PAS~T. c
13
THES T S
INVESTIGATION
OF
RED COPPER GLAZES
by
JL Hamilton Minnick
Course IX-B
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In accordance with t h e requirements for the
degree of Bachelor o f Science, I herewith submit f o r
your,
e n t i , t f ed:
"xnvestigstion
Red Copper Glazes
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ACKNOW.LEDGEMENT
The author wishes to acknow-ledge his
indebtedness- to Profressor -.
71. Norton 'under
whose supervision this investigation was made.
The kiln used was his deaign, a.nd his advice
and suggestions tere invailuable.
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1.
INTRODUCTION
To we people of modern times, the many
masterpieces of the ceramic art which have been
preserved through the ages, never fail to gain
admiration for the work of these ancient master
craftsmen.
It is difficult, oftentimes to un-
derstand how they were able to produce such
beautiful china, porcelain, and glazes with the
very crude tools and facilities available to
them in those forgotten times.
We cannot doubt
that the technique and methods used by them were
flawless, for these examples of their art which
we know and admire are perfect specimens of design and execution.
To a student of artistic beauty, the
mere handling and visual inspection of a beautiful glaze or a rare porcelain carries with it a
certain satisfaction, but to anyone interested
in reproduction of such ware, the charm of the
piece lies in being able to understand the background of research and methods of Technology
employed.
2.
It is safe to say that the Chinese and
Japanese were the first master potters and it
is from them that the rest of the world has
learned much of this art.
Their porcelains
and glazes are treasured even today for their
exquisite beauty and texture.
Among these
glazes is one known variously as "rouge flambe",
Chinese-red, sang de boeuf oY1 ox blood.
It is
the peculiar red glaze which is found on old
Chinese porcelain and because of its unique
color and difficulty of production has always
been a source of particular interest to potters
of many ages.
Many have produced it with vary-
ing degrees of success, but each one has offered
conflicting reports of the methods used.
It is
still worthy of much added research and has been
made the subject for investigation in this thesis.
3,
OBJECT OF THE INVESTIGATION
The object of this thesis was to investigate the Chinese red, or red copper glazes.
In
making such an investigation it is the conditions
under which the glaze is fired which are of primary importance, as it is these firing conditions
on which investigators disagree.
With this in
mind it was made the object of the thesis to investigate these conditions and study the changes
wrought in the glaze by variation in their
character.
DESCRIPTION OF APPARATUS
The investigator was very fortunate in
having at his disposal a design and materials
for building an electric furnace to be used as
a kiln in the investigation.
This furnace was
the design of Professor F. H. Norton and was already in process of fabrication.
It consisted of a round asbestos board
base on which two pieces of pipe were attached
in a vertical position
as a frame.
iiThe
kiln
itself was a cylindrical galvanized iron shell
into which were fitted special refractory bricks
of high kaolin content.
These bricks are also
excellent insulators and gave the kiln ideal heat
retaining qualities.
The open spaces between
brick and shell were filled with a powder made by
crushing pieces of this same type of brick.
The
base and top pieces of the kiln were round pieces
of asbestos board and whole brick were also placed
between these and the kiln chamber to form a completed closed and insulated srace.
The kiln
5.
chamber was made accessible from the bottom by a
hole cut through the asbestos base and base bricks,
and a frame sliding on the frame pipes carried a
round piece of asbestos and brick which completely
sealed the hole when in the closed position.
At each corner of the hexagonal shaped
kiln chamber a hole was drilled through from top
and bottom each top hole thus coming in a vertical
line with the one below.
These were for the aus-
tenitic stainless steel electrodes which made contact with the GlobaY heating elements within the
furnace.
After drilling these holes the interior
of the kiln was coated with a semi liquid mixture
of refractory cement which made a uniform surface
and protected the refractory.
This cement was
made up of 50% burned clay grog and 50% Bennington
kaolin, with sufficient water to give it a very
soft consistency,
Any expansion in the iron shell was relieved by springs which executed their force on the
top cover bearing on the shell.
