder and
another pipe is formed.
- The trimming and finishing machine is
simply a rotating horizontal metal cylinder
that fits the inside diameter of the pipe. A
leather-hard pipe is pushed onto it, and then
is cut to the correct length with a special knife
tool, that also makes grooves at the end of
the barrel for gripping the mortar better.
93
FIGURE 4.6.3-K Pipe trimming machine.
- Drying the pipes is normally done in the
open air, with the pipes lying vertically on
the floor. It is important to rotate them several times a day, as they will bend if they
dry unevenly. Factories located in monsoon
countries have difficulty with drying during
the rainy season. If a continuous kiln is
used, it can be built under a high roof so that
the top of the kiln can be used as a hot floor
dryer. More sophisticated drying systems
are normally not used, as they increase the
cost of the pipes too much.
- Grog: Pipe clay body normally contains
about 10 % grog. This grog is obtained from
pipe that is damaged in firing, which in a
successfully running factory is normally
about 10 %. The grog can be made in the
same pan grinder used for grinding clay body.
94
5. SEMIDRY FORMING
This type of fanning is done with press machines, which work on the principle of applying high pressure to force the clay into a
mould. Press moulding of soft clay is mainly used for fanning roofing tiles with complicated interlocking shapes, but there are
drawbacks with soft clay pressing: clay easi-
ly sticks to steel dies, the soft products are
difficult to handle, high drying shrinkage
causes warping and cracking during drying.
Dies for soft clay pressing are often made
from plaster provided with air holes so release is done by forcing compressed air
through the mould.
FIGURE 5-A Press moulding of roofmg tiles. The mould slides in and out and the operator is placing a wooden
frame on top of the pressed tile. He will tum the die and the tile will then rest on the frame. Taungbi in China.
95
Semidry forming processes were developed
to reduce some of these problems associated
with plastic clay forming.
Semidry powder pressing
Semidry powder means clay with 7 to 12 %
water content that is prepared in powder
form. The clay looks like powder, but when
some of it is squeezed in the hand, it sticks
together and holds its form. Because of this
characteristic, semi dry forming is limited to
products that can be made under pressure in
a mould or die, such as tiles, simple electrical articles (fuse holders, low tension insulators) and kiln shelves.
leather-hard pressing
Leather-hard means clay with about 15 %
water content. It is prepared to approximately the final shape when plastic, then allowed
to dry until leather-hard before pressing.
Semidry forming either requires beating by
hand, or the use of a press machine with a
metal die.
dry pressing
This uses clay with only 2 to 7 % water content. Because this requires great pressure,
the machinery is too expensive for small
producers.
5.1. ADVANTAGES AND
DISADVANTAGES
The major single user of semidry pressing
is the wall and floor tile industry. Tile making is an attractive industry, as use is growing rapidly all over the world, and the
market is increasing for exports from developing countries. Tiles at first glance seem a
simple product to make, since they are
96
just flat squares of clay. However, the big
problem is keeping them flat and square
through the various stages of drying and firing, and keeping them the same size in firing (unless the temperature is perfectly
even, there will be differences in firing
shrinkage).
In order to reduce these problems as much
as possible, there is clearly an advantage in
semidry pressing, which at least prevents
most drying shrinkage problems.
large tile industries:
However, as mentioned in the introduction,
the small producer cannot compete with
highly-automated, large-scale industries.
The state of the art for large-scale wall tile
production is based on using local red-firing
clays, automatically dry pressing them under tremendous pressure, and then fast-firing them (under 45 minutes) in roller hearth
kilns. Very accurate temperature control
produces accurately-sized tiles, with a very
small rejection rate. These production lines
are often computerized, and all the handling
of tiles is done on automated conveyor belts
- even grading and selecting and packaging
of the finished products.
market niches:
This leaves the small tile producer with the
possibility of other market niches, such as
hand-decorated tile (relief, engobe, underglaze, overglaze) and especially unglazed
floor and wall tile made from local red-firing clay. These tiles can be produced with
relatively inexpensive machinery, and periodic kilns can be used for firing with profitable results. Semidry pressing is then feasible (either powder or leather-hard system).
Marketing can be aimed either at the local
construction industry, or at the specialty export sector.
Nowadays in developing countries, there is
a rapidly growing middle and upper class
that is interested in adding status to their
homes. This means that there is a growing
market for items like tiles for the outside of
houses, either glazed or unglazed, facing
bricks (which are tiles made to imitate good
quality bricks), floor tiles for interior and
exterior, and even glazed roofing tiles. All
of these are made by semidry pressing.
is made of steel. This type of mould is
called a "die".
Because the work flow for semidry pressing is very different from plastic clay, and
different clay preparation machinery is
used, it usually is not combined in the same
factory as plastic clay forming, or else a
separate section of the factory is set up
exclusively for this process.
Better dies are made from hardened steel,
and are correspondingly expensive. They
make up for their cost by having a long working life, and can be machined to have sophisticated rounded edges.
electrical insulators:
For making simple electrical goods like fuse
holders, where a nonhollow form is required, semidry pressing is also the method
of choice. Hollow electrical goods are
generally slip-cast. A successful fuse holder
factory can make use of cottage industry, as
described in the introduction. By only producing clay body and firing finished goods,
this type of industry reduces labor problems
and factory space, by simply purchasing the
unfired fuse holders.
tile dies
Inexpensive dies are made from mild steel,
which is easy to machine but does not have
a long life (1-2 months for dies that are used
daily). They are suitable for very simple tiles
that do not have details like curved edges,
and for small pressings like fuse holders.
A set of dies consists of a top die, also called
a plunger, and a bottom die. The bottom die
is shaped like a shallow box. It has a metal
frame of mild steel which makes up the
sides, and in good dies, this frame is removable from its support, which is designed to
fasten to the base of the press. The frame
wears down after many thousands of pressings, and periodically must be replaced.
The floor of the die can slide up and down
in the frame, and is provided with a lifting
FIGURE 5.2.1-A Set of dies for pressing hexagonal
tiles consists of: top die or plunger (I), die frame (2),
bottom plate (3). The die frame is set in a tray (4). A
pressed tile (5) is shown.
5.2. PRESS MACHINES
Because press machines are needed for most
of these products, this section will describe
various types that are available.
5.2.1. DIES
All press machines depend on a mould,
which, because of the heavy pressure used,
97
device that fits into it from the bottom. After pressing a tile, the lifter is operated, and
the tile is raised up to the level of the frame
for easy removal.
The top die is fitted to the screw in such a
way that it does not rotate.
Correct
alignment of top and bottom dies is EXTREMEL Y important, as an expensive die
can be damaged beyond repair very easily.
If the press is operated without any clay in
the die, this will also damage the die.
In some tile dies the frame is lifted for releasing the pressed tile instead of the bottom
plate. This arrangement is used on the friction press in Fig. 5.2.3-A.
40000 kg = 178 kg per cm2
225 cm2
This may not be sufficient pressure to produce a strong 15 x 15 cm tile.
However, a tile die measuring 10 x 10 cm
has an area of 100 cm2 . Then:
40000 kg = 400 kg per cm2
100cm2
This would make a compact and strong tile
on the same press.
For smaller tiles, dies capable of pressing up
to 4 tiles at the same time can be made.
pressure requirements
The size of tile that can be powder-pressed
depends on the amount of pressure applied.
One source! gives the following data:
dry
semidry
plastic
2-7 % water
9-12 % water
15-20 % water
300-500 kg/cm 2
100-250 kg/cm2
100-150 kg/cm2
Successful pressing also depends on the
type of clay body used, and needs to be tried
out.
Standard friction presses are rated at 40 to
80 tonnes total pressure. This equals 80000
kg force applied to the die. Because the
pressure of the die is distributed across the
total area to be pressed, the kg per square
centimeter will be less with larger dies. For
example, a tile die measuring 15 cm x 15
cm has an area of 225 cm2. Then:
1. Hoffmann "Technologie der Feinkeramik", page
121.
98
other dies
Dies for fuse holders and other small shapes
sometimes are fairly complex, and are made
from mild steel by machining. They often
have removable pins for producing necessary holes in the pressing. These pins are removed after each pressing, and replaced for
the next.
Dies are also used for pressing "thimbles" small devices for separating tiles during firing.
Large dies are used for pressing saggars,
firebricks, and items like flowerpots.
semiautomatic presses
For semi-automatic production, presses are
equipped with rotating heads that have up to
6 bottom dies. As one die is pressed, the
next one is being filled, and the previouslypressed tile is removed. This increases production quantities greatly. This type of press
is often used for roofing tiles.
manual forming
For making kiln shelves (setter slabs),
powder clay is used in a metal frame, which
is made from reinforced angle iron. It is fitted with a sheet metal bottom plate. This is
not used in a press machine, because the
size requires more pressure than ordinary
presses can supply. Instead, the clay is
beaten into the frame by hand (see kiln
shelves, below).
5.2.2. HAND-OPERATED SCREW
PRESS
This machine uses a long screw to supply
pressure to a metal die. The screw turns in a
brass bearing, and the quality of this bearing
determines the usefulness of the machine.
The machine is available in a variety of
sizes, and depends on a heavy hand-turned
flywheel to supply the necessary pressure.
FIGURE 5.2.2-A Hand-operated screw press.
FIGURE 5.2.2- B Press moulding grooved plate for
electric cookers.
teamwork
It is quite a heavy machine, and must be
fixed to a foundation to keep it from shifting
in use. This type of press can be used for
any product, with either powder or leatherhard clay. Small screw presses are used for
tiles, and have a two- or three-man team: one
man manages the clay and die, and the other
two tum the flywheel. The largest sizes
press saggars and firebricks, and may have
as many as 3 workers turning the flywheel,
while two workers manage the clay and
heavy die.
production capacity
Daily production figures quoted from Europe (8-hour day) can be up to 2000 tiles, or
one every 14 seconds. This is with a very
experienced and motivated work crew. An
inexpert crew might manage half that
amount. It also assumes that there are no
delays, such as lack of clay, machinery
breakdowns, etc.
99
5.2.3. FRICTION PRESS
This is simply a motorized version of the
screw press, which has two movable "friction wheels" which can be moved horizontally. These wheels are driven by a motor
through a belt system, and in one position
move the press down, and in the opposite
mode move it up. They are usually made of
cast iron or mild steel, and the flywheel is
provided with a leather or fiber surface (the
same as used for flat belt drives) which provides the necessary friction.
FIGURE 5.2.3-A Friction press with friction wheels
(I), flywheel (2), die (3), handle (4) for moving the
screw (5) up and down.
so· tonne press
Friction presses also come in a variety of
sizes. They normally are rated according to
"tonnes", which means the amount of tonnes/force they can supply. A common size for
up to 15 by 15 cm tiles is "80 tonnes". This
size of press commonly uses a 3-5 HP, 3-phase
motor. Larger presses are available. Friction
presses do not necessarily apply more pressure than hand screw presses. The advantage is that they do it with less manual labor.
When selecting any screw press, it is important to be sure that it has sufficient stroke
length. This is the distance the top die travels from top to bottom of each stroke. For
pressing saggars, there needs to be enough
distance to remove the saggar easily.
Typical production figures for friction
presses with inexperienced workers are
about 125 presses per hour (double stroke).
Friction presses are often equipped with
double bottom dies. In this system, the dies
FIGURE 5.2.4-A Toggle press used for pressing fuse
holders.
100
slide back and forth, so that one is being
pressed while the other is emptied and refilled. This greatly increases production quantities.
5.2.4. TOGGLE PRESS
The toggle press is used for light pressing,
such as fuse holders and other very small
items. It works on a rack and pinion gear
principle.
hard clay. It works on the principle of an eccentric shaft which rotates and activates a
piston on which the die is mounted. This
supplies a very high mechanical advantage,
so high pressure can be supplied with a relatively small motor. However, it has the
drawback of a short stroke length. For leather-hard tiles, it is an efficient and low-cost
machine. It cannot be used for powder pressing, because it is too difficult to fill the die.
5.2.6. SIMPLE LEVER PRESS
5.2.5. ECCENTRIC PRESS
The eccentric press is also commonly used
for tile production, but only with leather-
There are two types of lever press which can
be used for pressing soft clay, as for tiles
with relief designs. These are used for low
production, custom-made tiles.
FIGURE S.2.S-A Simple manual eccentric press used
for pressing stilts for kiln settings.
FIGURE S.2.6-A Hand-operated lever press.
101
lever press
One is simply a long lever arm attached to a
fulcrum, with a plaster mould or a metal die
connected. A slab of clay is placed on the
table, the mould is pressed into it, and excess clay is then trimmed from the sides.
5.3. SEMIDRY PRESSING WITH
LEATHER-HARD CLAY
This process is used for all types of tiles floor, wall, and roof. The general work flow
is shown in Fig. 5.3-A.
ratchet press
The other type uses a mechanical principle
which is the same as an automobile jack, except that it is operated by a lever arm rather
than a screw. Mechanical advantage comes
at the top of the stroke, where rather high
pressure is obtained. This is also used with
plastic clay.
FIGURE 5.3-A Work flow for leather-hard pressing.
soil block press
Another type of lever press is commonly
used for pressing soil blocks for house construction. There are many variations on this,
with the original known as the "CinvaRam". This type of press could be used for
firebrick pressing, and even for tiles, but
those who have tried it say that it is slow
and produces uneven work.
A) Plastic clay is taken from store. The clay is prepared
as described in chapter 4.1.
5.2.7. HYDRAULIC PRESSES
Hydraulic presses use hydraulic cylinders to
provide pressure. They have the advantage
of producing very high pressure, but also are
very expensive, and unless specially
equipped, are slower than friction presses.
B-1) Blanks are formed in a mould. Excess clay is cut
off by a wire.
hydraulic ram press
A specialized type of hydraulic press is
known as the "ram press". It uses plastic
clay and special moulds made of hydrostone, that are fitted with air tubes to release
the pressing. This was a patented process
until recently, and only used in the U.S.
102
B·2) Blanks can also be extruded and then cut.
Figure 5.3-A continued
C) Blanks are placed on ware boards and left to dry
E) After pressing the tiles are placed on boards and
until leather-hard.
then finished by hand.
F) Drying completely.
D) The blanks are taken to the press machine for press-
-
ing.
G) Kiln firing is started with a long smoking period.
leather-hard clay requirements
method. However, since the tile blanks
should all be a uniform size, frequently a
wooden or metal mould is used.
Clay for leather-hard pressing varies according to the type of product. As usual, large
tiles (such as roof tiles) should have a higher
percentage of nonplastic materials like sand
or grog added. The clay can be prepared by
hand, or for larger production by machines
(see chapter 4.1).
blanks:
As seen in the work flow, the first step is
preparing "blanks". A blank is simply a
slab of clay, which can be made in a number
of ways. Frequently, slabs are cut from a
clay block using the sticks and cutting wire
dry separator:
The mould is sprinkled with a dry separator,
which usually is dry clay powder kept in an
open-weave cloth bag (such as a jute bag).
This is quickly done by shaking the bag in
the mould. Then a piece of clay is pressed
into the mould, and the excess is cut from
the top with a cutting wire. The mould is
inverted onto a ware board, so that the blank
neatly falls out.
103
wet separator:
The water release method is also used,
where the blank is formed in a metal frame
with water used as a separator.
finishing:
The leather-hard tiles, after pressing, are set
on ware boards to dry further. In some cases
the edges of the tiles are made smooth immediately after pressing, but this may also
be done just before firing. This process is
called fettling.
warping:
FIGURE S.3-B Making blanks from plastic clay in a
two-piece metal mould. Clay is thrown into the mould
and excess clay is cut off with the bow string.
During drying the tiles may need to be turned once or twice to avoid warping. When
they are distinctly leather-hard, they are
checked for warping. Warping can be corrected by placing the tile in a single bottom
die, and gently beating it. They then are
dried completely.
FIGURE S.3-C Pressing of leather-hard blanks. The pressed tiles are placed on a turntable ready for fettling.
104
5.4. PRESSING WITH POWDER
CLAY
As mentioned before, the advantage of pressing with semidry powder is that the pressed product is almost dry, and has almost no
shrinkage, and thus minimum warping. Besides the actual pressing, the clay body
needs to be specially prepared in the form of
granules containing 6 to 10% water. Various methods for doing this are described below. The work flow for semidry powder
pressing is:
C) The pressed tiles are placed on boards.
5.4.1. SEMIDRY POWDER PRESSING
WORKFLOW
FIGURE S.4.1-A Work flow of semidry pressing
A) Granules of clay with right moisture content are
taken from store to press machine.
D) Tile edges are fettled with a damp sponge or by
moving the tile back and forth on a coarse screen.
E) The tiles are dried completely.
B) Measured amount of body is filled in the die,
leveled off and pressed twice.
F) Firing after a long soaking period.
105
granulation
~~:~\:..~'J;.~.;.. ..
There are three main processes for preparing
clay granules, with the following work flows:
---~~~:2*,,.~t~,---
- DRY PROCESS
This is the simplest of the processes in terms
of equipment required and number of steps
taken.
"
D) Powdered clay is screened. This may not be necessary.
FIGURE S.4.I-B Work flow of dry process granulation
E) Clay is mixed with possible other materials in a
drum mixer or with a shovel.
A) Clay from storage.
F) Clay body with addition of water is mixed in a pan
.,..
.
mill.
G) Screening through 10 mesh and 40-60 mesh. Material coarser than 10 mesh and finer than 40-60 mesh
is returned to the pan mill.
4'.::
• ••
~
•
.~
.
B) Clay is dried to less than 10% moisture content.
H) Granules with a moisture content slightly higher
than needed for pressing are left to mature for several
days,
I) Moisture content is checked before pressing. GraC) Clay is powdered in a hammer mill 'or pin mill.
106
nules are taken to press machines.
Many variations in this work flow are
possible. The optimum process depends on
the nature of the clay, the force of the press
and the size and shape of the tile. It may be
possible just to grind the clay, adjust its
moisture content and then press it. Granulation can also be done in a pin mill without a
screen. The pan mill and the final screening
to remove the fines may be left out if the
press body does not tend to cause lamination. Granules can also be produced in a pan
mill with perforated bottom. The mixing
can be done in the pan mill if this is operated in batches instead of continuously.
