Lead Glazes for Ceramic Foodware
An ILMC Handbook
Lead Glazes
for Ceramic
Foodware
Richard L. Lehman
Rutgers University
The
International
Lead
Management
Center
i
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Lead Glazes
for Ceramic Foodware
Richard L. Lehman
Rutgers University
The International Lead Management Center
Research Triangle Park, NC USA
ii
Lead Glazes for Ceramic Foodware
An ILMC Handbook
A publication of the
International Lead Management Center
Research Triangle Park, NC
United States of America
Copyright 2002
The International Lead Management Center
All rights reserved. No part of this publication may be reproduced or
transmitted in any form or by any means, electronic or mechanical, including
photocopy, recording or any information storage or retrieval system, without
permission in writing from the International Lead Management Center
Printed in the USA by
International Lead Management Center, Inc.
P.O. Box 14189 Research Triangle Park, NC 27709-4189 USA
Author: Lehman, R. L. (Richard Long), 1949 –
Lead Glazes for
Ceramic Foodware
2002 First Edition
iii
Lead Glazes for Ceramic Foodware
An ILMC Handbook
ABOUT THE AUTHOR
Dr. Richard Lehman is Professor in the Department of Ceramics and
Materials Engineering where he conducts basic and applied research in new
and traditional forms of glass, ceramics, and polymers. He has particular
interest in lead-containing glasses and glazes, glass raw materials and melting
reactions, and the chemical durability of glasses and glazes. Dr. Lehman is
also active in the field of immiscible polymer blends and is Director of the
Center for Advanced Materials via Immiscible Polymer Blends at Rutgers. In
addition to research activities, Professor Lehman is active in graduate and
undergraduate instruction and he participates in external consulting activities
with local, national, and international industries and organizations. Prior to
joining the Rutgers faculty in 1982, Dr. Lehman gained eight years
experience at Johns-Manville and FMC Corporations where he worked in the
fiberglass and industrial chemical fields. Dr. Lehman has over 100
publications and 25 patents on the processing and properties of materials and
he is a Fellow of the American Ceramic Society. He received his BS., MS.,
and Ph.D. degrees from Rutgers University.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
ABOUT THE INTERNATIONAL LEAD MANAGEMENT
CENTER
The International Lead Management Center was founded in July 1996 and is
based in Research Triangle Park, North Carolina, in the United States of
America. The Center was established by the international lead industry in
response to the need for coordinated international action on the issue of lead
risk management.
Although ILMC was founded and is sponsored by the lead-producing
industry, liaison and cooperation has been established with the lead products
applications sectors. ILMC expertise and advice is therefore available across
the full range of issues associated with production, applications, recycling
and disposal.
ILMC complements and supports existing international risk management
activities and responds to the individual needs of countries who wish to
introduce such projects in either industry or their local communities.
The Center welcomes inquiries and requests for either advice or assistance.
Requests for assistance are assessed through the Center's considerable
network of technical, metallurgical and occupational expertise. ILMC
provides assistance by working with national governments and interested
parties to identify the most appropriate risk management options.
ILMC activities are supported by an extensive and growing database
containing consensus health materials and detailed lead risk information for
mining, refining, manufacturing, recycling and associated case studies. The
database will also maintain a register of those agencies and organizations
able to assist with the funding of community projects.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
PREFACE
Glaze development and design has always been one of the more delicate
and daunting tasks in traditional ceramic technology. So many oxides from
which to choose and the properties of the glaze, always changing with firing
conditions and body interactions, must match the underlying ceramic within
close tolerances to give a professional, defect-free, and functional glazed
surface. No wonder glaze formulation has maintained its artistic mystique
even in the face of comprehensive scientific study and understanding. Lead
glazes, in particular, offer a serious challenge to the formulator. Few other
glaze constituents provide such outstanding properties to the glaze and
forgive more processing transgressions, yet must be carefully formulated,
handled, and processed to avoid exposing workers and end-users to lead.