Both top and bottom asbestos boards were
grooved to accommodate the shell and form a good
joint.
As a matter of fact it was found necessary
6.
to cement this joint in order to give an airtight seal.
Constant pressure was executed on top
and bottom of each electrode by a spring in the
These
form of a flat piece of phosphor bronze.
also formed the electrical contacts to each
electrode.
These contacts were all connected at
top and bottom by an insulated wire and from each
of these a lead was taken to the source of power.
This constituted parallel operation of the Globar
elements which were designed to run on 115 volts,
and draw 4.5 amperes at this voltage.
Heating
control was furnished by a variable resistance of
the coil type which regulated the current flowing
to the Globars.
A Platinum - Platinum 10% Rhod
m thermo-
couple was used in conjunction with a Leeds and
Northrop indicating potentiometer for measuring the
temperatures in the kiln.
The thermo-couple was
inserted in the kiln through a hole bored through
from the top.
79
Power was taken from the Institute system through an auto transformer delivering voltage
to a switchboard in 15 volt steps.
It was thus
possible to choose any particular primary heating
potential.
Although not absolutely essential to the
experiment in a quantitative way an ammeter was
used.in series with the load.
It furnished an
excellent indicator of the effectiveness of the
electrical contacts within the kiln and under
severe reducing conditions showed whether an excess of carbon was being deposited on the walls of
the kiln.
8.
METHOD OF PROCEDURE
In conducting this investigation it was
necessary to have a kiln in which to fire the
glazes under observation and to this end work
was started on the kiln already described and illustrated.
In the meantime, and while working
on the kiln, research work in the literature
available on this subject was resorted to in an
effort to discover glazes and methods which had
produced the desired result.
When the kiln was completed two glazes
had been selected and frits were made of these.
The sample tiles on which glaze was applied were
then fired to determine the maturing point of the
glaze.
of the
When this was ascertained the remainder
runs
were made under varying conditions
of atmosphere and heating rate with the object
of finding those conditions under which the best
glaze was produced.
In doing this,
runs were
made with slow heating, fast heating, oxidizing
from start, reducing from start, and combinations
of all, both under steady conditions of atmosphere and alternating between reducing and oxidizing.
It is on these firing conditions which
most investigators disagree but that reducing
conditions are essential at some time during the
burn is conceded by all.
Before firing any glazes in the kiln
a heat run was made to determine the heating
characteristic of the kiln, and following this
the furnace was allowed to cool naturally while
closed to determine its cooling characteristics.
BASIC PRINCIPLE INVOLVED
A glaze containing copper oxide will
fire to a light blue or blueish green when
fired under completely oxidizing conditions.
In order to produce the red color in the glaze
it is necessary to fire the glaze under conditions which will reduce the cupric oxide and
leave finally divided metallic copper dispersed
throughout the glaze.
It is not enough, however,
to simply reduce the cupric oxide but to maintain
it in its reduced state and to prevent the other
10,
chemical reactions between the metal thus formed
and the silicates present in the mo1lten glaze.
It
is
this consideration which requires
adjustment and control of the firing conditions.
11.
DATA
The glazes used were the following:
1.
Keramos glaze maturing at a low temperature.
.5
K2 0
.5
Ca 0
2.5 Si 02
.1 Al 0
2 3
1.O B
0
3
2
Add .5% C u 0, and frit all but .1 clay
Molecular weight
4W
305.2
Frit recipe:
Parts
Constituent
Si
138
02
K2 0C0 3
69
Ca C 03
50
124
B2 03
C u 0
.005
x
1.5
305.2
Yield of frit
Maturing temperature of glaze
-
125 grams
S17750 F
12.
2.