C) The clay with a water content of 10 - 15% is granulated in a pan mill or pin mill.
D) Granules are matured for a few days until the moisture content is correct.
- WET PROCESS
This starts with the standard wet process of
preparing clay (see chapter 4.1.2). Following that process the plastic clay is here used
as the starting point of the granulation
process.
FIGURE 5.4.I-C Work flow of wet process granulation
E) Moisture content is checked before taking the granules to pressing machines.
Although complicated and requmng expensive equipment (filter press, blunger,
agitator, diaphragm pump) this prepares reliable body that works with any suitable
body composition.
A) Clay cakes are taken from filter press to drying area.
B) The clay is air-dried to a water content slightly
higher than needed for pressing.
spray drying
Clay body is first prepared as in the wet process, but instead of being pumped into a filter press, it is pumped into a spray drying
tower, where it is sprayed into a stream of
hot air that dries it to the correct moisture
content, as well as producing more or less
spherical granules. It then goes directly to
the press. This is the state of the art system
which is appropriate only for relatively
large-scale industries, due to cost and
sophistication of the machinery.
107
5.4.2. HINTS FOR SUCCESSFUL
PRESSING
There are some aspects of the process which
are worth discussing in detail, as they are
problems that have come up in our experience and that are not discussed in available
books.
granulating
The main purpose of granulating is to get
the clay particles into coarse condition, so
that they consist of small "clumps" or
"balls" of clay body larger than 40 to
60 mesh. This is important to prevent lamination problems (discussed in pressing,
below).
Granulating is not necessary for all clay
bodies. Generally, bodies which are fairly coarse - containing coarse fireclay and
grog (ranging approximately up to 30 mesh)
- can be dry-blended and then mixed with
the correct amount of water. With such
easy-to-press bodies, the water is usually estimated by volume, and is sprinkled evenly
into the dry body while stirring and turning .
it with a shovel. It is correct when body
squeezed in the hand holds its shape.
A simple form of granules can be produced
by adding all water to the grog which is then
mixed with the dry clay body. The clay will
stick to the grog and form small granules
and at the same time the water will be distributed evenly.
The body is often mixed with kerosene oil
about 1-2% (sometimes up to 5% is necessary) as well as water, which prevents sticking to the die. This type of body may also
press successfully without maturing, although it is always better to mature clay be108
fore forming it.
Both green and fired
strength of the pressed tile increase considerably if the body is allowed to mature for
some days.
Bodies that contain high amounts of fine
clay (ball clay, clay high in montmorillonite), or slippery materials such as some
kinds of talc, cause greater problems for
powder pressing, and require granulation.
For tiles, this is c?rrected in two ways: by
adding grog to the body, and by adding wetting agents (soap) to the water.
pressing work flow
Pressing itself has a fairly complicated work
flow :
- Inspection: Checking that die is clean.
Clay tends to stick to the die, and needs to
be removed with a stiff brush (fiber or brass
wire type) periodically.
- lubrication of die: The die needs to be
lubricated periodically with light oil to prevent clay from sticking. Frequency of lubrication and type of oil that is successful depend on the clay body. Commonly used
FIGURE 5.4.2-A A frame containing clay powder is
moved forward and back again over the die, filling and
leveling in one operation.
oils are kerosene, diesel oil, waste crankcase
oil (either plain or thinned with kerosene),
coconut oil, palm oil thinned with kerosene,
etc. This can be applied with a piece of
cloth.
- filling the die: In order to get the same
thickness of tile every time, the die must be
filled the same amount. This is done by adjusting the bottom plate, so that when filled
to the top and leveled, the tile is the correct
thickness. It is easy to make a filling device, which is simply a metal or wooden
frame that contains clay powder. Sliding
the frame over the bottom die automatically
fills the die, and sliding it back on the press
table scrapes the clay level.
- pressing stroke: With coarse bodies, it is
possible to press only one time. However,
with most bodies, two pressings are required
for each tile. The first press stroke is done
rather slowly - its purpose is to allow air to
come out of the clay powder. Then, a
second press stroke is done rapidly, which
fully compresses the clay.
FIGURE 5.4.2-B Steps in tile pressing
A) The die is wiped with a cloth soaked in oil.
B) A measured portion of clay powder is filled in the die.
C) The flywheel is turned and the tile is pressed in two
D) The bottom die is lifted and the pressed tile is care-
strokes.
fully lifted off and placed on a board.
109
- lifting the bottom die: This is done as
the top die is raised to its highest position.
two sliding dies), there needs to be a system
of signaling between the workers.
- removing the tile: The tile is lifted carefully from the die, and placed on a ware
board. If the bottom die is plain, without
relief pattern or rounded edges, the filling
frame can be used to push the tiles to the
other side, where it is picked up by an
assistant or by a conveyor belt.
safety switches
5.4.3. SAFETY SAFETY SAFETY!
ALL PRESS MACHINES ARE DANGEROUS! It is very easy to lose a finger or
even a hand. Workers need to be instructed
in correct use of the machine. The main
point is that BOTH HANDS MUST BE
USED TO PULL THE PRESS LEVER, so
that there is no chance of having a hand under the die. It is much safer for the same
person who fills the die to also pull the press
lever. If two persons are working with the
press (as is common with presses having
Many countries now have safety regulations
for workers using dangerous machinery. It
is common for press machines to be
equipped with two switches, which must be
held down in order for the press to move
-one is on the press lever, and the other is
located some distance away so that both
hands are out of the way before the machine
will operate. This is difficult to do with a
friction press, but it is highly recommended
that the manufacturer be required to equip
his machinery with such a safety device.
dust
Semidry bodies produce a lot of dust during
body preparation and during pressing and
fettling . Inhalation of clay dust over a long
period of time will damage the lungs.
Therefore, workers should be equipped with
dust masks and good ventilation is needed
in work areas.
FIGURE S.4.3-A Clay dust is very bad for your lungs. Wear a dust mask.
110
5.4.4. PRESSING PROBLEMS AND
SOLUTIONS
- Adding organic binders, such as starch,
sugar, CMC gum.
lamination problem
The main problem with powder pressing is
lamination in tiles, which means that the tile
will split apart in horizontal layers. These
layers are caused mainly by sideways slipping of the clay under pressure, which
causes clay particles to line up in the same
direction and not to get good adhesion. It is
made worse when air cannot escape quickly
enough from the body, which also causes
separation of clay layers. Another factor in
lamination is suction caused when the die is
lifted: if the die does not release easily from
the clay, it will actually tear the tile apart.
- Adding wetting agents to the water, such
as liquid soap, or sodium lauryl sulfate
(which is the common wetting agent in
liquid soap).
Lamination can be difficult to cure.
Possible solutions that can be tried are:
- Changing the clay body, which means
reducing the amount of slippery substances
such as talc, or changing the type of talc
from "plate-type particles" to "rough particles" (there are many varieties of talc), or
using talc with a rougher particle size.
- Adding coarser clays and/or grog,
which gives the clay more "tooth" and helps
prevent it from slipping.
- Maturing the clay for a longer period.
- Changing from dry clay processing to
the leather-hard method.
- Prepressing the clay body into bricks or
blocks (with a slightly higher water content), then drying them to the necessary water percentage, and breaking and granulating
them.
- increasing the granule size.
cracking problem
Cracking of tiles normally only shows after
firing. Its main cause is lamination. Other
causes: the green strength is too low to withstand stresses from handling and drying.
The cure is to increase: thickness of the tile,
moisture content of press body, the proportion of plastic clay, the pressing force or
maturing time of body. The pressure can be
increased by reducing the size of the tile, by
FIGURE 5.4.4-A Lamination in powder-pressed tile. The tile was broken after pressing to check for lamination.
111
increasing the speed of the flywheel or by
pressing and extra time.
FIGURE 5.4.5-A Semidry forming of refractories.
- Uneven thickness is corrected by ensuring that the same amount of clay is used for
each tile, by keeping moisture content constant, and by applying the same pressure to
each tile.
- Crumbling corners are caused by too
low moisture content, by attempting to press
too thin tiles, by lack of pressure or by
uneven filling of the die.
- Top center section separating from the
rest of the tile is a form of lamination. The
pressure in the center is always less than at
the edges, where the clay powder is better
compacted when the material bounces back
from the die frame. The cure is to strengthen bonding by increasing moisture content
and if that is not sufficient by granulating
the press body.
A) The semidry clay powder is beaten with a wooden
mallet into a mould placed on a sturdy moulding table.
5.4.5. REFRACTORIES
B) The moulded brick is lifted by lifting the bottom die
with a lever under the table.
Kiln shelves (sometimes called setter slabs)
for low and medium temperatures can be
made from a simple body of 50% fireclay
and 50% grog (with the dust fraction removed). This is prepared by blending by hand
with about 10% water - as described above,
a small amount squeezed in the hand will
stick together and hold its shape. This is
aged for at least 24 hours, and then is filled
into the metal frame described in dies, above.
The clay is beaten with a wooden mallet,
taking care to beat it well into the comers.
More clay is added, and again beaten until
the frame is filled level and the clay is as
compressed as possible. Then the frame is
turned over, and the opposite side is also
112
Other refractory products for kiln construction can be made in a similar way.
Fig. 5.4.5-A shows a die and moulding table
for production of large refractory firebricks.
A similar system could be used for kiln
shelf production.
C) Close-up of die frame (1) and bottom die (2) which is
placed inside the frame with the lifting rod downward.
beaten thoroughly. The shelf is removed
from the frame, and set aside for drying.
SuffIcient beating is very important, in order
to closely pack all the clay particles. This
makes a shelf that will not easily bend in firing or crack during handling. Standard
press machines do not provide enough
pressure for the large surface area of kiln
shelves (an 80-tonne press used for a 30 x
30 cm shelf can supply only 88 kg per cm2).
For more information on making refractories see "The Self-Reliant Potter: Refractories and Kilns" also in this series.
113
6. MOULDMAKING AND
PLASTER OF PARIS
The use of moulds in ceramics is as old as
ceramics itself. In fact, it seems likely that
people were pressing soft clay onto hard objects even before they discovered how to
fire it (baskets sealed with clay, etc.), or
pressing hard objects into soft clay to make
designs. When useful objects started to be
made from clay, it was a logical step to use
simple moulds so as to easily get the same
shape every time.
6.1. STAMPS
Stamps are also called "chops", and are the
oldest tools used for decorating or writing
on clay products. Usually, they are made
from wood or biscuit-fired clay.
FIGURE 6.I-A Making a plaster roller stamp.
What is a mould?
A mould is simply the shape of the space
which surrounds an object to be produced.
Because space itself has no form, the mould
must be made from a rigid material such as
wood, metal, biscuit-fired clay, stone, or,
most commonly, plaster of paris. Moulds include those which are used to actually form
a product (moulds for containers, sculpture,
etc.) and stamps, which are used to produce
a relief design on an already-formed product.
porous moulds
A) Make a long smooth clay slab.
B) A pattern is impressed by hand or by a stamp as shown.
Moulds made from plaster absorb moisture
from plastic clay or from liquid clay.
This causes the clay to shrink after forming
in the mould and it makes it easier to remove the moulded item.
Biscuit clay
moulds are also porous but to a much lesser
extent.
114
C) The clay slab is left to dry until it can be easily
removed.
F) After setting. the clay is removed and the cylindrical
plaster roller is ready to be fitted with a handle.
G) The roller used for decorating a clay surface.
D) Form the slab into a cylinder with a stick set in a
clay slab exactly in the center.
roller stamps
E) Fill the cylinder with plaster.
These are special stamping devices, which
are used for making bands of decoration
quickly. They are made from biscuit-fired
clay or plaster, and mounted on a wooden or
wire axle so they can be rolled on a pot.
metal stamps
It is also possible to use metal stamps for
impressing your company name, for example. This is usually a good idea, because
customers like to buy names that they know
or that their friends have recommended to
them. These stamps can be ordered from
printing shops.
115
6.2. MOULDS FOR PLASTIC
CLAY PRESSING,
BISCUIT CLAY MOULDS
6.2.1. ONE-PIECE MOULDS
moulds for simple forms
These are moulds used to produce simple
open forms, such as bowls, plates and tiles.
They consist of one simple curve, and often
have relief designs carved into them. Hump
moulds are used for forming slabs of clay.
They are mostly used for forming shallow
bowls, or they may be used on the potter's
wheel for making plates.
Moulds for jigger use are also one-piece
moulds, and are described below.
FIGURE 6.2.1-A.1 Press mould used for forming bowls.
116
fired pots as moulds
The simplest kind of one-piece biscuit
mould is another fired pot. Most traditional
potters who use the coil and beating method
use curved sections of broken pots to shape
new ones. In parts of Nepal, water jars are
still made by covering an old water jar with
cloth, then applying clay all around it. When
the clay is leather-hard, the pot is cut in two
pieces, removed from the mould, and joined
back together. This makes quite a rough
product, but it works successfully in small
villages where women make pots only for
their own household.
detailed biscuit moulds
More sophisticated and detailed one-piece
moulds are made from biscuit-fired clay.
FIGURE 6.2.l-A.2 A hump mould for making serving plates. A clay slab is placed on the mould and pressed into
shape. With a wire excess clay is cut off and the edges are made smooth with a sponge.
The process is not complicated. First, the
object to be moulded (model) is selected:
this may be a model in stone, wood, or usually also biscuit-fired clay. Stone or wood
usually needs a light dusting of talcum powder
or clay powder to keep the mould from
sticking to it. A slab of clay about 3 cm
thick is carefully pressed around the model,
making sure that it is pushed into all the details. Excess clay is then cut off, and the
mould is left to dry until it can be carefully
removed. After drying completely, it is biscuit-fired in the usual way and is then ready
for use. As with all moulds, the model has
to be free from undercuts that will prevent
the mould from being removed. Undercuts
are places that hold the clay, such as deeply
carved lines, or parts that stick out from the
model.
10 % or more, so the total shrinkage compared to the model can be 20-25 %! For this
reason, the size of the model needs to be
considered carefully.
FIGURE 6.2.1-B A mould (1) without undercuts allows
easy release. The other mould (2) has undercuts and
cannot be released. This shape has to be made with a
two- piece mould.
biscuit mould shrinkage
It should be remembered that the mould will
shrink (up to 10 %) and that the casting
taken from the mould will also shrink up to
117
FIGURE 6.2.1-D Hinged wooden mould for flowerpot
making.
wood hinged together. The pot is formed by
joining slabs inside the mould. The pot is released by unfolding the hinged mould.
FIGURE 6.2.1-C Cylindrical wooden mould for making water filter container.
wooden moulds
Moulds can be made from wood. Fig. 6.2.1C shows a cylindrical wooden mould that
forms the inside of a water filter container.
First the bottom is made from a slab of clay
and the mould is placed on this. Another
slab of clay is wrapped around the mould
and the slabs are joined together (see chapter 4.2.2). After joining from the outside the
wooden mould is pulled out and the joins
are worked over from the inside. The outside finishing can be done on a wheel, while
the mould is still in place.
hinged mould
A mould forming the outside of a flowerpot
(Fig. 6.2.l-D) can be made from pieces of
118
6.2.2. PIECE MOULDS
two-piece-mould
Simple closed shapes can be formed in twopiece moulds, such as vases, bottles and
even more complicated forms. Early South
American pottery was formed exclusively in
two-piece biscuit moulds.
As above, the mould is made from a model,
which is either bone-dry clay or biscuit clay.
The main thing is to carefully locate the
midline of the model, and to actually draw
this line on the model. Then, a 3-cm thick slab
of clay is wrapped and pressed around half
of the model, extending slightly beyond the
midline. This is allowed to dry until leatherhard, and then is accurately cut with a knife
along the midline. Several slight indentations
are made on the edge of the mould, to serve
as "keys" to lock in the second half. The
second half is then pressed onto the model,
and also must be carefully pressed up
against the other mould half. It is next dried
until leather-hard (it may be necessary to
cover the first half in plastic) and both
halves are carefully removed, checked to see
that they still fit correctly, dried and fired.
using two-piece moulds
Forming in a two-piece mould is the same
as described for pipes, above. Slabs are carefully pressed in both halves of the mould,
then are trimmed off just above the mould
edge. After they are slipped and scratched,
the mould is pressed together, and the inside
seam is worked together with a wooden tool
(if possible).
FIGURE 6.2.2-A Making moulds for pipe forming . The
moulds are made from a mixture of cement, clay. grog
and fibers.
two-piece water jar mould
An interesting variation on this process is
used for making water jars in parts of northern India. The mould consists of two halves
of the jar, divided along the belly. One half
has an opening in its center the size of the
water jar neck. This mould half is fastened
on the potter's wheel, and clay is centered
and thrown in it. The same is done in the
other half. The neck half is then joined to
the bottom (on the wheel), and the neck is
opened up and pulled to form the mouth.
The assembly is allowed to dry until leatherhard, when the mould is removed and any
finishing is done. The mould for the shoulder
of the pot usually has a relief design carved
into it.
plaster moulds
These simple moulds can also be made from
plaster of paris. The only difference is that
the model needs to be leather-hard clay, or it
has to be coated with a separator (see plaster
moulds below). Usually the easiest is leather-hard clay, which the plaster can be removed from easily - note, however, that in
this process you lose your model.
The midline is located as above, and a strip
of clay is placed with one edge coming just
up to it. This strip has keys pushed into it.
Plaster is applied on this half, and after setting, the clay divider is removed and the
plaster is coated with liquid soap separator.