The ability to formulate and engineer lead glazes to be safe to all those
involved, from the potter to the end-user, has been a major technological
advance of the past 100 years.
In assembling this book, my goal was to collect a comprehensive body of
technical data to enable all those interested in lead glazes for foodware to
easily access relevant information in a single source. In striving for this goal
several levels of information have been incorporated to address the needs of
various readers. Detailed glaze compositions and lead extraction data are
provided for the ceramic technologist, material handling and safety
information for the producer, and an overview section for the non-ceramist.
Hopefully this information assists in the safe use of lead-containing ceramic
glazes and decorations in a range of industries and geographies from the
large manufacturers of the industrialized world to smaller cottage industries
in less developed areas.
Richard Lehman
Princeton, New Jersey
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
ACKNOWLEDGEMENTS
As with all books, this book was written and assembled only with a great
deal of assistance. I wish to thank all of those who helped in the preparation
of the notes and the assembly of the text. I am particularly grateful to the
authors of the 1974 ILZRO Handbook on Lead Glazes for dinnerware who
made much of the original material available for the present text. In addition,
contributions from several key sources were invaluable, including Ferro
Corporation, Lead Industries Association, US Borax, Hammond Lead and the
American Ceramic Society.
Sections of Chapter 2 are adapted from, J. B. Wachtman, Ceramic
Innovations in the 20th Century, pp 103 – 105. Reprinted with permission of
The American Ceramic Society, PO Box 6136, Westerville, OH 43086-6136.
Copyright 1999 by the American Ceramic Society. All rights reserved.
Sections of Chapter 3 are adapted from, F. Singer and W. German, Ceramic
Glazes, Borax Consolidated Limited, London. Reprinted with permission of
Borax Limited.
Sections of Chapter 6 are adapted from, Lead Industries Association Manual,
Lead In The Ceramic Industries, Section 10. Reprinted with permission of
Lead Industries Association.
Sections of Chapter 9 are adapted from, A. Huber, "Decoration of Porcelain,
Earthenware and Bone China”, Ferro Corporation, West Wylie Avenue,
Washington, PA 412-223-5900.. Reprinted with permission of A. Huber and
the Ferro Corporation.
ISO Test methods in Appendix B are reprinted with permission of the
International Standards Organization, Geneva, SW.
Sections of Appendix D are from Hammond Lead Company, reprinted with
permission.
vii
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Table of Contents
About the Author
About the International Lead Management Center
Preface
Acknowledgements
iv
v
vi
vii
Chapter 1
Lead Glazes for Foodware – An Overview A stand-alone
1
primer for the non-ceramist
Use of lead glazes in contact with food.
Measuring Lead Migration from Glazes
The formulation and application of lead glazes
Historical Development
Ceramic Ware
Raw Materials
Safe Lead Glaze Compositions
Glaze Application Methods
Firing Method and Temperature Ranges
Decorations
Chapter 2
Introduction
1
2
2
10
Foodware Safety
Lead Glazes
Lead Free Glazes
Glazes Function and Texture
Clear Glazes
Opacified, Matt, and Special Texture Glazes
Chapter 3
Lead Glazes and Their Development
Historical
Function of lead in glazes
The Use of Lead Frits
The Use of Boric Oxide in Lead Glazes
Applications for Lead Glazes
viii
10
10
11
12
13
13
16
16
16
17
18
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Chapter 4
Lead Migration From Vitreous Surfaces
Factors Influencing Lead Migration