Sprechsaal glaze
.9 Na 2 0
.15 B2 0
3
0
.10 Al 2 0 3
.1 Ba
Add
.5% 0 u 0, and frit all but
Molecular weight
:
4.0 Si 02
.1 clay
332.0
Frit recipe:
Parts
Constituent
57.1
Borax - Na 2 B 4 07 .10H2 0
C 0
Na
3
2
63.6
19.7
Ba C 03
Si
C u 0 ,
228.0
0
.005
x
1.7
332
82.0 grams
Yield of frit
Maturing temperature of glaze
-
2200 o F
13.
Keramos
Molecular weight
.5
x
94.2
47. 1
.5
x
56.1
28. 05
2.5
x
60.1
1.0
x 101.9
.1
x 101.9
-
150.
S
305. 2
Addition of C u 0
.005 x 305.2 = 153 grams.
:
125 grams
Molecular weight of frit
Clay addition
-
69. 6
10. 19
-
Molecular weight
Yield of frit
25
-
283
(.1 x 258.1) ( 125 )
283
- 11.5 grams
14.
Sprechsaal
Molecular weight
.9 x 62
.1 x 153.
4
55. 8
S15.
34
.15 x 69.6
: 10. 44
.10 x 101.9
-
4.00 x
10. 19
S240.4
60.1
Molecular weight
=
332.17
Addition of C u 0
-
.005 x 332.17
-
Yield of frit
-
1.66 grams
82 grams
Molecular weight of frit - 155
(.1 x 258.1) (82)
Clay addition
S6.8 grams.
S
2
x
155
After fritting the glaze was ball milled to
pass 100 mesh screen and excess water dried off.
Portions of the frit were next in water with the
proper portion of the clay to be added and applied
to small pieces of tile with a brush or spray.
The tiles were placed on small kiln plates
moulded especially for the purpose from a 50-50 mix
of 80 grog and kaolin with a little dextrin added for
bonding.
Specimen Tiles
Designation of specimens is as follows:
#3 S F designates that a tile glazed with the
third (#3) batch of the Sprechsaal (S) glqze
mixed from the frit, the letter (F) distinguishing it from other tiles glazed with the
same batch.
#3 K B follows the same plan of designation for
the Keramos glaze.
16.
When coke was used for the reducing medium
it was placed on a furnace plate and entered the
kiln with the specimens.
Air, gas, and hydrogen were blown into
the kiln chamber through a 3/16" hole drilled
through the shell about in the middle of the kiln.
Slow cooling was at a rate of 7.2 my. per
hour and natural cooling by cutting the power off
completely.
Mature glaze
#1 K B
Run completely oxidizing.
#2 K B
Glaze matured oxidizing and cooled to
1480oF.
Reduced at 14800F for 20 min.
Cooled oxidizing.
#3 K A
Oxidizing to 60000.
reducing.
Rest of run completely
Hold at maturing temperature for
1 hr.
#4 K A
Alternating conditions starting at 1100 0 F
to maturity.
Cycle of 5 min. reducing,
15 min. oxidizing.
17.
#4 K B
Coke for reducing.
turity.
#4 K D
Heat rapidly to ma-
Remove from kiln.
Coke for reducing.
Soak at 1775OF for
25 min.
#4 K E
Coke and gas reducing.
start.
Coke in oven from
Run rapidly to maturity.
One
cycle of 1 min. gas reducing and air for
5 min.
#4 K E2
Coke reducing to maturity, gas reducing
for 1-1/2 min., soak for 1/2 hr. oxidizing.
#4 K F
Cool naturally to 12300 F.
Hydrogen reducing during complete run.
Soak 1/2 hr. at maturity, cool slowly.
#4 K H
Mature oxidizing.
Cool to 1450 F. Re-
duce alternating - gas 1 min. air 2 min.
for 4 cycles.
#4 K M
Mature and cool to 1450 0F.
Reduce at
1450oF for 5 min., oxidize 1 min., reduce 10 min.
#1 SB
Not mature.
0
Heat to 2000sF, cool to
1500 OF.
(1). Reduce 5 min.
(2). Oxidize 2 min.
(3). Reduce 10 min.