Plaster is then mixed and applied to the
other side. After it sets, the mould can be
smoothed and the plaster carefully removed.
plaster throwing
The main trick is applying the plaster so that
it gets all the details. This is done by "throwing" the wet plaster on the model, which is
done "backhanded" while the plaster is still
119
FIGURE 6.2.2-8 A four-piece mould used for plastic forming and the leather-hard piece released from the mould.
thin. This first thin coat is allowed to set,
and when the plaster is a bit thicker, a second coat is applied the same way. When
the plaster starts to become plastic, it is applied and smoothed by the handful, until the
mould is 3-5 cm thick.
multi piece mould
Complicated forms with many undercuts
can be made in a mould made up of many
pieces. The mould is made in a similar way
as described for casting moulds (chapter
6.5.2). Forming in the mould is done by
building up a layer of soft plastic clay inside
the mould. The mould is opened when the
clay is hard enough to support itself.
moulds for pressing plastic clay can be
made as follows:
cement mould recipe
PARTS BY VOLUME
Common cement 1 part
Common clay 40 mesh 1 part
Common brick grog 40 mesh 5 parts
Fiber 0.5% by weight
Almost any fiber can be used (jute, coir,
cotton, animal hair, nylon, etc). This should
be in the form of separate fibers, cut into
about 3-6-cm lengths. The purpose of the
fiber is to give the mould strength.
6.3. PLASTER SUBSTITUTES
The materials are mixed with water to
standard cement consistency, and this is
then applied to the model like plaster. It is
not suitable for very detailed moulds.
There is no really good substitute for plaster
of paris, but in areas where this is very expensive, of poor quality, or unavailable,
These moulds are useful for large items,
such as pipes. For small items, it is probably
better to use biscuit clay.
120
6.4. PLASTER OF PARIS
The word "plaster" means a substance
which, like clay, can be moulded to almost
any shape when soft, but either dries or
cures to a hard material. Clay and cow dung
mixtures used for surfacing walls and floors
are called "clay plaster", and this material
simply dries (with some shrinkage) to a hard
material. However, it can be made plastic
again by adding water. Lime plaster and
gypsum plaster have the special quality of
"curing" after they are mixed, which means
they undergo a chemical reaction that makes
them hard, so that even if placed in water,
they will not become soft again. On heating
to 120°C gypsum releases about 20 % of its
weight as water. This chemical reaction can
be written thus:
In words: gypsum + heat = plaster (and water which goes into the air).
When the plaster is mixed with water the
process is reversed and the plaster returns to
its original gypsum state and becomes hard.
"Plaster of Paris" is the type of plaster used
for making moulds for ceramics. It was developed in France around 1770, which gives
it its name. With the use of plaster of paris,
the ceramics industry was revolutionized,
because it made possible mass production
by slip casting, which was previously unknown. Plaster of paris has the special property of easily absorbing large quantities of
water, and there is no other material that can
be substituted for slip casting.
6.4.1. GYPSUM
The raw material for making plaster of paris
is gypsum, which is hydrous calcium sulfate (CA2S04.2H20). Gypsum deposits are
found in most countries. Very simply, making plaster is done by grinding the gypsum,
and then heating it until chemically-bonded
water evaporates at 110°C. This is similar
to the process of making lime, which is
known in most countries. It can be done
on a very simple level, but industrially is
done on a large scale and controlled in a
sophisticated way.
FIGURE 6.4.1-A Gypsum mining (courtesy of British Gypsum Ltd.)
121
6.4.2. PLASTER OF PARIS
PRODUCTION
It is nonnally not worthwhile to produce
your own plaster if ready-made plaster is
available. In several countries this is not the
case and the potter may have to make his
own plaster from gypsum. Often he will be
able to sell his plaster in the local market to
other potters or to manufacturers of writing
chalk.
Production moulds get contaminated with
deflocculants and clay. First the moulds are
cleaned of clay, dust and deflocculant crystals deposited on the mould surface. The
moulds are then crushed and the fine lumps
are washed in water, which will remove the
soluble deflocculants. After drying, the material is processed as described below. The
quality of recycled plaster is not as good as
plaster made from raw gypsum and the extra
cost of cleaning the old plaster should be
compared with the cost of fresh raw gypsum.
raw gypsum
Gypsum is a soft rock that may occur as
transparent or slightly white to gray crystals,
and sometimes as fibrous granules. Several
geological varieties exist: selenite is a clear
crystal; satin spar is white and fibrous; alabaster is massive and nonnally white; and
gypsite is a mixture of gypsum and white
sand. The crystal fonn can be scratched with
a finger nail, which produces a white line.
Gypsum is also used by the cement industry,
and in some places fanners use it as a
source of calcium for the soil. Deposits of gypsum are rarely pure and the gypsum content
may be only 65 % of the bulk material, the
rest being impurities of lime, clay or sand.
The purer the raw gypsum the better the finished plaster product will be. If possible,
check the deposit of gypsum before taking
delivery and explain to the supplier that you
only want the clear crystals.
recalcining plaster
Old plaster moulds can be ground and recalcined, since already-set plaster is gypsum
(chemically speaking). This should only be
tried if raw gypsum is not easily available or
very costly, since the quality will not be as
good as with freshly-calcined gypsum.
122
gypsum calcination
First, wash away sand, clay and other impurities from the raw gypsum. The gypsum is
then pulverized in a hammer mill. Smaller
quantities are simply heated in a pan and
when the gypsum starts to boil it is stirred
gently until all of it has been boiled. It is
difficult to judge when the gypsum has been
heated enough - it is better to overheat
slightly.
first & second set
Gypsum starts boiling around 120 ·C s.
Original S. 112 and more water is released.
FIGURE 6.4.2-B Calcining gypsum in a pan.
Plaster that has been through this "second
set" is less plastic than "first set" plaster on the other hand it is stronger. With simple
equipment like a pan it is not possible to
control when the "first set" is over, because
some of the plaster in the bottom of the pan
starts boiling again before all gypsum has
been through the "first set". Generally, it is
better to have some "second set" plaster,
rather than risking uncalcined gypsum,
which will weaken the plaster moulds and
shorten the time it takes for the plaster to
harden after mixing with water.
FIGURE 6.4.2-C.1 A steel kettle (I) is placed on top of
a firebox (2) inside a brickwork cylinder. Gypsum is
loaded at (3) and plaster is discharged through a door at
(4). Stirrers (5) rotate at 15 r.p.m.
FIGURE 6.4.2-C.2 Oil-fired gypsum calciner similar to
the one shown above.
calcining kettle
For making larger quantities, a kettle for
calcining can be made locally. A cylindrical
pan made of brickwork or sheet metal is set
inside an oven heated from below. Mechanical agitators stir the gypsum to ensure even
heating. A thermometer inserted in the gypsum enables the operator to know how the
calcination is progressing. At around 120 °c
boiling starts and the temperature in the upper layers will drop to around 100 °c until
the first set is finished. Then the temperature will rise again to 170 °c when the second boiling starts. If heating is too fast, it
may be difficult to recognize that the boiling
of the "first set" has stopped before the second starts. In Burma we went through the
"first set", and after seeing the temperature
rising again we heated until the gypsum
started to boil. We then let this second
boiling go on for about 15 minutes (600kg batch) before ending the heating. The
agitators were left to stir for another 20
minutes to allow all moisture to evaporate.
screening & storing
After cooling, the plaster is screened
through 60-100 mesh and packed in bags,
which are stored in a dry place. If the plas123
ter becomes moist it will set and be spoiled
in the bag.
Freshly-boiled plaster requires more water
to become fluid, resulting in moulds that are
porous and weak. After some weeks ageing
the plaster will absorb a little moisture from
the air and this will enable a plaster mixture
to be produced with less water. For this reason it is better to store the freshly calcined
plaster for 1-4 months before using it. In industrial production, a small amount of dissolved deliquescent salt is added to the plaster during calcination and this has the same
effect.
C) Gypsum is changed to plaster in a calcining kettle.
FIGURE 6.4.2-D Work flow of gypsum calcining
D) Screening on a fine sieve. Residue is reground in a
hammer mill.
A) The raw gypsum is cleaned.
E) Screened plaster is packed in bags.
B) Gypsum is crushed in a hammer mill.
124
F) Plaster in stored in a dry place.
FIGURE 6.4.2-E Rotary kiln calcining 14 tonnes gypsum per hour (courtesy of Heidelberger Zement).
industrial production
Plaster is widely used in industrialized
countries. Large-scale plants either use big
kettles holding up to 20 tonnes or rotary
kilns. In principle the process is the same as
the one taking place in a simple pan, only
much better controlled and yielding a
plaster of consistent quality.
setting times
Plaster is often available with different setting times. For example, U.S. suppliers can
provide "20-minute casting plaster" or "30minute casting plaster", and offer more than
30 different types of plaster and gypsum cement. For specialized model making, these
products are often useful.
Setting time can be retarded and speeded up
by addition of chemicals. Retardants are
sodium carbonate, vinegar, dextrin. Accelerators are warm water and salt. For specialized model making, this is often useful, but
normally it is better to use the plaster without any addition since this reduces its
strength. Small amounts of uncalcined gypsum will speed up the setting time. Shorter
setting time is a sign that your plaster is not
stored properly and has become too old.
FIGURE 6.4.2-F Autoclaves heated with steam produce batches of 10 tonnes alpha plaster (courtesy of
Heidelberger Zement).
gypsum cement
A special plaster is produced by calcining
the gypsum under pressure in an autoclave
to 120°C. The calcining thus takes place under humid conditions and this produces plaster with a special crystal structure (alpha
plaster) that makes the finished set plaster
much stronger, as more plaster can be added
to the water. This type of plaster is, of
course, more expensive, but the extra cost is
justified when used for model making and
for block and case mould production. It is
essential for moulds used in the ram press.
Plaster suppliers offer many different qualities made from mixtures of ordinary plaster
(beta plaster) and gypsum-cement plaster
(alpha plaster). However, in many countries
only one quality is available.
6.4.3. SHIPPING AND STORAGE
Because plaster of paris is easily damaged
by water, care should be taken with shipping and storage.
125
If you must order plaster of paris in some
plaster to water ratio
quantity, it is better not to order more than 6
months' supply at a time. Request the supplier to provide it in polythene-lined bags,
or in sealed drums. Plaster should be stored
above the floor: bags may be placed on top
of boards, to allow air circulation below and
prevent any water from getting into the
bags. It should not touch the wall. It also
should be stored so that the oldest plaster is
used up first.
The strongest mixture of standard quality
pottery plaster is 70 parts (by weight) water
to 100 parts plaster. This is used for case
moulds. Working moulds used for jiggering
and casting are mixed with 75 - 80 parts
water to 100 parts plaster. Where plaster is
expensive it is better to use a high proport-ion of plaster, because the working moulds
will last longer.
Plaster used for bats or dewatering trays are
mix with 90 parts water to 100 parts plaster.
6.4.4. MIXING PLASTER OF PARIS
Gypsum cement (alpha plaster) can be mixed
with only 40 parts water to 100 parts plaster.
This produces a very hard plaster with high
expansion and low absorption, ideal for master and case moulds.
When mixing plaster, it is very important to
always use the same amounts of water and
plaster powder. If more water is used, the
strength will be less and the water absorption greater. Less water will make a thicker
mixture, which will be stronger and which
will have less water absorption. This will
cause problems in the slip casting section,
with moulds casting at different speeds.
The right ratio of water to plaster cannot be
fixed. It will vary from one source to the
next and from batch to batch. So every new
batch should be tested and compared with
the last batch.
FIGURE 6.4.4-A Graph of plaster porosity and expansion. With increase of plaster content expansion increases, but
porosity decreases.
.....
....,
60 %
0.30 % <>:
CI1
Z
H
n.
0.25 % ~
z
.
..
0
H
~
CI1
40 %
0.20 %
30 %
0.15 %
20 %
0.10 %
90
80
70
60
WATER : PLASTER
126
50
RATIO
40
z
~
FIGURE 6.4.4-B Plaster mixing work flow
A) Measured amount of water
B) Correct amount of plaster is
C) The mixture is left to soak
is filled in the mixing bucket.
added to the water.
for 2 minutes.
E) The plaster is poured onto a piece of
wood to release air bubbles.
D) The mixture is stirred for 2
minutes. Stir with upward
movements that release air bubbles.
F) The bucket and other tools are cleaned immediately.
127
rules for plaster work
RULE 1: ALWAYS WEIGH THE PLASTER AND THE WATER!
Most mould makers get lazy after a time,
and start to just estimate the amounts.
Sooner or later, they will make mistakes,
and the casting section will have problems.
PROBLEMS COST MONEY!
RULE 2: NEVER PUT WET HANDS
OR TOOLS IN THE PLASTER BAG!
Always use a dry bowl or spoon when taking plaster from the bag. Any moisture will
cause the plaster to set in the bag.
RULE 3: AFTER STARTING TO MIX,
NEVER ADD MORE WATER!
After you start to mix plaster, the chemical
reaction begins immediately. Sometimes, if
the plaster starts to get thick too soon, the
mould maker will try to add water to make
it thinner. This doesn't work - the mould
will be soft and may never set.
RULE 4: DO NOT PUT AIR BUBBLES
IN THE PLASTER!
Air bubbles are Enemy Number One of the
mould maker. If they are on the surface of
the mould, the quality will be very bad. Follow the instructions below for mixing plaster very carefully. Air bubbles do not come
from the plaster - they come from the hands
of the mould maker.
Here is the detailed work process for mixing plaster of ~aris:
1 containers
3 sieving
For good quality moulds, it is important to
have clean tools. Always use a clean container to mix the plaster. Plastic buckets are
best, because they are easy to clean. Dirty
tools and mixing containers will result in
poor quality moulds. Tools should be washed immediately after use.
Plaster sho\lld be sifted into the water
through a coafse sieve to remove big particles.
2 weighing
Always weigh the water first, and then add
the correct amount of plaster by weight. Experienced mould makers think that they can
estimate amounts correctly. This is not true:
usually mistakes will be made, and the quality of moulds will be uneven. Temperature
of water is also important. Cool water
means slow setting; hot water means fast
setting.
128
4 mixing time
The time for mixing and pouring should always be the same. After plaster is added to
water, it should be left without stirring for 2
minutes. This allows the plaster particles to
absorb water. At the same time, air bubbles
rise to the surface.
5 by hand or machine
Hand-mixing is satisfactory for amounts under 3 kg. Larger amounts should be powermixed. The mixer should force plaster from
the bottom up - not suck air into the mixture.
In hand-mixing plaster, it is important to stir
the plaster quickly, but not to mix air into
the plaster. First put your hand slowly into
the plaster. All mixing should be done from
the bottom with the fingers. The hand
should not come above the surface of the
plaster.
6 pouring plaster
Pouring the plaster into the mould should be
smooth and steady, so that air cannot enter.
It is best to place a small board in the mould
first, and to pour the plaster over it - this
spreads out the stream of plaster without introducing air bubbles. Experienced mould
makers hit the table with their body while
pouring plaster - this helps to release bubbles. The best way to remove bubbles is by
using a brush, which is gently moved about
the surface of the model immediately after
pouring in the plaster.
plaster in a suitable container or old plaster
bag. Buckets and tools should be washed
immediately in a bucket of water always
kept ready for that purpose. NEVER put
plaster in a sink or drain, as this will block
it.
Old, hardened plaster that falls from tools or
buckets into fresh plaster will speed up the
setting time, and cause bad quality moulds.
6.5. PLASTER MOULD MAKING
like "cheese", and can still be shaped.
Mould making from plaster of paris is a
highly specialized skill, and small-scale producers who produce by slip casting or by
jigger often have a full-time mould maker,
and an area that is set aside specifically for
mould making. Because the same mould
needs to be reproduced sometimes hundreds
of times, there is a well-established system
for doing this, which involves several stages.
It is a bit complicated to remember the different names for each stage. When we talk
about "positive", this means the shape itself:
for example, the positive of an apple looks
just like the apple. "Negative" means the
shape of the space around the apple, just as
the inside of a shoe is the "negative" of the
foot.
- solid: After pouring, the plaster surface is
steps in mould making
7 setting of plaster
Plaster goes through several stages of becoming hard:
- liquid: As the chemical reaction starts to
occur, the liquid will become gradually stiffer.
- plastic: This is the stage when plaster is
glossy. When it starts to become solid, the
surface will appear dull.
- final crystallization: The plaster becomes
hot, and expands slightly (about 0.3 %).
8 - clean-up
Remember that it is easiest to clean up plaster before it gets hard. Discard any excess
- model (positive): This is the original design from which all the other moulds are
made. It looks exactly like the intended product. It is usually made from plaster of
paris, which may be built up by hand and
carved, formed on a variety of hand-operated devices, or turned on a special plaster
wheel.
129
FIGURE 6.5-A A duck in clay (positive) and its print in plaster (negative).
- master mould (negative): This is the
mould made from the model, and is also
called the "block". Both the model and the
master mould are kept in a special place - if
they are lost or damaged an entirely new set
of moulds needs to be made.
- double case mould: This is identical except it has an additional outer casing for
making case moulds. If large-scale production is planned, several double case moulds
are made, since frequently a mould is damaged.
130
- case mould (positive): This is made from
the master mould or the double case mould,
and looks exactly like the model, except that
it also includes the outer casing. For largescale production several hundred working
moulds are needed and when producing
these it is practical to have as many case
moulds as can be cast from one plaster mix
batch.
- working moulds (negative): As many of
these as are needed can be produced from
the case mould.
FIGURE 6.S-B The 4 moulds used in producing working moulds for a jigger cup. Model (I), master mould (2), case
mould (3), double case mould (4). The mould faces are covered with shellac.
6.5.1. MODEL
In detail, the process of making the different
moulds is:
The way of making the model depends
mostly on whether it is to be
- asymmetrical, free-form, or not round:
for example, sculpture, rectangular objects,
handles, spouts, etc.;
- circular and symmetrical: for example,
plates, bowls, vases and other standard container shapes.
not round model
Complicated sculptural objects (human
figures, etc.) are usually made first in plastic
clay (as above) because it is easier to model
small details and to make alterations. These
models are generally made from solid clay
(as opposed to hollow), which is built up
around a wire armature to give it strength.
This process will not be described here in
detail, as there are entire books on this subject alone.
If the object is to be produced on a large
scale, a master mould is then made.
Another material used for models is wax,
which is usually the same kind of soft modeling wax that is normally used by bronze
casters. This is built up and worked with hot
metal tools.