Effect of Glass Structure
Role of Specific Oxides
Chapter 5
Glaze Raw Materials
21
21
22
24
25
Frits
25
Description
Types
Lead Oxides
Lead Monoxide -- PbO
Lead Orthoplumbate or Red Lead, Pb304
Lead Hydroxides and Carbonates
Crystalline Lead Oxide-Hydroxide
Pb(OH)2
2PbCO3 Pb(OH)2
Lead Silicates
Chapter 6
Materials Handling
26
28
30
32
Safe Handling Practice
Hygiene and Medical Monitoring
Proper Plant Hygiene
Proper Instruction and Supervision
Regular Medical Monitoring
Material Safety Data Sheets (MSDS)
32
33
Chapter 7
Glaze Compositions and Lead Migration Behavior
37
Summary of Experimental Results
Tests on Cone 3-5 Clear Production Glazes
Cone 5 Clear Glazes for Institutional Dinnerware
Effect of Coloring Oxides on Lead Release
Cu, Cr, and Co in a Clear Cone 4 Glaze
Fe and Mn in a Clear Cone 4 Glaze
Fe, Mn and Zircon in a Clear, Cone 02-4 Glaze
Cu Fe, Mn and Zircon in a Clear, Cone 02-4 Glaze
Cu, Cr and Co in a Low Temperature Cone 05 Glaze
ix
34
37
39
41
42
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Effect of Commercial Stains on Lead Release
Pb-Sb Yellow Stain In Cone 08-04 Glaze
Cr-Al Pink Stain in Cone 06-02 Glaze
Sn-Sb Gray Stain in Cone 02-4 Glaze
Co-Cr-Fe Black Stain in Cone 1 Glaze
Cr-Al Pink Stain in Cone 02-4 Glaze
Various Commercial Stains in a Cone 02-4 Glaze
Effect of Opacifying Oxides on Lead Release
Standard Glaze Fired to Cone 4 and 01
High Lead Glaze Fired to Cone 4 and 01.
Effect of Variations in Alkali, Alkaline Earth, Boric and
Zinc Oxides and Beryl in a Cone 4 Glaze
Effect of Alkali Oxides
Effect of Alkaline Earth Oxides
Effect of Boric Oxide
Effect of Zinc Oxide
Effect of Beryl
Effect of Base Glaze Variations: Lead Silicate (PbO·1.3SiO2)
Additions of Alkali Oxides to Base Glaze
Additions of Alkaline Earth Oxides to Base Glaze
Additions of Opacifying Oxides
Effect of Base Glaze Variations: Lead Silicate (PbO·1.5SiO2)
Latin Square Experiment
Knoop Hardness
Effects of Al2O3, B2O3 and ZrO2
Repeated Extractions on the Same Glaze Surface
Repeated Tests on Clear, Cone 4-5 Glaze
Repeated Tests on Low Temperature Cone 05 Glaze
Repeated Tests on Low Temperature Uranium Red Glaze
47
54
56
62
68
74
Chapter 8
Effect of Glaze Processing Variables
77
Introduction
Relationship Between Lead Release and Glaze Thickness
Effect of Glaze Thickness of a Cone 4 Fritted Lead Glaze
Effect of Glaze Thickness in Cone 07 Glazes
Effect of Different Bisque on Lead Release
Clear Cone 4 Glaze with Coloring Oxides
Lower Temperature Glaze with Coloring Oxides
Clear Cone 4 Glaze with Opacifying Oxides
General Effects of Varying Firing Time and Temperature
77
78
80
81
82
x
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Effect of Glaze Thickness, Firing Time and
Temperature: Commercial Glazes
Factorial Experiment Design
Results
Effect of Glaze Thickness, Firing Time and Temperature:
Laboratory Fritted Glazes
Effect of Under-Firing on Lead Release
Properties of Underfired Production Glazes
Chapter 9
Decoration of Dinnerware
88
98
Requirements for Ceramic Colors
Composition and Preparation of Ceramic Colors and Inks
Overview
Composition and Preparation of Pigments
Frits, Composition and Manufacture
Composition and Preparation of Media
Application of Ceramic Decoration
Overview
Preparation of Printing Materials
Preparations of Printing Inks
Application
Firing
Summary
Appendix A:
Fritting Approaches to Control Lead Solubility
98
99
101
102
102
105
107
109
Solubility Tests On Frit Powders
Lead Extraction From Glazed Surfaces
Relation of Glaze Structure to Durability
115
119
120
Appendix B:
Tests For Lead Extracted From Glazed Surfaces
127
ASTM C738 - 94 (Reapproved 1999) Standard Test
Method for Lead and Cadmium Extracted from
Glazed Ceramic Surfaces.