(4). Oxidize 1 m in.
18.
#1 S C
Soak 1/2 hr. at 2000 0 F, cool to 1500 0F.
(1). Reduce 5 min.
(2).
Oxidize 1 min.
(3). Reduce 10 min.
(4).
Oxidize I min.
Cool oxidizing to 1200 F.
#1 S D
Soak for 1/2 hr. at 2150 0 F, cool to 1550oF
(1).
Reduce 20 min.
(3).
Reduce 15 min.
Cool reducing
(2).
Oxidize 2 rmin.
to 1215oF.
Cool oxidizing to 1000F.
#2 S B
Straight oxidizing, no matured. 1800oF
#3 S B
Soak 1/2 hr. at 2000 0 F, cool slowly to
15000F
#3 S C
(1.) Reduce 10 min.
(2).
Oxidize 1 min.
(3.) Reduce 10 min.
(4).
Oxidize 1 min.
Soak 1/2 hr. at 2075 0 F, cool to 1450oF.
(1).
Reduce 20 min.
(2).
Oxidize 2 min.
(3). Reduce 15 min.
Cool reducing to 12250F.
Cool oxidizing to 1050F.
#3 S E
Over burned.
Soaked 20 min. at 2150 0 F,
cool to 1600F.
(1). Reduce 30 min.
(2).
(3). Reduce 15 min.
Cool reducing to 12000F
Cool oxidizing to 10500F.
Oxidize 3 rmin.
O
0
#4 S B
Heat
to 2150 F for 10 min., cool to 1450 F
(1). Reduce 20 min.
(3).
(2). Oxidize 2 min.
Reduce 20 min.
Cool reducing to 1200oF.
Cool oxidizing to 1050Fo
#4 S H
Heat to 22000F for 5 min., cool to 15000F.
(1).Reduce 5 min.
(3).
Reduce 10 min.
Cool
reducing to 1550°F
Cool
oxidizing to 1350OF
(2).
Oxidize 1 min.
These glazed specimens may be inspected at the
office of Professor F. H. Norton.
20.
CONCLUSIONS
1.
Successful production of the red glaze
depends first on complete maturity of the glaze at
the lowest possible maturing temperature.
This
can only be done without overburning by holding the
kiln temperature at a steady value for sufficient
time to insure complete maturity.
Factors which
influence the length of time are thickness of body,
thickness of glaze, and composition of glaze.
If
the glaze is rich in easily fusible oxides then
both the temperature at which it matures and the
length of soaking time at this temperature will be
reduced.
Without this complete maturing of the
glaze it is difficult for the reducing reactions to
take place in the glaze and the resulting colors are
muddy and lustreless at best.
From the results obtained red glazes will
be produced when first fully matured and then reduced at some lower temperature.
Both the maturing
temperature and that for reduction are dependent on
the glaze composition.
2.
The manner of applying the glaze to the
pile does not seem to influence the result.
3.
The rapidity of heating to maturity and
cooling from it likewise does not seem to influence
the results.
4.
The black color of some of the glazes is
due to large amounts of metallic copper from excessive reduction.
5.
The greenish spots dispersed in the red
are caused by a cupric oxide glass which is rich in
insoluble silicates of copper.
These segregate out
and appear as the green spots.
6.
Long soaking of the glaze at the maturing
temperature has not effect on the result.
RE COMMENDATI ONS
The results of this investigation lead
the author to believe that useful information could
be obtained from further study carried on in the
following lines:
1.
More varied reducing atmospheres at differ-
ent temperature ranges.
2.
Trial of other glazes with methods used by
successful investigators.
3.
Measurement of kiln temperatures with an
optical or radiation pyrometer because it was found
that at high temperatures under reducing condition
the thermocouple was far from accurate.
4.
Application of the glaze on different
bodies.
5.
Glazes with foreign oxides such as tin,
uranium, iron, titanium and others.
23.