Simpler forms, such as rectangular dishes,
oval vases, and the like, are often made by
casting a block of plaster and then carving it
to shape, or by building up plaster on an armature to the rough form, and then filing it
to shape.
round model
- plastic clay process: Round models can
be made in plastic clay, as above. The disadvantage of this is that the model is destroyed
131
in making the master mould. However, the
master mould can be used to make as many
case moulds as needed. Using plastic clay
is very fast, and allows a number of variations on a design to be made quickly, before selecting the final one. Because only
the outside of the model is used for mould
making, throwing is normally done thicker
than usual. This method is very useful for
larger items, or for items that do not have a
lot of fine detailing. The model is set up
leather-hard before the master mould is
made. Remember that the model needs to be
about 10 % larger than the final product - if
accuracy is required, the drying plus firing
shrinkage (total shrinkage) of your production clay should be used.
the plaster wheel
The most common method of making round
models is in plaster of paris. There are several advantages to this method:
- If the original design has been made as a
drawing on paper, the size can be calculated
very accurately, and a cardboard profile
made of the contour, which is then used to
check the model.
- No shrinkage to consider since the original model can be made to the same size as
the working mould.
- The plaster model can be kept permanently in storage, and, if needed, can be
used to produce a new master mould.
FIGURE 6.S.1·A A vase is modeled in plaster on a
plaster wheel. A strong stick attached to the wall is
used to support a cutting tool.
- The model can be carefully detailed,
and altered millimeter by millimeter if required.
Normally, this work is done on a special
plaster wheel, which is basically a motor-
FIGURE 6.5.1-8 The wheelhead of a plaster wheel serves as a base for model working. It holds the form that
is cast over it. but allows it to be lifted off afterwards.
132
ized potter's wheel, with either fixed or variable speed. The wheelhead is fitted so that
plaster can be attached to it, and a special
brace is attached so that the worker can hold
his arms steady.
making the model
To start work, a cylinder of flexible material
called a "cottle" is mounted on the wheelhead.
This is filled with plaster of paris, and as
soon as it has set enough to hold its shape,
the cottle is removed and rough shaping is
done immediately while the plaster is still
wet and soft. The shaping is done with a set
of special tools, which can be made by a
welding shop, and can be worked on a grinder when special shapes are needed.
The work is more or less the same as using a
wood lathe, except that the plaster is turning
FIGURE 6.5.1-C Cottle for casting on a plaster wheelhead.
FIGURE 6.5.I-D Various shapes of tools for plaster wheel.
133
vertically. Usually, an experienced wood or
metal turner can easily learn to tum plaster,
as it is softer and easier to work than other
materials. A master plaster turner is amazing to watch, as plaster literally flies in all
directions as he quickly develops the shape.
Once the rough shaping is done, it does not
matter if the plaster gets hard, although it is
best to keep it covered with plastic so it
does not dry out until the shape is final. The
shape can be checked very accurately if a
stiff paper or sheet metal profile is made. In
this way, fine shaping can even be done
over a period of several days if necessary.
In this process, plaster is always removed
down to the correct curve, and new plaster
should not be added. If too much plaster is
removed, it usually is better and saves time
to start over again. Even though more plaster can be added to already-set plaster, it often will not join really well, may be harder
or softer, and usually results in a rough area
when cutting through the joined place. If it
cannot be avoided, plaster can be joined by
scratching the already-set plaster surface
and soaking it completely with water before
pouring fresh plaster onto it.
BLOW OUT PIECES! So make a separate
area for working with plaster, and be careful
that plaster does not get into the clay.
the turning box
This is a wooden frame which is fitted with
a metal shaft that can be rotated by a hand
crank. It is used in combination with a profile. The metal shaft is first prepared by wrapping it with paper or cloth, and providing it
with a metal pin. The paper prevents plaster
from sticking to the shaft, and the pin is fixed so that it keeps the plaster from spinning freely while working, but it can be removed when finished so the plaster can
slide off the shaft. Plaster is first built up on
the shaft by applying it and rotating at the
same time. When it is a little smaller than
the profile, more plaster is applied, and the
profile shapes it to the final form. WARNING: This seems like a simple device, but
in fact requires a lot of skill to operate correctly. It is not recommended unless you can
find an experienced person to teach you.
FIGURE 6.5.I-E Plaster turning box with a profile of a
If necessary, a standard potter's wheel can
be used as a plaster wheel. This is not really
recommended, but if you produce only a
few models, there is no point in investing in
a special plaster wheel.
WARNING! ALL PLASTER WORK
SHOULD BE DONE AWA Y FROM
CLA Y WORKING AREAS. EVEN
SMALL PINHEAD SIZE PIECES OF
PLASTER THAT GET INTO THE CLAY
WILL CAUSE YOUR POTTERY TO
134
vase to be modeled.
6.5.2. MAKING MOULDS FOR
PRODUCTION
Although making moulds is virtually always
the same process: MODEL, MASTER
MOULD, CASE MOULD, WORKING
MOULDS, the specific steps are different
for making jigger moulds, two-piece
moulds, and multiple-piece moulds.
Some important points to remember are:
- Mixing and pouring plaster: according
to section on plaster.
- Cottle: This is the word used for the device that holds the liquid plaster around the
mould.
Rectangular cottles are made from wooden
planks, or, for smaller sizes, pieces of glass
or rigid plastic. Wood should be soaped to
keep the plaster from sticking. For most
purposes, a "universal cottle" is used, which
can be adjusted to any shape. Special
clamps can be made to hold the cottle in position and are very convenient if many
moulds are to be made, but this is rarely
done. In practice, bricks, clay, etc., are
generally used to hold the cottle.
Cottles are sealed with plastic clay to prevent plaster from leaking out. Take care
with this step, as the pressure of liquid plaster is sometimes surprising, and a lot of
work can be lost if all the plaster runs out of
the cottle.
- Separator: Plaster can be poured onto
Round cottles are made from sheet metal or
linoleum (Fig. 6.S.I-C), which is rolled into
a cylinder with the appropriate diameter,
and held in shape with wire. A separator is
not required for these materials. The height
is not important - it must be higher than the
intended depth of plaster, and not so high
that pouring the plaster is difficult to control.
FIGURE 6.5.2-A Universal cottle with clamps made
from iron bars.
clay, linoleum, glass and wet wood without
sticking. But if it is poured onto another plaster surface, it will stick unless a separator
has been used. Modem separators are often
made from silicon compounds in spray
cans, which are very expensive. However,
liquid soap is the standard one and works
very well if correctly applied. The standard
soap is thick brown potash soap diluted with
water (about 0.25 kg soap to 1 liter water).
If this is not available, bar soap used for
laundry can be prepared by boiling 500 g
soap in 2 liters water. After it has cooled
another 2 liters water are added.
- Applying separator: The separator can
be applied with a soft brush or with a
sponge. The first coat is done with a very
thin separator which will reduce the absorption of the plaster. Wipe the soap off with a
sponge. The next applications are done with
a thicker soap separator. After each applica135
FIGURE 6.5.2-B Shellac is applied to a cup model before the master mould is cast.
tion, excess soap is wiped off with a sponge.
Do not clean the sponge or your hands in
water. This is done 5 times on a fresh mould
and 3 times on a mould that has already
been used. The last coat is wiped off very
carefully so that no excess soap is left on the
surface because that will spoil the surface of
the new cast.
- Mould sealer: Normally five coats of
soap will be enough to seal the plaster surface.
However, some potter's prefer to apply a
special sealer to the mould surface before
using a separator. The sealer is usually shellac, which is thinned with denatured alcohol. 3 to 4 coats of shellac are needed, but
each application should be allowed to dry
completely before applying the next, otherwise the earlier shellac layers will be partly
dissolved by the new coat and form bumps
on the mould surface.
If shellac is not available thin enamel paint
can be used instead. The mould should be
136
dry before applying the paint. Either dry it
in the sun for several days, or in a heated cabinet that is below 50 "C.
- Pouring plaster: Never pour plaster directly onto the face of the case mould. The
point of pouring will become more dense
and for a slip casting mould this would produce an uneven cast. Pouring should be
done without splashing so that no air bubbles
are trapped. Place a piece of wood inside the
cottle and pour onto that. Immediately after
pouring, air bubbles can be released by moving a soft brush gently over the mould surface.
- Opening the mould: This is a step which
often can cause difficulty, especially with
single-piece moulds with vertical sides
(such as cup moulds for the jigger). If you
have correctly done the sealing and soap application, and the mould does not have any
undercuts, it should be possible to separate
the mould. The mould should be opened
when the plaster is hottest. The heat makes
the mould "sweat" a little, softening the
soap separator and this helps release.
Normally this can be done by inserting a
thin knife blade in the seam, and gently tapping it with a wooden mallet. Do this all
along the seam, and repeat it all around until
the mould comes away. The greatest difficulty is often found with jigger moulds,
which are rather deep and often create a
strong suction which resists all attempts to
open the mould. If you have compressed air
available, this is the best solution to the
problem. A hole (quite small, about 2 mm
will do) is drilled through the mould up to
FIGURE 6.5.2-e
A) A nail is inserted in the soft plaster of a jigger cup
mould.
B) Compressed air enters between the case mould (1)
and the working mould (2) and forces them apart.
the model- don't worry if you penetrate the
model, as this can be filled with a bit of
plaster later. The hole can also be made by
inserting a nail in the plaster before it stiffens (Fig. 6.S.2-C. Then apply a rubber or
plastic hose to the hole and force in air. If
nothing else is available, a bicycle pump
often works, but the metal end should be
removed.
- Case mould: Although case moulds are
normally made from plaster of paris, wax is
sometimes used for limited production. It
has the advantage of being easier to remove
from the master mould, and can be used to
produce a large number of working moulds.
Plain paraffin wax (the same used for making candles) can be used, but a better mixture is 2 parts paraffin to 1 part beeswax (if
available). The master mould is set up as
usual, and sufficient wax is melted to fill the
mould. Hot wax shrinks when it cools, so to
minimize this, the wax should be cooled in
its melting container until a crust starts to
form at the edge. It is then ready for pouring. As with slip clay, pouring must be done
quickly and without hesitation - otherwise
rings will form on the wax surface.
In modem European practice, case moulds
are made from silicone rubber (a 2-component product, which when mixed sets to
make a rubber-like material that is flexible).
one-piece moulds
- jigger mould: One-piece working moulds
are very frequently used for jigger work.
However, in order to produce them, it requires a fairly complicated master mould. All
steps from making the model to producing
working moulds for a jigger cup are shown
in Fig. 6.5.2-E.
137
FIGURE 6.S.2-D Case moulds of silicone rubber for making multiple-piece plaster moulds for slip casting porcelain
figures (courtesy of Royal Copenhagen).
FIGURE 6.S.2-E Work flow of making a jigger mould
A) Place a cottle on the plaster wheelhead and cast a
B) Tum a model of the cup on the wheel, seal it with
rough cylinder.
shellac and apply a soap separator.
138
0) Cut the CASE MOULD so it is slightly sloping and
cut the base of the model down a bit further. Apply
sealer and separator.
C) Place a cottle around the model and make a cast.
D) The master mould, also called block mould, is cut
on its outside to fit the jigger head on the jigger ma-
H) Make a cast of the case mould. This is the DOUBLE
chine.
CASE MOULD.
E) Cut the base of the model down, slightly sloping as
shown at (1). Apply sealer and separator.
I) After turning the outside of the double case mould
F) Make a cast of the master mould. This is the CASE
J) Take the moulds #2-4 off the plaster wheel, turn
MOULD.
them upside down and cast an extra model.
the whole set is ready: model (1), master mould (2),
case mould (3), double case mould (4).
139
Figure 6.5.2-E continued
K) Cast an extra set of model, master mould and case
mould and store them for future reproduction. Then
cast a number of case moulds from the other set of
moulds.
L) Produce working moulds from the case moulds.
- plaster expansion: The plaster expands
about 0.2 % on setting. The expansion of the
working mould may crack the case mould.
There are two solutions to this. The ftrst is
to make the case mould stronger by using
more plaster for the same amount of water
and by inserting reinforcing metal bands in
the case mould during its casting. If extra
gypsum cement plaster (alpha plaster) is
available this is used for case moulds.
The second option is to allow the case
mould to crack. Before casting the ftrst working mould a vertical incision is made in the
case mould from the outside so that a neat,
straight crack is produced during the ftrst
casting. A metal wire is then tightened around
the case mould, but during each casting the
case mould will still yield a little to the expanding working mould. The advantage is
that it is easy to release the working mould,
but the disadvantage is that the working
mould is not running perfectly true when
placed in the jigger head. The working
moulds can be trimmed on the plaster wheel
FIGURE 6.5.2-F A double case mould is reinforced
with metal wire, which is inserted before pouring plaster.
M) Before the plaster starts setting tum the case mould
on a potter's wheel while touching the mould surface
with a soft brush.
N) Dry the working moulds completely before use. The
first few cups formed in the new moulds may be difficult to release.
140
FIGURE 6.5.2-G Cracked case moulds reinforced with wires on the outside. A wire (I) set in the working mould provides an air inlet for use of compressed air when separating the moulds.
to run true, but this of course adds to the
cost of production. However, in many
countries this method is the only practical
solution, when plaster is of poor quality.
two-piece mould for slip casting
This is probably the most common type of
mould for slip casting most simple shapes,
like flower vases, small decorative objects,
etc. Although moulds for slip casting should
be of uniform thickness so that water is absorbed equally from all areas, in fact they
are often made square or rectangular in section for ease of producing and handling the
working moulds.
FIGURE 6.S.2-H A three-piece slip casting mould with
"spare" and a cast vase ready for finishing.
The model may sometimes be prepared
from clay, but normally is made from plaster of paris, as above. It is important to remember that moulds for slip casting require
a "spare", which is the slip pouring spout
that holds an extra reservoir of slip and permits accurate finishing of the mouth.
141
FIGURE 6.5.2-1 Steps in making a two-piece slip castingmould
A) Mark the midline of the vase with a pencil. Seal the
model.
D) Pour plaster slowly into one corner. Vibrate the
mould gently or move a soft brush over the mould surface to remove air bubbles.
B) Set the model in plastic clay up to the marked midline. Make the clay surface smooth and make a clay
plug at the mouth to fonn the spare.
E) Remove cottle and clay and tum the new mould
over, keeping the model in place. Clean clay from all
plaster surfaces. Make keys or notches in the plaster,
place a new clay plug at the mouth.
C) Place a casting cottle around the model. Allow
about 4 cm between cottle and largest diameter of the
vase. Seal gaps with clay. Apply separator to the
model.
142
F) Replace the cottle and soap the model as usual, taking extra care to soap the notch holes.
Figure 6.5.2-1 continued
multiple-piece mould
The vase we made above in the two-piece
mould had to have a flat bottom, because
otherwise it would have an undercut in the
bottom and could not be released from the
mould. An indented bottom can easily be
made with the help of a third mould piece.
This type of mould is shown in Fig. 6.S.2-H.
G) After separating the two moulds, we have a set of
master moulds also called block moulds. Make an extra
set of these to keep in store together with the model.
FIGURE 6.5.2-J Steps in making a three-piece casting
mould.
H) From the two master moulds make as many sets of
case moulds as needed.
A) Set the model in clay as before, but with the bottom
at the edge as shown. Otherwise proceed as above.
I) From the case moulds working moulds are produced.
B) The two master moulds without bottom part.
143
Figure 6.5.2-J continued
multiple moulds for irregular shapes
Pottery shapes can also be square, triangular, oval or even completely irregular as for
example a sculpture. Slip casting of such
items requires moulds divided into many
sections.
C) Assemble the two master moulds with the model in-
side. Hold it together with rubber bands, tum it upside
down, cut notches and place a cottle around it.
The first step is to determine how many
pieces of mould are required, and where the
separation lines will be. This is largely a
matter of experience and good judgement.
There must not be any "undercuts" which
prevent the mould from being pulled away
from the model. When the lines have been
determined, they can be drawn on the model
with pencil.
FIGURE 6.5.2-K Steps in making a mUltiple-piece
mould
D) Soap well and pour plaster to cast the third mould
piece.
A) Draw separation lines on the model.
E) The three master mould pieces after separation.
144
B) Build up clay walls along the separation lines.
C) Place a cottle around it and extend the clay walls to
the walls of the cottle.
F) Replace the cottle, apply separator and pour plaster
into the other sections.
D) Pour plaster into sections not bordering each other.
G) Remove the cottle, tum the mould upside down, cut
key notches and pour plaster into the last section.
E) Open the cottle, remove clay walls and cut key not-
H) The finished casting mould. The casting slip is
ches in the plaster.
poured through the four legs of the figure.
145
A multiple-piece mould can also be made
without using a cottle. First the largest area
defined by the separation lines has a clay
dam built around it, which normally can be
about 4 cm high. The plaster is applied by
splashing it on layer by layer until about
4 cm thick. The clay is then removed and
the plaster face is "keyed" as described above.
The area next to the new cast is then dammed in by clay and the process is repeated.
6.5.3. MOULD PROBLEMS AND
SOLUTIONS
Problem:
Cause:
Plaster sets too fast.
i. plaster is not properly
calcined, contains gypsum
ii. equipment dirty
Cure:
iii. high ratio of plaster to water
FIGURE 6.5.3-A Store one set of models and master
iv. mixing water too hot.
moulds of all production moulds for future reproduction.
i. replace the plaster
ii. clean equipment
iii. increase water content
6.4.4. In particular, let plaster
iv. try with cooler water.
ring. Vibrate mould gently or
soak completely before stirmove a soft brush over the
Problem:
Cause:
Cure:
Plaster sets too slowly.
i. plaster is contaminated with
clay, sand, etc.
ii. too much water in the mixture.
i. discard the plaster
ii. reduce water content of the
mixture.
mould face after pouring.
Problem
Air bubbles on the mould sur-
Rough mould surface, or soft
mould surface.
i. soap used as separator was not
wiped off before pouring
ii. coarse grains in the plaster.
i. wipe the separator off with a
squeezed sponge
face. This is one of the most
ii. screen the plaster through
Cause:
Cure:
Problem:
Cause:
Cure:
common problems with
100 mesh, regrind the plaster,
plaster moulds.
complain to supplier. Make a
improper mixing and pouring.
thicker mix so the large par-
Follow instructions on mixing
ticles do not settle on the
surface.
and pouring procedure given in
146
Problem:
Cause:
Cure:
Mould is too soft and breaks
down easily.
i. too much water in the mixture
ii. plaster is contaminated or too
old
iii. moulds were dried at high
temperature.
i. correct water/plaster ratio
ii. discard plaster
iii. dry moulds below 50°C.