1. Scope
2. Summary of Test Method
3. Interferences
4. Apparatus
xi
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
5. Reagents
6. Procedure
7. Report
8. Precision and Bias
ISO Standard 6486: Ceramic Ware, Glass-Ceramic Ware,
and Glass Dinnerware In Contact With Food -- Release of
Lead and Cadmium
0 Introduction
1 Scope
2 Normative References
3 Definitions
4 Principle
5 Reagents and Materials
6 Apparatus
7 Sampling
8 Procedure
9 Expression of results
10 Reproducibility And Variability
11 Test Report
12 Permissible limits
Appendix C:
Materials Handling
131
146
Proper Plant Hygiene
Proper Instruction and Supervision
Regular Checks By Plant Physician or Medical Director
Health Risks of Lead Compounds
Assessing Personal Exposure
Health Hazard Information
Physical Properties
Major Health Effects Noted from Lead Exposure
Occupational Exposure to Lead:
146
148
148
148
152
Appendix D:
Sections of Materials Safety Data Sheets
for Selected Lead Compounds
154
Appendix E:
Pyrometric Cone Properties
176
Temperature Equivalents of Orton Large Pyrometric Cones
xii
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Pyrometric Cone Table Notes:
Cone Position Diagram
177
178
Appendix F:
Glossary Of Terms
179
Appendix G:
Bibliography and References
183
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
CHAPTER 1
Lead Glazes For Foodware – An Overview
A stand-alone primer for the non-ceramist
Uses of Lead Glazes in Contact With Food.
Lead glazes can be safely used on a wide variety of ceramic ware, such as
earthenware pottery, stoneware, and a range of porcelain type bodies. See
Appendix F for a glossary of these and other terms. In fact, the excellent
properties and wide processing latitude
provided by lead oxide [PbO] in the
glaze structure make it an ideal glaze
component over a wide range of ceramic
ware compositions and firing ranges.
When properly processes, the lead
oxide, which is typically present in
concentrations ranging from a few
percent up to nearly 50%, is chemically
combined in the glass structure and the
Glazed earthenware bowl, cone 06
amounts that can be extracted by food
substances or other common acidic media are extremely low, well below
established limits set by the FDA, ISO, and other regulatory groups. When
isolated instances of high lead release have been measured for ceramic ware,
the causes typically fall into one of three categories: [1] the glaze was
improperly formulated and did not contain the proper mixture of ceramic
oxides as described subsequently in this text, [2] proper firing practices were
not followed and the lead oxide was incompletely combined with the silicate
glaze matrix, or [3] decorations and/or coloring agents were inappropriately
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
used such that the lead in these agents was easily extracted, or they were
applied on inappropriate parts of the ceramic ware.
Measuring Lead Migration from Glazes
Strict international standards exist that limit the amount of lead and
cadmium that can be released from ceramic ware in contact with food. Such
standards are defined by the International Standards organization [ISO] and
are discussed in greater detail in later chapters. Generally, the tendency of
lead to migrate from the glaze is greatest under acidic conditions and the
standard ISO test defines a 24 h exposure of the ware to 4% acetic acid at
room temperature, about 22o C. Such
tests must be conducted under
controlled laboratory conditions and
numerous laboratories are equipped to
conduct these procedures. Ware passes
this test if the acetic acid contains less
than 0.5 – 2 parts per million [mg/l] of
lead after the test, depending on the size Atomic absorption instrument used
and shape of the ceramic ware. Flat in measurement of lead release.
plates pass if they release less than 0.8 mg/dm2 according to the established
procedure. Well-manufactured foodware with well-designed lead glazes will
easily pass these limits. In addition to the formalized procedures established
by ISO, there are other methods for assessing the lead migration potential of
foodware, ranging from alternative laboratory methods such as
potentiometric electrode methods to much simpler test kits available to the
consumer.