BIBLIOGRAPHY
"On the Change In Colored Glazes by Reducing at Low
Temperatures" by Hans Eska, Sprechsaal, May 22 and
May 29, 1930.
"Coloring Agents in Glasses and Glazes" by Sir
Herbert Jackson, Proceedings of the Royal Institution
of Great Lritain, Vol 25, 1927, pp. 230-241.
"Collected Writings of H. A. Seger", Vol. 2, pp. 733746.
"China Red Glazes" by F. Wurts, Keramos 9 (4), 1930,
pp. 121-126.
"Experiments on Red Copper Glazes" by Eberhard
Breitenfeldt, Keram. Rund., 37, 1929, pp. 393-394.
24.
LITERATURE ABSTRACTS
The abstracts included here are from those
works which seem to be the most straightforward in
their treatment of the subject.
PROCEEDINGS OF THE ROYAL INSTITUTION
March 4, 1927.
Coloring Agents in Glasses and Glazes.
Sir Herbert Jackson.
It is worth while to refer to one property
of cuprous oxide.
This oxide does not form salts
with acids except the halogen acids.
With oxygen
containing acids, if it reacts with them, metallic
copper is produced along with a cupric salt of the
acid.
This action takes place in the glaze under
the influence of silica with production of metallic
copper dispersed through a greenish cupric oxide
glass.
The brilliant 'sang de boeuf" glazes are
made under conditions which tend to reduce any cupric
oxide which may be formed so that the glass is not
25.
rendered dingy by a green tent due to dissolve cupric
oxide.
Very little copper is used - in the neighbor-
hood of .5%.
Under the microscope some pieces are red,
others purple or blue and some like neutral glass.
A black glass might be a mixture of red copper glass
and green cupric oxide glass through which but little
light is transmitted.
"Change in Colored Glazes by Reducing at Low Temperatures" by Hans Eska - Sprechsaal, May 29, 1930.
Foreign oxides as well as material used
for reducing, influences the production of the red
color.
The color is produced by the little copper
crystals which lie in the surface of the glass.
In the glaze the fields still free near
the crystalized places are light green to dark green.
Reducing substances and cooling play a role.
A finer
grinding of the glazing materials increases t1ne ability
of dissolving and thereby the regularity of the melt.
26.
The formation will proceed more evenly
and the color of the glaze appears more uniform.
Some materials are not wholly miscible in their
liquid states, which is proven by the formation of
the green segregations.
"Collected Writings of H. A. Seger"
-
Vol. 2 pp. 733-
746.
Segar does not agree with many of his contemporaries regarding the production of the red color.
According to him it is not sufficient to reduce by a
simple continuous reduction, but rather to alternate
between reduction and oxidation.
If reduction is
accomplished with illuminating gas a part of its
carbon is deposited due to decomposition of the gas,
which surrounds the particles preventing fusion.
If
glazes, reduced in this manner are heated in air, the
carbon is burned out, but at the same time the copper
segregated out is also oxidized to cuprous oxide and
the glaze is
enabled to fuse.
glazes are always produced.
In this manner red
The quantity of copper oxide which is used is also
a great importance for the beauty of the color.
In
his experiment he used a content of corper between
.5 and 1% and always obtained very good results,
With a content of .5% of copper oxide to 100 parts
of a porcelain glaze, the glaze is colored red and
perfectly opaque.
After the ware was glazed and had been
set so that the smoke could reach it freely at first
a fire as oxidizing as possible is maintained.
As
soon as a dark red heat shows in the kiln, as much
smoke as possible is produced and continued up to a
temperature at which the glaze begins to vitrify.
This is followed by short periods of oxidation kept
up at short intervals - about 1 to 2 min. for each
1/4 of an hour; between these, hoVever, a strongly
reducing kiln atmosphere must prevail.
This mode
of firing must be continued until the glaze has become densed and somewhat glossy.
There upon the
burning may proceed with oxidizing or reducing kiln
conditions up to the close of the burn.
C-).
APPEND
.I X
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Furnace
Diagram.