Problem:
Cause:
Cure:
Casting or jigger items stick
to the mould the fIrst few
times in use.
salts in the plaster deposited on
the mould surface during
drying.
Dust the mould with talc or
grog dust before using it the
fIrst few times. Drying moulds
from the outside only will reduce the problem
147
7. SLIP CASTING
Slip casting is a specialized way of using
plaster moulds for mass production of ceramics. The word "slip" means a single clay
or a mixture of clays and other materials
that has been mixed with enough water to
make it liquid, so that it can be poured into a
plaster mould.
drain casting
Slip casting is a fairly new process in the history of ceramics, as it only became practical
after the invention of plaster of paris 200
years ago.
solid casting
The slip casting process shown in Fig. 7-A
produces a hollow item which outside has
the shape of the mould. This process is called drain casting because the moulds are
emptied of excess slip.
In this method the mould forms both the inner and outer face of the cast. The advantage is, that all casts will be identical and
FIGURE 7-A The slip casting process
C) The mould absorbs more
A) A three-piece plaster mould
is filled with casting slip.
Water is absorbed by the pI aster and the clay that was mixed
with that water forms a layer of
clay on the inside.
148
water .from the clay and when
B) After 20-40 minutes
the mould is emptied. A
layer of clay, forming a
vase, remains inside the
mould.
D) The vase is placed
on a board for finishing
the vase is hard enough to
and the mould is reas-
handle the "spare" is trimmed
and the vase is released from
the mould.
sembled and used for
another casting.
FIGURE 7-B Mould for solid casting of kiln setters.
the cast can have walls of varied thickness.
Solid casting moulds are more complicated
to make and they are mainly used for high
quality ware and for items that need to be of
uniform size. Fig. 4.4.l-D shows an example of solid casting of handles.
7.1. CASTING BODY
Slip casting gives potters more problems
than other forming techniques. A good
casting slip is often difficult to develop, and
preparation of the clay slip has to be done
carefully following the same procedures
each time. The following factors playa role
in making a casting slip:
fluidity
The clay body needs to be fluid enough to
be poured into the plaster moulds, and after
the clay sets on the mould surface, the remaining clay must drain out without leaving
"runs" or "tears" on the inner surface of the
cast. Fluidity is the opposite of viscosity; a
stiff liquid is said to have high viscosity,
and a very fluid liquid has low viscosity.
The viscosity of a clay slip is adjusted by
addition of water or deflocculating chemicals like soda ash (sodium carbonate) and
water glass (sodium silicate).
density
The clay slip should contain as little water
as possible. A high amount of water pro149
FIGURE 7.I-A A casting shop with casting tables and racks. It is important to keep work area clean and well organized. (courtesy of Royal Copenhagen)
duces a less dense cast and increases drying
shrinkage of the clay. It will also lengthen
the casting time, and the plaster moulds will
have to absorb more water, which reduces
the number of items that can be cast per day
and the life of the moulds. Water content of
the slip should be 25 % to 45 % by weight
of the slip. Density is measured by weighing
1 liter of the slip. 1 liter of water weighs
1 kg, and we say it has a density of 1; clay
slip will have a density higher than 1.
ter to draw water from the slip, so the casting rate is lowered. If the casting slip has a
high proportion of fine plastic clay, it will
lengthen the casting time considerably.
Therefore, casting slips should normally
contain less plastic clay than plastic bodies.
As a rule, a casting slip should contain 50 %
clay and 50 % nonplastic materials (by dry
weight).
clay body strength
permeability
As soon as the clay slip is poured into the
mould, the plaster starts to suck water from
the slip, and a denser layer of clay body is
built up next to the plaster. As this layer
thickens, it takes a longer time for the plas150
The finished cast should be strong enough
to handle when it is taken out of the mould,
and there should be no sticking to the
mould. The green strength of the clay body
needs to be enough for handling without
breaking.
detlocculation
Clay particles are extremely small and
shaped like plates. In a plastic clay a thin
film of water surrounds each clay particle
and enables the particles to slide over one
another when pressure is applied during forming. As more and more water is added, the
clay particles start to move more freely, and
gradually the clay becomes a slip. An increase of water lowers viscosity. Equal
amounts of water and clay make a slip.
So much water makes the slip impractical to
cast. This problem is solved by adding chemicals, which changes the behavior of the clay
particles.
slip is "flocculated". With addition of an alkali (e. g. soda ash), the clay particles will
repel each other and this state is called "deflocculated".
Fig. 7.1-B shows clay particles enlarged
approximately 100,000 times. In an acid environment, clay particles carry electrostatic
charges on their surface: positive charges at
their edges and negative charges on their
two faces. The clay particles behave like
small magnets, and the positive edges are attracted to the negative faces. This results in
packing of the clay particles, and in increased viscosity.
Clay particles behave in two different ways
when freely suspended in water. If an acid is
added to the water the clay particles will attract each other, and it is said that the clay
When the clay slip is deflocculated by addition of an alkali the particles become negatively charged all over and they will then repulse one another (Fig. 7.1-C). This repulsion makes the slip fluid with less water.
FIGURE 7.l-B Clay particles are attracted to one an-
FIGURE 7.l-C Clay particles are repelled by each other
other in a flocculated slip.
in a deflocculated slip.
151
thixotropy
This means the property of clay to become
more viscous when it is left undisturbed.
When a slip is left all night, it often
becomes thick and viscous. If it is stirred, it
again will become fluid. Thixotropy is useful in a casting slip because it helps the
newly-cast items to keep their shape in the
mould. However, too high thixotropy makes
it difficult to empty the moulds. Water glass
decreases thixotropy, whereas soda ash does
not affect it.
7.1.1. GUIDELINES FOR
DEVELOPING A CASTING
BODY
As a starting point, a casting body can be
developed from a plastic body. However,
the body needs to be adjusted keeping the
following points in mind:
- A casting body should contain approximately 50 % clay material and 50 % nonplastic materials.
- Very fine clay particles will reduce the
casting rate by blocking the flow of water
from the slip into the plaster mould. Plastic
clays should be substituted for less plastic
ones and the clay part should be made up
from more than one type of clay.
- Natural clays contain soluble salts, and
these will act as flocculants, increasing the
necessary amount of deflocculants. Some
clays prove impossible to deflocculate. The
salts may come from the water, and, as a rule,
you should never use recycled water from
clay preparation for casting slip mixing.
152
FIGURE 7.1.1-A In large casting shops slip is often delivered by pipes. (courtesy of Royal Copenhagen)
- Clays containing sulfates are also difficult to deflocculate. Sulfates can be neutralized by additions of barium carbonate,
usually 0.2-1 %.
- Addition of frit as a body flux may also
introduce flocculants, since the frit may be
slightly soluble. As a remedy try to use another frit or use glass powder.
- Clays containing montmorillonite (like
bentonite) contain high amounts of colloidal
silica that may cause the slip to gel and in
general are difficult to deflocculate. This is
called "livering".
- The standard deOocculant is a 3 to 1
mixture of sodium silicate and soda ash.
The ratio of sodium silicate should be higher for a body of low plasticity, whereas
more soda ash is used for bodies containing
higher amounts of plastic clay.
Sodium silicate tends to cause the cast item
to stick to the mould, produces stringy slip
that drains unevenly and results in a hard
cast. Too much soda ash produces casts that
never get hard, clay bodies that thicken up
more quickly, and causes the mould to wear
out faster. Soda ash should be purchased
fresh, and kept in an airtight container. Old
soda ash absorbs water from the air and
becomes soda bicarbonate, which will no
long-er work as a deflocculant.
FIGURE 7.1.1-B Funnel used for testing fluidity of
casting slip.
- The amount of deOocculant should be as
low as possible and not higher than 0.3 %
(based on dry weight of body). High
amounts cause excessive wear of moulds
and make the cast stick to the mould.
testing
After developing a suitable body, the right
amount of deflocculant is found by doing a
simple test. First, weigh all ingredients accurately. Mix the measured body with water
until the body becomes just fluid. Then, dissolve a mixture of sodium silicate and soda
ash in hot water.
Example:
5 g soda ash + 15 g sodium silicate in
200 ml water equals 0.01 g deflocculant for
each I ml water. So each 1 ml of solution
added to 1 kg body means an addition of
0.01 % deflocculant.
What we want to find is the amount of deflocculant producing the most fluid slip. The
fluidity is tested by filling a large funnel to
the rim with slip while closing the outlet
with a fmger. The time (in seconds) it takes
for the slip to drain out after releasing one's
fmger is a relative measure of its fluidity.
First the run-through time for slip without
deflocculants is found and then the deflocculant solution is added to the slip 5 ml at a
time. With each addition the slip is stirred
well and run through the funnel. Initially,
the slip will run faster with increase of deflocculant, but as the concentration of deflocculant increases, the slip will become
less fluid. At that point the test is stopped
and the results are plotted on a graph as
shown in Fig. 7.1.1-B. A line is drawn between the dots, and from this graph the
amount of deflocculant with lowest viscosity is read. Normally a point a little to the
left of this is used, since it is important to
use as little deflocculant as possible.
153
SECONDS
20
18
..... 16
~
~
14
"~z
8
'"
6
~
4
z::>
:g
<>
bl
!Jl
2
5
10
15
20
25
ML
30
35
40
45
50
55
60
ML
DEFLOCCULANT SOLUTION ADDED TO TEST SLIP
FIGURE 7.1.1-C Results of a fluidity test presented as a graph. 25 ml solution (0.25 % of dry body) is the right
amount of deflocculant in this example.
trial batch
FIGURE 7.1.1-D A blunger with slowly rotating stir-
Now prepare a lO-kg test batch of casting
body following the standard procedure and
using the amount of deflocculant found
from our funnel test. In the above test we
did not find the right amount of water for
the slip. This is now done by adding only
33 % water and the deflocculants to the dry
body (which means a water content of 25 %
of slip). Leave it for 24 hours. After that,
additional water is added during blunging
until the slip has the right consistency. The
slip is tested by casting some items. If it
works and if you have measured all components accurately, you have the final recipe
for your casting slip.
rers (10-15 r.p.m.) is placed high for gravity feeding
ageing
A deflocculated casting slip should be used
within 2-3 weeks. Carbon dioxide from the
air enters the slip and this acts as a flocculant
which will make the slip less fluid. The ageing can be slowed down by keeping the slip
154
of slip by pipes to the casting tables. The gentle agitation of the slip releases air bubbles.
in an airtight container or by sealing the slip
surface with a thin layer of oil.
4 Join the moulds tightly together with rubber.
grog
Grog can be added to a casting slip. It will
stay in suspension in the deflocculated slip.
The finished surface will be as smooth as if
no grog was used. Grog is used for casting
large items, especially sanitary ware. It is also
used for casting of refractory kiln furniture.
5 Stir the slip just before casting and check
that it does not contain air bubbles.
6 Pour the slip slowly and evenly into the
mould without stopping and splashing. The
slip should hit the bottom of the mould.
7 Top up the mould every 5-10 minutes.
7.1.2. CASTING SLIP PREPARATION
1) Dry all body materials and weigh them
accurately.
2) Fill the right amount of water in the blunger. Gradually add soda ash dissolved in hot
water, then add half of the body. After some
blunging add the remaining body and the
sodium silicate.
3) Leave the slip to age for 24 hours.
4) Screen it through 60 mesh. Check fluidity
with the funnel test and density by weighing
1 liter of slip. Cast a few items to be sure the
slip is working correctly.
8 Let the mould set until the casting has
reached correct thickness.
9 A good mould can be used 2 to 4 times
per day, depending on weather conditions.
Before each casting, the mould must be
dried in the sun or in the drying cupboard.
Otherwise moulds wear out fast.
10 After correct thickness is reached, drain
out the excess slip. If the slip is drained into
its main container, do this through a coarse
screen that will retain pieces of plaster and
dry clay.
FIGURE 7.1.2-A Excess slip from the casting moulds
is emptied into a slip tank trolley that moves under the
casting tables.
recommended procedure for casting
1 Before casting examine the moulds for
clay or dust on the inside.
2 Clean dirt from the moulds with an air
hose or a soft dry brush. Never use water it will damage the mould.
3 Arrange the moulds in the order of cas-ting: heavy moulds first, then medium size,
then small size.
155
7.1.3. CASTING PROBLEMS, CAUSES
AND CURES
Problem: Slip gelling in the mould.
"To gef' or "gelling" means slip becoming
too thick in the mould, so that excess slip
cannot be poured out.
Cause: Underdeflocculated slip.
Cure: Adjust the slip batch by adding more
sodium silicate in very small amounts (to
prevent overdeflocculation).
Cause: Slip that has not been mixed sufficiently.
Cure: Mix the batch for a longer time.
FIGURE 7.1.2-B A large number of moulds for solid
casting of plates are arranged in a row.
11 Then keep the mould upsidedown until
the clay is stiff enough to remove.
12 Take the cast product out when the
clay starts to pull away from the plaster
mould.
Cause: Too much free iron or alkaline in
the clay itself, or too much organic matter in
the clay (black ball clays often cause this
problem).
Cure: Reduce the amount of clay causing
the problem, by experimenting with variations on the body recipe.
Problem: Uneven casting thickness.
This means the walls of the casting are thick
and thin.
Cause: Difference in mould absorption due
to difference in the thickness of a mould thick areas absorb water faster than thin
areas, causing excess buildup of slip.
Cure: Check the quality of your mould. It
may be necessary to change the thickness of
the mould - thick areas in the mould may
need to be made thinner.
helpful hints
Costing: In costing greenware, the guideline
is to charge 10 % of the mould cost for each
casting. This can be adjusted according to
how complicated the mould is.
156
Cause: Moulds are too old and are starting
to harden.
Cure: Throw out nonabsorbing moulds.
Average mould life is 150-200 castings.
Problem: Greenware cracking in the mould.
This means the greenware cracks before it is
removed from the mould.
Cause: The most common cause is too
much water in the slip.
Cure: The amount of deflocculant needs to
be corrected, or there may be problems with
some of the clay ingredients - in this case, it
may be necessary to change raw materials.
Cause: Slip with insufficient plastic ingredients.
Cure: It may be necessary to add more plastic clay to the body.
Cause: Poorly designed moulds.
Cure: If this problem seems to occur with
one or two particular moulds, then the
moulds are probably the cause of cracking.
They may have undercuts which catch the
slip.
Cause: Greenware is allowed to remain in
the mould too long.
Cure: Check your casting process. Are you
leaving the greenware in the mould too long
after draining it? It should be removed as
soon as it is stiff enough to be handled without damage.
Problem: Greenware sticking to the mould.
Cause: New moulds with greasy or soapy
surfaces.
Cure: Dust the inside of the moulds with
talcum powder. The problem should disappear
after making a few casts.
Cause: Too much organic material or large
amounts of fine particles in the clay. Black
ball clay often has too much organic material.
Cure: It may be necessary to substitute
some of the ball clay for china clay.
Cause: Underdeflocculated slip which does
not shrink and takes longer to dry in the
mould, or overdeflocculated slip with too
much sodium silicate (this is the most commoncause).
Cure: Adjust the amount of deflocculant.
Cause: Moulds are saturated with water.
Cure: Completely dry the mould at a temperature below 50 'C. (If mould becomes
too hot, the plaster will disintegrate).
Problem: Stiff and brittle greenware.
This means leather-hard or dry greenware
that breaks too easily.
Cause: Overdeflocculated slip. If you experience slow casting times and greenware
with a sandy surface, overdeflocculation is
usually the problem.
Cure: Overdeflocculated slip usually needs
to be thrown out. The amount of deflocculation, especially sodium silicate, must then
be reduced.
Cause: Clay particle size may be too
coarse.
Cure: Slip plasticity can be improved by
ageing it at least 24 hours. If this does not
solve the problem, then the recipe needs to
be adjusted with more plastic clay.
Problem: Pinholes in the surface of green-
ware or fired ware.
Cause: Trapped air bubbles in the slip.
Bubbles in slip occur if the slip is mixed at
too high a speed.
Cure: Bubbles in slip can be prevented if
slower mixing speed is used.
157
Cause: The slip has too low fluidity.
Cure: Increase deflocculants and/or water.
Cause: There is a high carbonate content in
the water used to make the slip.
Cure: It may be necessary to use water
from a different source.
Cause: There is a high content of organic
material in the clay which releases gas
when it decomposes. Especially black clay
contains a high amount of organic materials.
Cure: Screen the slip through a 60-mesh
sieve.This will release the trapped gas.
Cause: The soda ash in slip decomposes because of too long storage time.
Cure: Throw out the slip.
Cause: Fast pouring into the mould may introduce air bubbles and the return of excess
slip after casting may do the same.
Cure: Pour the slip slowly without stopping
until the mould is full and stir the slip before
casting to release air bubbles.
Cause: Poor quality moulds with pinholes
on the surface.
Cure: Improve mould-making process, by
mixing plaster more carefully to prevent any
air bubbles.
7.2. SLIP BUILDING AND
FIBERSLIP
There are a few specialized ways of working with slip that are not found in most ceramics books: one is using deflocculated
slip for joining plastic clay, and the other is
making large slabs by mixing fiber into the
slip, called "fiberslip".
158
As already mentioned in the section on plastic clay, there is an advantage in using deflocculated slip for joining leather-hard
clay, because the shrinkage is the same as
plastic clay - this is especially true when
joining large pieces, which tend to crack at
the seams in drying. The process is:
- Mix deOocculated slip, using the same
body recipe as your plastic clay. This type
of slip is most easily deflocculated with
"Calgon" (sodium hexametaphosphate) in
amounts up to 1 %. Calgon is mixed first
with water, and then the clay body is gradually added while blunging.
- When joining, it is not necessary to
scratch the surfaces to be joined (this may
not be true with all clay bodies, and needs to
be tested). Apply slip to both surfaces with a
brush, and press them together immediately.