The Formulation and Application of Lead Glazes
Historical Development
According to written accounts and from the analysis of archeological
artifacts, lead has been used in foodware from ancient Egyptian times to the
present. The broad popularity of lead as a glass and glaze constituent stems
from the numerous processing and property characteristics it imparts to the
glassy state. Among other attributes, lead lowers the melting point of glazes,
widens the processing range, imparts surface smoothness and brilliance, and
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
gives the glaze the ability to heal manufacturing defects such as blisters and
pinholes during processing.
The main disadvantage of using lead in glazes is
the high toxicity of lead when absorbed into the
body. This can occur when the glaze is not properly
designed and/or is improperly applied to the ware, or
poisoning can occur in the factory where lead
chemicals are being handling in the glaze preparation
process. The historical use of highly soluble lead
compounds, such as white lead, exacerbated this
hazard. Beginning in the late 1800’s, new lead
19th Century Pottery
compounds were developed that were much less bioavailable and thus reduced the uptake of lead by pottery workers. Most
successful of these approaches was the fritting of lead, a process by which
lead oxide is chemically combined in a glassy matrix and crushed to a fine
powder. These glassy powders, or frits, keep the lead compounds tightly
bonded and have greatly reduced the hazards of working with lead. They are
a key element of standard lead glaze practice today.
Ceramic Ware
A wide range of ceramic pottery processes exist today, ranging from the
traditional methods of hand fabrication and glazing followed by one or more
firings in simple periodic kilns -- much as has been done for centuries -- to
modern efficient processes where dinnerware is automatically formed by
jiggering machines, fired, glazed and fired again in high speed kilns. In all
instances two of the most important variables, as will be repeatedly discussed
in later chapters, are the nature of the underlying ceramic body and the
temperature to which the ware is fired. Ceramic body types fall into several
categories as indicated in the table:
Ceramic Body Typical Firing
Earthenware
Low, up to cone
03
3
Description
Often called pottery, these
materials are porous, usually
colored or pale white and were
originally prepared from natural
mixtures of clay and other
ceramic materials.
Lead Glazes for Ceramic Foodware
Stoneware
Porcelain
An ILMC Handbook
Moderate to
High
Cone 2 – 10
Moderate to
High
Cone 2 – 10
Dense, impermeable ware,
usually colored to pale.
Dense, white ware, often with
high translucency, made from
highly refined raw materials.
Raw Materials
The raw materials used for lead-containing glasses in contact with food
are principally the same raw materials used for other earthenware, stoneware
and porcelain glazes. These materials, mostly beneficiated mineral materials,
include clay as a source of SiO2 and Al2O3, flint [SiO2], feldspar [(Na,K)2O,
Al2O3, SiO2], whiting [CaO], dolomite [(Ca,Mg)O], zinc oxide, talc,
ulexite/colemanite [B2O3] and various frits. Frits are premelted glassy
materials that contain certain percentages of important glaze oxides and
which are crushed to a fine powder and sold as a commodity to the ceramic
industry. Frits are desirable because they enable water-soluble materials to
be combined in an insoluble glass matrix. Many alkali and borate oxides are
thus rendered more useful as a glaze ingredient by fritting. Lead compounds
are also often combined with silica and other ingredients and fritted. The
lead in these frits is much less bio-available than the lead in more traditional
and more soluble lead oxides and carbonates. Modern practice for
manufacturing lead glazed foodware minimizes the use of highly soluble
compounds such as white lead and favors the use of more insoluble
compounds such as lead oxide or lead silicates.