Excess slip can be removed from the seams
after it has set a bit.
- For extra reinforcement of the joints,
they can actually be "taped" together. The
tape is made by cutting strips of very open
weave fabric, such as cotton mosquito netting or cotton gauze; or if available, a very
open weave fiberglass cloth. These tapes are
dipped in slip, and applied to the joined
pieces. The tape provides very high green
strength and resistance to drying cracks. In
the firing, it bums up, leaving only the slip.
The disadvantage of this method is that the
tape produces a texture at the joints - however, if used on the inside of items such as
large planter boxes, this is not a problem. It
also is possible to incorporate it as decoration on the outside.
building forms with slip
It is also possible to build with slip, using
cause they are very fine and easy to separate.
One half percent seems like a very small
amount, but it is surprising how it increases
the viscosity of the slip, and it gives incredible green strength.
Alternatively, finely-chopped jute fiber can
be used. In India, this is often used
in unfired clay sculpture.
FIGURE 7.2-A Spreading fiberslip inside a plaster
- Building with this ''fiberslip'' is best
done inside plaster moulds. The slip is simply
spread on by hand or with a trowel, and built
up to the necessary thickness (which will
be about the same as plastic clay). The advantage is that slabs do not need to be made,
and there are no problems with joining. Drying shrinkage is less than plastic clay, and
cracking problems cannot occur because of
the fiber. The nylon fiber is so fine that even
though it bums up in firing, the finished appearance is no different from regular clay.
mould.
moulds. This process is suited for making
slabs, or for producing forms that are too
large for practical casting in the normal
way:
- Deflocculated slip is prepared as usual,
but as little water as possible is added. The
slip should be just thin enough for the mixer
still to work successfully - it will not pour
as easily as regular casting slip. It can be
used like this, or, for extra strong and more
easily workable slip, it can be mixed with
0.5 % chopped nylon fibers - these are
sometimes available in bulk, and should be
cut to about 2 cm. It is important that they
be single, unwoven fibers. It also is possible
to make these fibers yourself, by chopping
nylon rope. Nylon fibers work the best, be-
- Because the green strength is so great, it
is a useful technique for large pots that are
difficult to transport when green.
flberslip for sculpture
Another way of using fibers lip is for slab
making or for sculpture. Again, this is a process that ordinarily would not be used, but
for large-scale work, where there are shrinkage problems, it is worth trying:
- The same slip as above is used, preferably with the addition of chopped nylon fiber.
- The process of building up slabs is the
same as making fiberglass, where glass
cloth is used to reinforce plastic resin. With
fiberslip, various kinds of cloth can be used:
the main requirement is that they be very
159
open weave so the slip can penetrate
through them. Fiberglass cloth is good if it
is about the same weave as mosquito netting; nylon net or cotton mosquito netting
can be used; or "cheesecloth" (open-weave
cotton gauze) also works well.
- To make a slab, first a paper (newspaper)
separator is laid on a flat surface. Next, a
piece of cloth the desired size is laid on this.
Then, a layer of slip about 2-4 mm thick is
spread on the cloth. Another piece of cloth
is laid on the slip, and the process is continued until the slab is the desired thickness.
When the slab is leather-hard, it can be cut
(with a sharp knife) and assembled with slip
and tape as above.
160
- For making large SCUlpture, the process
can be quite useful. An armature (support)
can be made from wood, wire, etc. and
then covered with wire netting. This is
then cov-ered with cloth, and fiberslip is
built up on the surface as for making a
slab. Usually, the outer layers will be
fiberslip only (with no reinforcing cloth) so
that the surface can be carved and modeled.
The pieces can be fired together - the
armature will bum and melt in the process,
but as it is inside the sculpture, this does
not present a problem. Clay with low
shrinkage is best for the process, such as
plastic clay with a large grog percentage, or
low-firing white clay such as 50 % ball
clay/50 % talc.
8. FINISHING AND QUALITY
CONTROL
Finishing is the process of completing all
the details on a product - since this book is
about forming, we will discuss ftnishing only up to the stage when the product is ready
for ftring.
Quality control is an activity that ought to
be a part of the production process, but
which is very often neglected. First the producer has to decide what is an acceptable
quality of the ftnished product and then he
has to ftnd out where in the production process control is needed to ensure that his products are up to standard. Finishing should
always be followed by an inspection of quality.
8.1. IMPORTANCE OF FINISHING
Your customers will expect your products to
be ftnished correctly, so that each piece has
the same quality. It is the responsibility of
the producer to guarantee his products, and
to check the work at each production step.
Workers need to be instructed on their individual responsibilities for ftnishing, and the
supervisor should be strict in rejecting products that do not meet the expected standard. The ideal system is: "Every worker is a
quality controller", but this seldom works
without careful instructions and an incentive
scheme.
There are several ways of paying workers:
- fixed salary: Workers are paid a ftxed
monthly wage, regardless of production
quantity. This is not usually a satisfactory
system, as workers will tend to work as little
as possible.
- piece rate: This was the standard system
for many years in the ceramics industry.
However, with the introduction of labor unions, and the idea that factory owners should
provide better treatment for labor, the system is not used so much nowadays.
- basic salary plus incentive for high production: many factories now use this system. Each worker has a ftxed minimum
production quantity, which is the amount of
work he must produce in order to pay his
production costs. If he produces more than
the minimum quantity, he is paid extra according to the labor cost per piece.
These systems can be introduced by considering each main step in production as a
separate industry. For example, the jigger
section is responsible for producing ftnished
greenware. This greenware is inspected and
counted by the supervisor, and the workers
in the jigger section are paid according to
the amount of acceptable greenware. Similarly, the glazing section is paid according
to the amount of correctly-glazed ware, and
161
the firing section is paid according to the
percentage of successfully-fired ware.
Slip casting
- After the casting has started to pull
away from the mould, the spare line is cut.
8.2. STEPS IN FINISHING
The steps in finishing vary according to the
production method. For example:
Forming on the potter's wheel
- Before removing from the wheel: The
rim must be smoothed, and the bottom is
undercut to make trimming easy.
- Leather-hard: The foot ring is trimmed
and smoothed with a sponge. No further finishing should be required.
- Bone-dry: The product is inspected for
cracks and imperfections, and any poor quality products are thrown in the scrap
container.
FIGURE 8.2-A Cutting the seam on a cast vase.
162
- Leather-hard: The mould line is cut
away and smoothed with a sponge.
- Bone-dry: No further finishing should be
required - only inspection and rejection of
poor quality products.
Joining
- After forming of the individual parts:
They are joined and all excess clay from the
joining, fingerprints and marks are sponged
away.
- Drying: The items are turned or paired to
avoid cracking and warping.
- Bone-dry: inspection of quality.
FIGURE 8.2-B The edges of leather-hard pressed tiles
are trimmed with a knife.
Press moulding
- After pressing: Tile edges are fettled and
in the case of plastic-pressed products press
seams are cut and sponged.
- Bone-dry: inspection of quality.
8.3. METHODS OF FINISHING
The methods of finishing have been discussed according to various forming systems potter's wheel, extrusion, jigger, slip casting,
etc. Here are some general hints:
- Finish ware as soon as possible. It is usually easiest to finish ware as soon as it is
possible to pick it up without damage. Potter's wheel products should be trimmed as
soft as possible - waiting too long slows
down the process and causes damaged products. If ware becomes too dry, it is best to
throw it out, as trying to rewet it usually
takes longer than starting over again. Trying
to finish bone-dry ware with sandpaper is
rarely satisfactory - it takes a long time,
produces dust which is a health hazard, and
leaves a rough surface.
ly. Tools should be cleaned after use, and
kept in a convenient location.
8.4. QUALITY CONTROL
Acceptable quality varies according to the
product. Common ware, such as low-cost
storage jars, is often very roughly finished.
Fine quality porcelain is rejected for even
the slightest flaw, and often up to 50 % of
products are not acceptable as top quality.
setting a standard
The producer must keep a constant watch on
quality, and should try continuously to improve his product. A customer who receives
even one batch of poor quality products is
likely to look for another supplier. The level
of quality depends on what quality competitors in the market are supplying and the ex-
FIGURE 8.4-A Finishing of high quality porcelain.
For quality it is essential to have good working conditions: items placed at the right height, good tools and
plenty of light.
- Use the correct tool. Although one tool
can be used for many finishing jobs, it is
best to have specific tools in order to make
finishing easier. For example, special tools
can be easily made for jobs that need to be
repeated many times: if you need to produce
several hundred 2-inch posts, then it makes
good sense to make a wooden cutting box
which is set up for cutting the correct
length.
- Tools should be in good condition. Clay
trimming tools should be sharpened regular163
pectation of the customers. Small tea shops
will demand strong teacups, but will be
more concerned about the price of the cup
than its looks. So for that market quality
means a cup with a strong handle, a rounded
rim and no chipping or crazing of the glaze.
Quality does not mean that the products are
beautiful; it means that the customer gets
the product he expects. Good quality is not
necessarily more costly or difficult to produce, but it pays attention to details and
good management.
controlling quality
Once you have decided what your standard
is for a particular product you can simply
sort your finished products into two groups:
acceptable and unacceptable. The rejected
products could be sold at a lower price. This
method is costly, because you have invested
a lot of time, labor and materials in a product you will have to sell cheap or even
throw out The most cost-effective method is
to look at the whole production process and
decide where mistakes could be discovered
early. As soon as a mistake is discovered the
Slip-cast cup: quality control points
OPERATION
POSSIBLE FAULT
INSPECTION
Slip preparation
Pinholes
Air bubbles in slip
before casting
Slip casting
Cup too thick/thin
Casting time
Trimming rim
Uneven rim, sharp edge
Rims
Attaching handles
Handles easily come off
Dryness of cup
Finishing/drying
Warping, rough surface,
handles out of position, cracking
When dry, before
biscuit firing, all
rejects put in
clay scrap bin.
Unpacking biscuit
Cracking, overfiring
Cracks and warping
Glazing
Crawling, pinholes,
running, etc.
Dirty biscuit ware,
glaze thickness
Firing
Over/underfIring, etc.
Cones, draw trials
Transfer to store
All the above-mentioned
Sorting according to
standard.
164
product can be rejected and no more time
and money are wasted on a faulty product.
Let us look at an example that shows where
inspection could prevent faults in the finished product (table on p. 164).
In a small industry where the producer or
the supervisor is directly involved in the
production he would check the production
at most of the steps listed above. He could
make inspection a part of another activity to
save time. If the supervisor packs the biscuit
kiln all products will have to go through his
hands and this is a good opportunity to
check all products.
In larger industries more formal quality control procedures are needed. In the example
above inspection could be reduced to a
check of the casting slip and a check when
the products are completely dry. At that
point both the quality and the quantity can
be assessed as part of a piece rate pay system. The factory manager should make frequent spot checks at all steps in the production to make sure that the quality control
system works.
fulfilling orders
The biggest problem for small producers is
accepting more orders than they can complete in time. Rush orders are likely to be
poorly finished, and customers will never
accept excuses for poor quality. For that reason, producers must learn to correctly estimate production time, and should never telv
customers that they can do it faster. At the
time of taking orders the producers should
FIGURE 8.4-B Finishing of pressed tiles is checked
and marked for quality and piece rate payment.
also show their customers what quality they
can expect. In ceramics production, especially in small industries, there will always
be variations in color, texture, etc. The customer should be shown these and not only a
few chance samples, that will be hard to reproduce.
Customers come back again if they know
they can trust a producer to tell them the
truth.
165
9. DRYING OF WARE AND
MOULDS
Drying takes place when the water in the
clay transfers to the surrounding air. When
the air outside the clay contains as much
moisture as the air inside the clay, the process of drying stops.
place. This stage is termed leather-hard, and
there is still plenty of water left between the
clay particles. The graph shows the relationship between drying shrinkage and water content of a clay.
leather-hard
rate of drying
As the water leaves the spaces between the
clay particles, these move closer together.
That causes the clay to shrink as the water
disappears; but when the particles all touch
each other, little more shrinkage will take
Clay dries faster in warm, dry and windy
weather. The rate of drying can be slowed
down by covering the clay with plastic
sheets or wet cloth, which causes the air surrounding the clay to become more humid.
FIGURE 9-A Example of relationship between water content, shrinkage and pores. After 50 hours the clay has
become leather-hard and shrinkage stops. The condition of the clay at points 1-3 is illustrated in Fig. 9-8.
100
90
...
80
:.::
70
>-
60
<
u
SO
lJ...
0
40
UJ
z:
=>
-'
0
::-
clay
30
20
10
10
20
)0
40
50
60
70
80
90
100
110
120 h~~
DRY I NG T I ME I N HOURS
166
FIGURE 9-B Three stages of clay from forming to dry
warping
(enlarged about 100,000 times).
Warping is caused by uneven drying. A tile
will dry more on its upper surface so this
will shrink and bend the tile upward.
Cracking and warping are reduced by turning
the products frequently during drying. Warping of plates and bowls is avoided by
stacking them on top of each other rim to rim.
1) Water surrounds all particles, the clay is plastic.
2) The clay is leather hard. Particles touch one another.
Thin clay ware dries faster and more evenly
than thick ware, and is therefore less prone
to cracking and warping. Thick-walled designs should be allowed to dry extremely
slowly or should be made from clay with a
high content of coarse materials like grog
and sand.
Small products can be dried rapidly without
problems. Large items require special care
in drying, especially in dry weather or hot
climates.
FIGURE 9-C Drying hints
3) Clay is dry. Only a little water remains in the clay
pores. The water remaining between the clay particles
in the pores continues to move out of the clay to the
surface, from where it evaporates. When no water remains in the pores the clay is called bone-dry.
A) Keep drying items away from sun and wind.
The clay ware should dry evenly so that it
shrinks evenly. A handle on a cup tends to
dry faster than the cup itself, and the different rate of shrinkage will produce a crack
in the handle - unless care is taken to let the
whole cup dry slowly.
B) Rotate items several times a day if they seem to be
drying unevenly. Flatware like tiles are sure to warp if
nottumed.
167
Fig. 9-C continued
C) Placing bowls, plates, cups, etc., rim to rim prevents warping.
D) Large items should be dried very slowly, and can be
loosely covered with plastic sheets.
G) After attaching handles let the pot dry upside down.
Otherwise the rim will dry faster and this may crack the
handle.
H) Typical crack of plastic-formed tiles dried in bungs.
Edges dry first, but the center of the tile is under pressure and cannot accommodate the shrinkage of the
edges so they crack. Stack tiles more openly. DustE) Added parts, such as large handles, can be wrapped
pressed tiles crack in the same way, but more easily be-
in plastic or painted with liquid wax emulsion, so that
cause they have less green strength and additional prob-
they do not dry faster than the rest of the pot.
lems with lamination.
7
F) Items with large bottoms may crack across the cen-
I) For plastic-formed tiles: the edges dried first and the
ter of the bottom, because the bottom sticks to its sup-
center accommodated the shrinkage. When the center
port and when it shrinks the periphery cannot follow.
shrunk the rigid edges caused it to crack. Semidry
Sticking can be prevented by placing the pot on sheets
pressed tiles: the pressure in the center caused less
of paper or dusting its support with sand or fine grog.
stress due to differences in particle orientation.
168
FIGURE 9-D Drying boards for slabs are made with an open grill so the slabs will dry from both sides. Note that the
slabs on the floor are laid on old newspaper to avoid their sticking to the floor.
J) Pots with handles or spouts attached should be dried
slowly. Place them on the bottom of the rack where it is
cooler and drying is slower. Cover with plastic if
9.1. DRYING SYSTEMS
necessary.
Air can absorb water until saturated. How
much it can absorb depends on how much it
holds already and on its temperature. The
higher the temperature, the more water can
be contained in the same volume of air.
There are three basic approaches to air
drying of clay ware:
- A large amount of unheated air is circulated around the ware. Example: outside
drying sheds for drying of bricks and roofing tiles.
- A small amount of air is heated to a high
Fig. 9-C continued
temperature. This is seldom used except in
the ceramics laboratory.
169
FIGURE 9.1-A Shed with racks for drying roofing
FIGURE 9.1-B Drying chamber using heat from cool-
tiles.
ing kilns.
- A medium amount of air is heated
firing and cooling dries the ware. The warm
kiln will create a draught of warm air that
also helps drying.
moderately. Example: batch dryers for clay
ware and plaster moulds. Rapid circulation
of air is more energy-efficient for drying
than high temperature.
Thin-walled crockery will dry by itself on
the ware racks. Drying of heavy articles like
bricks, tiles or pipes may need the help of
dryers, especially in the rainy or cold seasons.
The simplest way of speeding up drying is
to carry the ware outside and leave it in the
sun to dry. Trolley racks make this easier.
Artificial drying is most often done by heating air and circulating it around the drying
ware. For heavy clay products like sanitary
ware, it is sometimes done by placing the
ware on a heated floor. Infrared light is a
technically good solution, but electricity is
expensive. The cheapest way is to make use
of the waste heat from kiln firings.
drying chamber
For medium-scale industries with large outputs, or for drying plaster moulds, a drying
chamber is useful. Inside the chamber a
ware rack is made for ware boards, or better
still, the chamber is made to hold one or
more ware trolleys so that drying goods can
be taken in and out quickly.
cross draught chamber
Warm air from a cooling kiln or from an additional stove is introduced from one side
and a fan provides a draught across the
chamber through the ware racks. A constant
movement of air over the drying ware is as
important as heat. The air is not saturated
with water by just passing through the
chamber once, so in order to economize,
most of the air should be recirculated by the
fan. A centrifugal fan is most appropriate
for this.
kiln drying racks
Racks for holding drying ware can be arranged around the kiln or on top of it, so
that the heat radiating from the kiln during
170
The centrifugal fan can at the same time
draw hot air from the kiln via a duct system.
However, combustion air should not be used
FIGURE 9.1-C Drying chamber with a fan (1) recirculating air and drawing waste heat through a duct (2) from the
kilns. A damper (3) regulates amount of air to be recirculated.
directly. It contains soot that will dirty the
ware, along with moisture and sulfuric acids
that may corrode the duct system.
tunnel dryer
The same principle can also be used in a
tunnel dryer where the ware trolleys enter at
one end and are taken out at the other. The
trolleys should move in the same direction
as the drying air.
cabinet dryer
A more simple drying chamber can be made
as a cabinet with a ware board rack inside, a
heating device in the bottom and a hole at
the top for moisture to escape. Heating can
be done by electricity or by a stove. Combustion air, except from burning gas, should
not enter the drying chamber since the ware
will be dirtied by soot. The hot combustion
gas can instead be led through the chamber
FIGURE 9.1-0 Trolley tunnel dryer. Waste heat enter at (A) and exits at (B). Ware trolleys enter at (C) and are taken
out at (D).
o
171
1
Jpo
..
lH~
LJ.
t6.
::;::
6.
1~
2
A
A
.8,
-
....
8.
A
l..
0
FIGURE 9.1-E Mangle dryer. The endless chain (I) carries shelves (2) with ware and moulds.
in stove pipes. Combustion itself takes place
in a small stove or firebox next to the drying
chamber.
mangle dryer
For large potteries a mangle dryer is a costeffective way of drying ware and moulds.
An endless chain is carrying shelves loaded
with ware inside a drying chamber. The
shelves are loaded at one end next to the
forming unit and emptied at the other end.
The size of the shelves and the speed and
length of the chain are designed according
to the capacity of the forming unit. This
type of dryer is mainly used in jigger ware
production and its initially higher cost is
justified by longer mould life, lower labor
cost, and saving on space, racks and ware
boards.
172
9.2. DRYING OF PLASTER
MOULDS
Plaster moulds used in casting or jigger production need to be dried regularly. Working
with wet moulds slows down production
and moulds that are constantly wet become
soft and "rotten". Hardness and strength of
wet moulds are only half of that of dry
moulds. This reduces the number of times
moulds can be used, and consequently increases production cost per unit. Moulds
need to be dried at least once a day if they
are to last long.
cost of drying:
Drying of plaster moulds follows the same
principles as for drying clay ware. However, the need for drying is more urgent, because the production requires a constant
supply of dry moulds. If no artificial drying
is used, it means that a large number of
moulds has to be in stock. This takes up a
lot of space, increases labor cost since the
workers will have to carries farther for the
moulds, and more money will be tied up
in a large stock of moulds. All of this costs
money, and with jigger or casting production of some scale, it may be more economical to use artificial drying.
recalcination of moulds:
Plaster should not be overheated, because it
will then start to recalcine. Gypsum starts to
calcine from around 100°C, and since set
plaster is actually gypsum, the plaster
moulds also become calcined on heating. At
temperatures above 50°C, the plaster
moulds will start to soften and become
chalky at the edges. This will of course
weaken the moulds and reduce their service
life.
dryer operation:
When operating the dryer it is very important to make sure that the temperature inside
the dryer does not exceed 50°C. An ordinary thermometer is placed inside the dryer
and its temperature is checked regularly. As
long as the moulds are wet, they do not become hot even if the air is above 50°C, but
once they are dry they will suffer damage,
especially on their edges and comers which
dry out first. Recalcination and cracking due
to thermal shock can be reduced by painting
mould edges with a 10 % solution of com
sugar. It is safer to take the moulds out of
the dryer when they are still slightly damp.
deflocculants:
Plaster moulds used in slip casting should
be dried only from their outer surface. That
means they should be kept assembled as for
casting so that only a little water will evaporate from the inner mould surface. The casting slip and therefore also the water in the
casting moulds contains deflocculants, and
when the water evaporates, the deflocculants remain on the mould surface. Deflocculants attack plaster, so it is better if they
are concentrated on the outside of the
mould, where they do little harm. If the
mould dries from its inner surface it will
wear out faster.
dryer location:
One advantage of using a mould dryer is
that less time is spent on handling moulds.
The dryer should be located close to the
forming area. Jigger moulds may be placed
in the dryer with the clay items inside, so
when the moulds are taken out of the dryer
they are emptied before returning to the jigger
operator. The jiggered items are then put on
ware boards which are brought to another
area for finishing. Before deciding on where
to place the dryer, the work flow of the
whole process should be worked out.
Additional information on drying is given in
sections 4.2.4. and 4.3.6.
173
10. WORK FLOW AND FACTORY
LAYOUT
Work flow is simply all the steps that are
necessary from beginning to end of any production process. For anybody engaged in
production, it is very useful to take the time
to list all the steps, and to find out where
time and energy are being wasted.
STEP
The various steps in a production work flow
can all be broken down into the following,
and shown in a graphic way with symbols:
SYMBOL
EXPLANATION
o
action which changes the form of a
material, like cutting, pressing, throwing, etc.
Raw materials' storage
Operation
Transport
moving material from one location to
another
Delay
D
necessary step which takes significant
time, like drying
Operation/delay
CD
operation that takes significant time,
like firing
Inspection
o
quality control check
Finished goods' storage
174
Analyzing your work flow is very helpful
for increasing production efficiency. When
you understand all the steps, you can often
make work easier and increase profitability
by:
- ORGANIZING THE WORKPLACE
The location where forming or finishing
work is done should be furnished with a
good source of light and enough space
around the workplace to allow easy
movement. Remove all unnecessary equipment and rubbish which will hamper
work. Only equipment that is needed
should be kept at the workplace. Frequently-used tools could be hung on the wall.
Keep racks for storing products nearby.
- TRANSPORTING EQUIPMENT
Ceramic products go through many operations so they are moved around a lot.
Carrying one pot at a time is a waste of
time. Most pottery can be carried on ware
boards about 120 cm long and 10 - 20 cm
wide. A ware board can carry 20 cups at a
time and they can be stored on stationary
racks or on trolley racks.
Raw materials and heavy products like
bricks and pipes can be moved on carts.
-
MAKING TRANSPORTATION DISTANCES AS SHORT AS POSSIBLE
If you can rearrange your workshop so
that clay only has to be carried 10 meters
FIGURE IO-A Ware boards for jigger moulds are placed next to the jigger machine. When the moulds on one side of
the trolley are used the operator turns the trolley around. Once all moulds are used the trolley is taken directly to a
drying chamber.
175
instead of 50 meters, this saves a lot of
wasted effort. Another example is a pipe
factory, where pipe has to be carried over
rough ground and around heaps of coal,
etc., to get to the kiln. This creates a lot of
extra work, and disturbs the workers,
as well as possibly being the cause of
injuries. It would make good economic
sense to take a day to construct a clear
pathway, as the work would go much
better afterwards.
FIGURE lO-B Stationary racks for ware boards.
- REDUCING DELAYS AS MUCH AS
POSSIBLE
Often there is not much that can be done
about delays. For example, it may not be
possible to shorten firing time. Another
frequent delay is drying - in this case, it is
worth getting some help to compare the
cost of artificial drying with air drying.
FIGURE lO-C A cheap alternative to racks: boards with tiles are stacked using bricks as spacers.
176
- reorganize work procedures and workplace
- increase number of workers
- introduce incentive pay scheme
- work in two or three shifts
- increase capacity of machinery.
Several options may be used at the same
time. Before you start to change anything
try to make two or three different plans and
then compare the cost of them.
consultants
FIGURE IO-D Cart for moving hollow bricks.
Even though the initial investment may be
high, sometimes the time saved makes it
financially advisable. This may be especially true for jigger production, where a
forced air dryer can cut down on the number of moulds required and increase daily
production capacity at the same time.
However, we do not recommend investing
in expensive equipment unless the benefits
have been carefully analyzed beforehand.
Factory owners often buy the latest equipment just because it is new, and regret it
later when it does not meet expectations.
production capacity
Once you have analyzed your production
flow and made a good plan for reducing
time wasted on internal transport, etc., you
will also find that some sections in your production are bottlenecks. That means they
delay the flow of products causing other
sections to be idle while they wait for materials or sernifinished products. There are
various possibilities for curing bottlenecks
or increasing production capacity in general:
There are actually industrial consultants
who make very detailed work flows, to the
point of finding out how many seconds each
step takes, and then try to find ways to
shorten the time. This is useful up to a point,
but most small producers either operate a
family business, or have quite a small number
of workers, who are not likely to cooperate
if they are asked to take 5 seconds for an
operation that normally takes them 6 seconds. People are not machines, after all,
even though management would often like
to see them that way.
Work flow is the starting point for planning
factory layout. This is true for planning new
factories, as well as remodeling old ones
(which often have expanded gradually and
have become inefficient in the process).
10.1. WORK FLOW EXAMPLE
The following work flow analysis of a jigger cup production in Burma is an example
of how production can be reorganized with
simple means.
177
original situation
moulds at a time. After being taken out of
the moulds, the jiggered cups were carried
by hand in stacks 15 to 20 cups high to fettling machines. This system was very laborintensive and the stacking caused many
cracks.
Before the reorganization of the production,
jigger moulds were placed on racks behind
the jigger machines and two assistants for
each jigger machine would run between the
rack and the jigger machine with 2 to 3
FIGURE lO.1-A Work flow plan of the jiggering production analyzed below.
,
RACKS
WITH
,
,.,.
MOULDS
DRYING
CHAMBER
,
MOULDS
AND
,
b8*
?P'
~
~
..
~9*
JI01
,
CUPS
'@9*
"
[iQ:
,,
l'
RACK
AND
GLAZING
FEHLING MACH II ES
~
9@
+
!
..
DRY
178
~
9~
SECT! ON
....
-t~"" '(0).
!
Tl*----+_
T
UNFINISHED
DECORATION
CUPS
o
RACK
OPERATOR
3* ACTIVITY INDICATOR
--+ FLOW OF CUPS/MOULDS
o
TROLLEY
reorganization
chines. The layout is shown in the process
flow plan in Fig. 10. I-A, which corresponds
with the work flow chart in Fig. 10.1-B.
A system for internal transport of products
was established. Ware boards for carrying
cups and moulds were introduced together
with stationary racks and trolleys to hold the
boards. One board could carry 6 cup
moulds. A trolley could hold 36 boards or a
total of 216 cup moulds. In the work flow
analysis 216 cups are used as the batch unit.
The jigger operator (B) would always have
one trolley next to her and she would pick
one mould at a time directly from the board
on the trolley. When the moulds on one side
were filled with jiggered cups she would
just tum the whole trolley and start with
fresh moulds. An assistant (C) would replace the boards with filled moulds with
fresh ones from the rack next to the jigger machines. During the rainy season the assistant
would take the whole trolley to the drying
A drying chamber was constructed so that it
could accommodate four trolleys with ware
and moulds. The jigger machines were relocated and placed so that they were close to
both drying chamber and the fettling ma-
FIGURE lO.1-B Work flow chart for jiggering and fettling cups
Product: small teacup 115 g dry weight
Batch: 216 cups equal to I trolley with moulds
ACTIVITIES
OPERATORS
A
B
C
MINUTES
D
1 clay storage
2 clay to jigger machines
C>
0
3 jiggering cups
64
4 replacing filled moulds
C>
15
5 emptying moulds of cups
0
15
6 cups to dryer
C>
2
7 cups drying
8 dryer to fettling rack
D
240
10
C>
C>
3
10 fettling cups
0
74
11 cups to decoration rack
C>
3
9 rack to fettling machine
Minutes per type of operator:
11
Number of respective operators:
Average batch time per operator
11
64
32
80
4
2
4
16
16
20
179
chamber and replace it with another. With
this system one assistant could service two
jigger machines.
bottleneck. A work flow chart shows such
bottlenecks and indicates how to solve
them.
Another worker (A) would supply clay to all
jigger machines and would also transfer
dried cups to the fettling machines. The stationary racks were placed between the jigger section and the fettling section so that
jiggered products placed in the rack from
one side could be taken out by the fettling
operators on the other side.
The longest time it takes on average for one
group of operators to finish one batch is 20
minutes. That means that during a workday
of 7 hours, 21 batches or 4536 cups can be
produced. If the fettling unit could produce
more by having an extra machine or working overtime, average batch time could be
16 minutes. This would increase production
to 5040 cups per day. This could be achieved by having the fettling unit work 45 minutes extra per day.
The work flow chart below shows the
minutes it takes for each operator to finish
one batch, equal to 216 cups. The production capacity is planned according to the
daily output of the four jigger machines. If
another step in the production flow cannot
finish this quantity per day it becomes a
180
The flow chart also shows that operator A
only spends 11 minutes per batch. He could
therefore be given an additional task like
looking after the drying chamber.
Appendix
GLOSSARY
Absorption
Takes place whr.n water is soaked up by biscuit-fired clay.
Accelerator
A material that increases speed of a chemical reaction.
ACmotor
Motor for alternating current, which is the type of current normally supplied to households.
Acrylic plastic
An artificial material made from carbon polymers.
Adhesion
Bonding between two surfaces.
Ageing
Also called maturing. Storing of plastic clay under moist conditions increases plasticity.
Armature
Internal framework to support a sculpture or other structure.
Ball clay
A fine-grained, plastic clay firing to a white or buff color.
Bat
A small slab of plaster of paris used as a base for pots during forming and drying. A
slab of clay used in production of saucers and plates on a jigger machine.
Batch
A quantity of products or a mixture of materials treated as one set.
Bone-dry clay
Clay that has been dried above 100°C. No moisture remains between the clay pores.
Bullring kiln
Also called bull trench kiln. A continuous kiln for firing bricks. Bricks are stacked in a
ring-shaped trench and covered with a sealing layer. Chimneys are placed on top of the
kiln and moved as the firing zone moves around continuously.
Calcine
To heat a (ceramic) material to a temperature high enough to release carbon dioxide,
chemical water or other gases.
Calgon
Deflocculant used in casting slips. Chemically termed sodium hexametaphosphate.
Chuck
A tool used to hold pots or other items while they are worked on.
Clay body
A mixture of different clays and other materials like grog, feldspar and talc. Clay is a
natural product; clay body is man-made.
CMC
An organic binder based on cellulose. Chemically termed carboxymethyl cellulose sodium salt.
181
Coir
The outer husk of coconut used for making ropes.
Continuous kiln
A kiln fired continuously. In tunnel kilns the ware is moved through the firing zone.
In ring kilns (Hoffmann kilns) the firing zone is moved.
Crockery
Clay pots used in the household.
DC motor
Direct current motor. A battery supplies direct current.
De-airing
Removing air from plastic clay by exposing it to a strong vacuum. It improves plasticity and reduces lamination problems.
Deliquescent
A material (salt) that easily absorbs moisture from the air.
Density
The modem term for specific gravity. The number of times a material is heavier than
the same amount of water. If a slip has a density of 1.7 it means that I liter slip
weighs 1.7 kg.
Dewatering
Removing water from a clay slip in order to get a plastic clay.
Extrude
Shaping of a plastic material like clay by forcing it through a die.
Fettling
Trimming rough edges of pottery before firing.
Fireclay
A clay that can withstand high temperatures though it may not fire to a white color.
It is used for making refractory materials.
Flux
Material that lowers the melting point of a clay body.
Frit
Is made by melting several glaze materials to a glassy mass which is ground and used
in a glaze. Frit added to a body or a glaze lowers its melting point.
Fulcrum
The point against which a lever turns.
Granule of clay
A small grainlike mass of clay particles.
Grog
Fired clay that has been crushed. Grog is added to clay to reduce drying cracks or to
reduce thermal shock cracks in kiln furniture or firebricks.
Household ware
Pottery used in the household.
HP
Abbreviation for horsepower, a measurement of how much work a machine can do.
I HP =0.7457 kilowatts.
Hydrostone
A special hard plaster that is capable of making plaster mixtures with much less water
than normal plaster. It is used for making models, master and case moulds and press
moulds.
Kiln furniture
Shelves or slabs, saggars, posts or stands, used for placing ware in the kiln during firing.
Kneading
Manually work up clay for the purpose of mixing it better, getting air pockets out of it,
and making it softer. Kneading is done immediately before clay is used for forming.
182
Lamination
Separation of materials in layers. Often seen in pressed tiles and in extruded clay products.
Low tension insulator
A glazed ceramic item used for insulating electric wires for currents below 440 volts.
Overglaze
Decoration with colorants on top of other already-fired glazes. The firing of overglaze decorations is done at a lower temperature than the original glaze firing.
Plastic
A plastic material (like clay) has the ability to be moulded easily. Acrylic plastic is
often just called plastic.
Porosity
Ability of fired clay to absorb water. After firing the pores between the clay particles
can hold water.
Refractories
Materials or products like firebricks, kiln slabs, etc., which can withstand high temperatures without melting.
Relief designs
A clay surface can be moulded so that some parts stand out from the rest of the surface. It is a picture in three dimensions.
r.p.m.
Abbreviation of revolutions per minute. A potter's wheel may rotate at 200 r.p.m.
Sanitary ware
Ceramic ware used in bathrooms and for disposal of sewage.
Slip
Mixture of clay and water. Used in casting and for joining leather-hard clay pieces.
Smokeless stove
A stove for cooking provided with a small chimney that takes the smoke from the cooking fire outside the house.
Smoking
A potter's term used for the first period of firing a kiln with greenware. During this
period all moisture is released from the clay and the water vapor looks like smoke.
Tableware
Items like plates and cups used for eating at the table.
Thixotropic
A clay slip is thixotropic when it becomes temporarily liquid when stirred, but returns
to a stiff gel when standing.
Thrower
A person who forms or throws pots on a potter's wheel.
Viscosity
A litjuid has high viscosity when it does not run easily.
Vitrified
When clay is fired to a high temperature it starts to melt. It is vitrified when after
firing it is hard and brittle and absorbs little water.
Warping
Deformation of ceramic items caused by uneven drying or overfiring.
Wax
A long strip of clay extruded from a die.
Wedging
A manual method of preparing plastic clay. A lump of clay is thrown forcefully onto
a solid table. It is then cut in two, one half then being thrown down on the other half.
This is repeated 20-50 times.