Safe Lead Glaze Compositions
The formulation and use of lead in glazes in contact with food requires the
development of a lead migration resistant composition and a proper firing
cycle to permit the glaze raw materials to fuse together into a homogeneous
acid resistant glassy surface coating. Although the fundamental design of
safe lead glazes is not trivial and requires skill in glass and ceramic
formulation technology, some general trends can be identified that will
enable the layperson to understand the general approach.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Glazes are glassy substances that are comprised of a silicate network
modified with alkalis and other elements to produce the desired properties.
Safe lead glazes are resistant to the attack of various food acids, which seek
to extract lead and other modifier elements from the silicate network.
Generally speaking, safe glazes have a large amount of silica and a low
amount of alkali network modifiers.
More specifically, oxides can be
categorized by their role in the glaze and their general effect on lead release,
as indicated below.
Glaze Network
Glaze Stabilizers
Glaze Fluxes
Formers
Strongly bind
Intermediate
Promotes lead
lead
diffusion
SiO2
CaO
PbO
ZrO2
MgO
Na2O
Al2O3
Al2O3
K2O
B2O3
B2O3
[note: Some oxides appear in more than one category and their role
depends on the type and amount of other oxides in the glaze]
The formulation of glazes is discussed in terms of the glaze molecular
formula and the relative number of moles of the various oxides. One mole of
SiO2, for example, is equal to the number of grams corresponding to the
molecular weight of the oxide. SiO2 has a molecular weight of 60.1, so one
mole of SiO2 equals 60.1 grams. The weight of one mole of other oxides
follows accordingly.
One class of foodware safe lead glazes contains low amounts of fluxes
such that the moles of the stabilizers and fluxes, as noted above, is
approximately 25% of the mole amount of the network formers. Examples of
three such glazes, maturing from Cone 06 to 5, are given in the table.
Example Glazes for Foodware Surfaces
[Firing temperatures for corresponding large cone are at 150o C/h
heating rate, no soak. Soaking will lower maximum required
temperature. Use cones to precisely measure heat work imparted to
ware]
Oxide
Cone 5
Cone 1
5
Cone 06
Lead Glazes for Ceramic Foodware
An ILMC Handbook
K2O
Na2O
CaO
PbO
CaF2
1.72%
3.06%
7.64%
16.07%
2.48%
1.63%
9.50%
15.65%
1.52%
1.60%
Al2O3
B2O3
9.57%
6.03%
9.57%
7.33%
6.48%
10.70%
SiO2
ZrO2
55.90%
53.84%
100.00%
100.00%
37.30%
0.98%
100.00%
Firing
Temperature, C.
1196
1154
999
Lead release,
base glaze, 24 h,
4% acetic acid,
22o C, mg/l.
0.16
0.02
0.11
23.10%
18.33%
Glaze Application Methods
A glaze slip, or water suspension, is prepared from the raw materials
selected for use in the glaze. In
so-called raw glazes, the
individual raw materials [clay,
flint, feldspar, whiting, lead
oxide] are dispersed directly in
water to produce a suspension
about the consistency of heavy
cream. Fritted glazes usually
contain all the necessary oxides
precombined in the frit, except
for the oxides corresponding to
a 10% addition of clay or
similar suspension agent that is
Applying glaze by the dipping process
added with the frit to the water.
Binders such as sucrose or carboxy methylcellulose are sometimes added to
6
Lead Glazes for Ceramic Foodware
An ILMC Handbook
improve the dry strength of the glaze and avoid defects. The glaze can then
be applied to the ware in a diverse number of ways, such as the brushing,
sponging and dripping methods favored by artists and some potters, to the
highly mechanized spraying and waterfall methods used in industry. Hand
dipping of ware is a widely used practice [photograph] in large industrial
plants and in small potteries. After sufficient drying at ambient or in a heated
dryer, the ware is ready for firing.