183
TABLES OF WEIGHTS AND MEASURES
Metric System:
I kilometer, km
1m
I cm
Imm
= 1000 meters, m
= 100 centimeters, cm
I cubic meter, m3
II
I ml
= 1000 liters, I
= 1000 cm3 or ml
= lOOOmm3
I tonne
I kg
Ig
= 1000 kilograms, kg
= 1000 grams, g
= 1000 milligrams, mg
to convert:
10 millimeters, mm
= 1000 micron, ~
to:
multiply
metric by:
to convert:
to:
multiply
U.K. & U.S. by:
feet
inches
inches
inches
3.280
39.370
.394
.039
feet
inches
inches
inches
m
m
cm
mm
.305
.025
2.54
25.400
acres
sq. feet
sq. inches
2.471
10.764
.155
acres
sq. feet
sq. inches
hectare
m2
cm2
.405
.093
6.451
cu. feet
cord (wood)
cu. inches
cu. inches
U.K. gallon
U.S. gallon
35.314
.276
.061
61.020
.219
.264
cu. feet
cord
cu. inches
cu. inches
U.K. gallon
U.S. gallon
m3
m3
cm3 (cc)
I
0.0283
3.625
16.387
.016
4.546
3.785
pounds
ounces
kilograms
grams
.453
28.349
length:
m
m
cm
mm
area:
hectare
m2
cm2
volume:
m3
m3
cm3 (cc)
I
weight:
kilograms
grams
184
pounds
ounces
2.205
0.035
TABLE OF SIEVE MESH SIZES
Table of Standard Sieves
Gennany
Britain
United States
France
DIN
B.S.I.
U.S. standard
AFNOR
Openings
mrn
Openings
mesh/cm
mesh/in.
Openings
mm
mesh/in.
Openings
mrn
6.000
5.613
3
4.76
4
5.000
5.000
4.000
4.00
5
3.353
5
3.36
6
2.812
6
2.83
7
3.000
4.000
3.150
2.500
2.411
7
2.38
8
2.500
2.000
2.057
8
2.00
10
2.000
1.676
10
1.68
12
1.600
1.402
12
1.41
14
1.500
1.200
1.000
4
5
1.204
14
1.19
16
6
1.003
16
1.00
18
1.000
0.853
18
0.84
20
0.800
0.630
0.750
8
0.699
22
0.71
25
0.600
10
0.599
25
0.59
30
1.250
0.500
12
0.500
30
0.50
35
0.500
0.430
14
0.422
36
0.42
40
0.400
0.400
16
0.340
18
20
0.353
0.295
44
0.35
45
52
24
0.251
60
0.297
0.250
0.211
72
50
60
70
0.178
0.152
85
0.300
0.250
0.200
0.177
0.150
0.120
30
35
40
0.210
0.177
0.315
0.250
0.200
80
100
0.149
0.125
100
120
0.125
0.104
120
150
0.100
170
0.105
0.088
140
0.089
170
0.124
0.100
50
60
0.090
70
0.160
0.075
80
0.076
200
0.074
200
0.060
100
0.066
240
0.062
230
0.063
0.053
110
0.053
300
0.053
270
0.050
0.040
130
0.037
400
0.040
0.080
Mesh means the number of threads per linear cm or inch of sieve cloth. Openings indicate the distance in mrn
between. two threads. I
I
After F. and 5.5. Singer, Industrial Ceramics.
185
DENSITY
Specific gravity (SG) of a material, a mixture of materials or a clay slip is expressed as how
many times it is heavier than the same amount of water, i.e. how many kg per 1 liter volume
or gram per cm3. Density is the weight per volume unit and in the metric system this equals
specific gravity (glcc or kg/l) but in many countries slip densities are still measured in ounces
per pint.
The density of a clay slip is found by weighing 1 liter of the slip. If it weighs 1.6 kg the slip
has a density of 1.6.
TWADDELL SCALE
Clay and glaze suspensions have normally densities between 1.0 and 2.0. On hydrometers
used for measuring glaze and slip densities the densities between 1.0 and 2.0 have been divided into 200 units. These units are called degrees Twaddell and the formula for calculating
these is:
°TW = (density - 1) x 200
°TW
Density = 200 + 1
MAJ. BROGNIART'S FORMULA FOR DRY CONTENT OF LIQUID
If we know the specific gravity of the material in a liquid suspension and the density of the
suspension we can calculate the dry weight of this material from:
Dry weight = (lq - 1000) x S
S - 1
W = dry weight of material in 1 liter of liquid
lq = weight in grams of 1 liter liquid
S = specific gravity of dry material
Most clays have a specific gravity of 2.5.
186
Example:
This formula is very useful. Let us suppose we have an already-prepared clay slip and we
want to add 5 % iron oxide based on dry clay content. First we measure exactly 1 liter slip
and we find it weighs 1.7 kg. Now we calculate dry content of 1 liter slip using Brogniart's
formula:
Dry weight =
(1700 - 1000) x 2.5
2.5 _ I
= 1,166 grams
We have, say, 30 liters slip so dry clay content in 30 liters is:
30 x 1.166 kg =35 kg. Addition of 5% iron oxide to dry clay content =1.75 kg.
Conversion Table for Pint Weights
ozlptUK
ozlptUS
S.G.
°TW
22
22.8
23
24
25
25.2
26
26.4
27
27.6
28
28.8
29
30
31
31.2
32
32.4
33
33.6
34
34.8
35
36
37
37.2
38
18.3
19
19.2
20
20.8
21
21.7
22
22.5
23
23.3
24
24.2
25
25.8
26
26.7
27
27.5
28
28.3
29
29.2
30
30.8
31
31.6
1.10
1.14
1.15
1.20
1.25
1.26
1.30
1.32
1.35
1.38
1.40
1.44
1.45
1.50
1.55
1.56
1.60
1.62
1.65
1.68
1.70
1.74
1.75
1.80
1.85
1.86
1.89
20
28
30
40
50
52
60
64
70
76
80
88
90
100
110
112
120
124
130
136
140
148
150
160
170
171
179
187
BmLIOGRAPHY
Hugh Allen. The Kenya Ceramic Jiko - A Stovemaker's Manual, London 1990.
H.N. Bose. Modern Pottery Manufacture, Bhagalpur 1947.
Barry Brickell. A New Zealand Potter's Dictionary, Auckland 1985.
Michael Cardew. Pioneer Pottery, New York 1971.
Charles Chaney and Stanley Skee. Plaster Mold and Model Making, New York 1981.
Kenneth Clark. The Potter's Manual, London 1983.
Harry Davis. The Potter's Alternative, Australia 1987.
E. Eipeltauer. The Preparation of Raw Gypsum and Its Conversion into Various Hemihydrates National Research
Council of Canada, Technical Translation 899, Ottawa 1960.
R.W. Ford. Ceramics Drying, Oxford 1986.
Robert Fournier. I\lustrated Dictionary of Practical Pottery, New York 1975.
Donald E. Frith. Mold Making for Ceramics, Radnor 1985.
Frank Hamer. The Potter's Dictionary of Materials and Techniques, New York 1975.
David Hamilton. The Thames and Hudson Manual of Architectural Ceramics, London 1978.
David Hamilton. The Thames and Hudson Manual of Pottery and Ceramics, London 1974.
Josef Hoffmann. Technologie der Feinkeramik, Leipzig 1987.
John B. Kenny. The Complete Book of Pottery Making, Radnor1974.
Glenn C. Nelson. Ceramics, New York 1960.
Henrik Norsker .The Self-Reliant Potter: Refractories and Kilns, Wiesbaden 1987.
Henrik Norsker. Clay Materials - for The Self-Reliant Potter, Wiesbaden 1990.
Ernest Albert. Sandeman Notes on the Manufacture of Earthenware, London 1921.
Alfred B. Searle. The Clayworker's Hand-Book, London 1929.
Felix Singer & Sonja S. Singer. Industrial Ceramics, London 1979.
CERAMICS JOURNALS
Ceramics Monthly, Box 12448, Columbus, Ohio, U.S.A.
Interceram, Verlag Schmid, P.O.
Box 6609, 7800 Freiburg, Germany
Tile & Brick International, Verlag Schmid
New Zealand Potter, P.O.
Box 12-162, Wellington, New Zealand.
Ceramic Review, 21 Camaby Street, London, U.K.
Studio Potter, Box 70, Goffstown, N.H., U.S.A.
188
Index
Agricultural items
Air
to open mould
Alabaster
Armature
Barium Carbonate
use in casting slip
Biscuit mould
shrinkage
Biscuit moulds
two piece
Block mould
Bottles
making in two pieces
Brick kilns
bull ring kiln
Cabinet dryer
Calcium sulfate
Calgon
as deflocculant
Case mould
making of
Casting slip (also see slip casting)
sulfate problems in
Chops
for pressing in clay
Chucks
use in trimming
Clay plaster
CMCgum
as clay binder
Coils
Cottle
Crockery
Cutting tools
used in throwing
Dam
in making plaster moulds
Deflocculant
amount
effect on plaster
Dies
for fuse holders
22
137
122
131
152
152
1I7
1I8
130
52
88
171
121
158
135
137
152
1I4
55
121
III
30,31
133, 135, 142
13
53
146
153
173
98
hardened steel
mild steel for tiles
top and bottom
Double shaft mixer
for pipe clay body
Drain casting
Dry pressing
requirements
Dryer
location of
operation of
Drying
cabinet dryer
chamber
cost of
effect of deflocculants
general rules
helpful hints
of plaster moulds
pipe
problems
process of
racks built on the kiln
rim to rim
systems of
use of fans
Drying chamber
workflow
Drying problems
items with handles
Earthenware
Electrical products
for small producer
forming methods
Extruder
dies
hand
lever system
machine
screw system
Extrusion
definition
handles
97
97
97
92
148
96
173
173
171
170
172
173
169
59
173
94
58
167
170
77
170
170
179
58
64
9
97
20
78
78
78
79
78
78
86
189
products
tiles
Factory layout
work flow
Fettling
workflow
Fiberglas
for joining
Fiberslip
building and modelling with
definition
for sculpture
green strenght
jute fiber additions
mixing chopped nylon
slabs made with
Filter candles
Finishing
care of tools
hints
quality control
tools for throwing
Foot rings
how to make
Friction press
pressure
production figures
Fuse holders
General shaping techniques
Goblets
Granulation
details
size of granules
spray drying
wet process
work flow
Granules
for semi dry pressing
Gypsite
Gypsum
alabaster
calcination of
first and second set
gypsite
purchasing
raw gypsum
satin spar
selenite
Gypsum plaster
Gypsum cement
Hand screw press
Hand-forming
tools
190
84
85
174
180
158
158
158
159
158
159
159
159
21
161
163
163
163
54
54
100
100
21
41
49
108
108
107
107
106
105
122
122
122
122
122
122
122
122
122
122
121
125
99
29
29
Handle making
coil
extruded
pressed
pulling
slip casting
Hollow bricks
clay requirements
extrusion
soft clay system
stiff clay system
Hollow brick kiln
Bull's ring type
rice husk fuel
Household ware
Hydrostone
for ram press dies
Import substitution
Jigger
arm
axle and bearings
clay for
clutch for
definition
design of machine
drive system
drying
dust health hazards
early type
finishing
finishing methods
making moulds for
mould life
moulds for
moulds with relief designs
number of moulds required
problems and solutions
production quantity
profile adjustment
profile or template
speed of rotation
variable speed
wheelhead and chucks for
Jigger chuck
metal. rubber. lead. reinforced
Jigger/jolly
Jiggering
work flow
Joining
casting slip
helpful hints
interlocked
using cloth tape
60
61
64
60
64
86
86
86
87
89
89
13
102
9
11
71, 78
71
68
71
66
70
71
77
69
67
75
69
137
67
72
72
68
77
67
72. 78
71
70
71
70
78
16
177
59.64
65
65
65
158
using deflocculated slip
Kiln
periodic
roller hearth
Kiln furniture
forming methods
stands
Kiln shelves
by semi-dry-pressing
making
Knobs
how to make
Lamination
in powder-pressed tiles
Leather-hard-pressing
description
clay requirements
Lids
different types
how to make
interlock
Lifters
removing pots from the wheel
Lime plaster
Low tension insulators
Lubrication
of dies for tile pressing
Machinery
making your own, ready-made, used
Machinery manufacturers
reliability of
Master mould
Measuring
tools for throwing
Model
clay
plaster
wax
Model making
plaster wheel
plastic clay process
Mould
definition
biscuit clay
biscuit pots
biscuit, how to make
breakage of
cement addition
clay models for
for water jar
hump
made from cement
midline
158
96
96
18
84
99
112
49
III
167
96,103
49
49
49
43
121
21
108
12
91
135
54
135
131
131
131
132
131
114
116
116
116
77
77
119
119
116
120
118,119
one-piece mould
preventing sticking
two-piece mould
types of
undercuts
wire reinforced
Mould making
2-piece mould for slip casting
avoiding bubbles
block mould
by throwing model
case mould
clay dam
cottle
general work flow
introduction
jigger mould
making the model
master mould
model
multiple piece mould
negative
not round model
one-piece mould
opening the mould
positive
rectangular cottle
rough surfaces
round cottle
separation lines
separators
soft surfaces
solid clay model
spare
turning box
uni versaI cottle
wax model
working mould
Mould problems
air bubbles
rough surface
soft moulds
sticking
Needle tool
for throwing
Nylon
mixing with slip
Overglanze
Pipe
barrel
by extrusion
clay requirements
collar
116
77
118
14
117
77
141
146
130
132
137, 139
146
133
135
129
137
133
130
129, I3I
143
129
131
137
136
129
135
146
135
144
135
146
131
144
134
135
I3I
130
146
146
147
147
57
159
9
89
91
92
89
191
drying
extruder operation
extruding machine
finishing machine
fittings
forming and appropriate products
grinding body
grog for
hand-thrown daily quantity
names of parts
production hints
screening clay for
slab construction in wooden moulds
socket
uses of
wheel-throwing
Plaster of Paris
alpha gypsum
autoclave
avoiding air bubbles
calcining kettle
definition
gypsumcement
history of
how to pour
industrial production of
mixing rules
mixing time
mould opening
recaIcining
retardants
rules for working with
screening & storing
sealer
separator
setting process
setting times
shipping
soft
spoiled
storage of
substitute
"throwing"
water/plaster ratio
weighing
Plaster moulds
recalcination of
Plaster wheel
profile
tools
Plastic clay
definition
dry work flow
192
94
93
93
93
91
89
92
94
89
89
92
92
89,91
89
19
89
125
125
129
123
121
125
121
129
125
126
128
136
122
125
128
123
136
135
129
125
125
147
124
125
120
ll9
126
128
173
132
134
133
23
23
grog
hydraulic ram pressing
preparation
press moulding
wet process
wet work flow
wet work flow by hand
Posts
Potter's Wheel
clay for
closed forms
finishing
goblets
joined forms
large bottles
lids
main parts of
motorized
open forms
principle
roofing tiles
trimming
unmotorized
Powder pressing
problems
workflow
Press machines
eccentric press
friction press
hydraulic press
lever press
pressing work flow
ram press
safety regulations
screw press
soil block presses
toggle press
Press moulding
definition
semiautomatic pressing
see semidry pressing
Pug mill
axle and blades
de-airing chamber
double shaft, de-airing
extruding mouth
for bricks
for hollow brick
for pipe extruding
horizontal
horizontal, de-airing
horizontal, single shaft
motor
35
102
23
14
25
26
26
84
14,41
35
43
54
49
49
52
49
35
39
41
34
20
54
35
III
105
97
101
100
102
101
108
102
llO
99
102
101
14
98
84
80
80
83
80
81
83
83
81
83
81
79
machine parts
reduction gear system
vertical
vertical, bottom extrusion
Quality control
Ram press
gypsum-cement moulds for
hydraulic ram pressing
Recalcination
of plaster moulds
Refractories
definition
forming methods
setter slabs
Relief designs
Ribs
for shaping on potters wheel
Roof tiles
semi-automatic pressing
Rotary screen
for pipe clay
Sanitary ware
Satin spar
Selenite
Semidry pressing
advantages of
blanks
clay requirements
dies
general work flow
granulation
leather hard
powder work flow
products
problems with talc
Separator
semi-dry pressing
wet method
Setter slabs
by semi-dry pressing
by casting
making
Sewage pipe
market for
Shaping
tools for throwing
Shellac
sd plaster sealer
Silicone
as plaster separator
Slab making
Slabs
joining
79
79
80
81
163
125
102
122,173
18
18
99,112
14
54
20
98
92
22
122
122
105
103
96
98
103
106
96
105
96
108
103
104
99
19
112
91
54
136
135
29
31
making with slip
Slip casting
barium carbonate in slips
body development guidelines
definition
deflocculant amount
deflocculation
density
electric charges on clay particles
flocculation
fluidity
fluidity test
"gelling"
green strength
greenware costing
greenware cracking in the mould
greenware sticking to the mould
helpful hints
history
"livering"
making a trial batch
permeability
pinholes
plastic clay in bodies
preparation of slip
Problem and cures
problems from old soda ash
soda ash
sodium silicate
stiff and brittle greenware
testing if slip is correct
thixotropy
uneven casting thickness
viscosity
Soap
as plaster separator
Soda ash
purchase it fresh, in deflocculation
Solid casting
Spare
for slip casting
Sponges
for throwing
Stamps
for pressing in clay
metal
roller stamps
Sulfates
problems in casting slip
Tableware
Throwing
definition
tools for
159
8, 148
152
152
IS, 148
153
151
149
151
151
149
153
156
150
156
157
157
156
148
152
154
150
157
150
155
156
153
153
153
157
153
152
156
149
135
153
148
141
53
114
115
115
152
9
35
53,54
193
Tile
decorative
definition
dies
extrusion of
fenling and finishing
handmade
market
plastic pressed
pressure requirements
products for small producer
semidry forming
Tile dies
life of
steel for dies
Transport equipment
ware boards, racks
Trimming
centering for
chuck
definition
194
16
16
97
85
104
16
17
17
98
96
17,96
97
97
175
55
55
55
finishing
high foot rings
judging thickness of bottom
lids
tap centering
tools
Turning
definition
Undercuts
mould making
Wages
fixed
incentive
piece rate
Warping
control of
Water filter
Wax
for case mould
Working moulds
56
56
56
49
55
55
54
35
144
161
161
161
167
21
137
135