Firing Method and Temperature Ranges
The firing of ceramics is defined and controlled by a seemingly archaic
method of temperature measurement that relies on observation of the
softening and melting of miniature
ceramic cone structures formulated
from various ceramic formulations
[see photo of partially and fully
softened cones in a kiln with fired
bowl]. This method, the method of
cones, is in reality a highly accurate
method for measuring heat work, the
product of time and temperature,
Pyrometric cones used to measure
achieved during the firing process. It
firing heat work. Slightly slumped
free-standing cones at left, Fully
is more accurate, reproducible, and
down plaque cones at right.
subject to fewer errors than many
electronic
methods
based
on
thermocouples and the like. Cones do, however, have an unusual labeling
sequence as illustrated in the table of cones and corresponding temperature
values given in Appendix E. Increasing temperature of firing is defined by
decreasing cone numbers that begin with a zero (i.e. 010, 09, 08, etc.) until
01 is reached. Subsequent cone numbers commence with cone 1, 2, 3, 4, etc.
Most ceramic dinnerware is firing in the cone 06 [1000o C] to cone 10
[1305o C] range, with earthenware and other porous or highly fluxed
compositions occupying the lower part of this range and stoneware and
porcelain the upper part.
Firing methods have varied greatly since antiquity and current practice
continues to demonstrate a great deal of diversity. The ancient methods
7
Lead Glazes for Ceramic Foodware
An ILMC Handbook
consisted principally of wood or coal firing in kilns insulated with refractory
stones or simple bricks made of fireclay. Temperatures were usually low and
most ware produced was earthenware. These kilns were often located on
hills or facing windward directions to promote combustion. The ancient
firing methods have gained renewed popularity in modern times among
artists and specialty potters. However, the vast majority of commercial ware
is produced in highly efficient periodic or continuous kilns fired with gas or
electricity. Periodic kilns are filled with ware during a production shift and
then fired over a period time ranging from eight to 36 h or more, depending
on the size of the load and the type of ware. Continuous kilns fire ware
continuously as it is loaded and pushed through the kiln on cars, belts or
rollers; the thermal profile of the kiln and the speed at which the ware is
pushed through determines the firing time/temperature cycle for the ware.
An efficient process developed in recent decades is that of fast firing. In this
process, the ware is advanced through a roller hearth kiln in a single layer
and at great speeds. The speedy process results in fast heating and cooling,
which requires special formulations to avoid the introduction of flaws during
the process, and the complete firing cycle can be just an hour to two.
Decorations
Decorations represent an important area of technology in the manufacture
of ceramic foodware. From an aesthetic and marketing perspective the use of
decorations is critical to the generation of beauty, consumer appeal, and
marketplace competitiveness. Principal types of decoration include decals,
painted images, direct ink transfer images, and metalization. Decorations can
contain lead, cadmium and other toxic materials that require careful
application to foodware ceramics in order to avoid consumer ingestion. The
three major issues that influence decorated foodware safety are the
composition of the decorations, the area of the foodware to which they are
affixed, and whether the decoration is underglaze or overglaze.
Decorations are mixtures of pigments and matrix media. The role of the
matrix media is to provide a substance into which the pigments can entirely
or partially dissolve and/or to provide a bonding agent that adheres the
pigments to the ware. The formulation of pigments and the matrix media
determines the stability of the decoration with respect to end-use conditions.
Although a definite trend exists in the industry to reformulate decorations
8
Lead Glazes for Ceramic Foodware
An ILMC Handbook
away from toxic materials, some colors (bright red for example) are difficult
to produce without toxic materials [Cd in the instance of red]. Special
methods, such as encapsulation, have been developed to address these issues.
When possible, it is desirable to position decorations away from direct
food contact areas or other high-risk vicinities, such as the rim of a glass or
cup that comes into contact with the
consumer.
Various
regulatory
guidelines either prohibit decoration
within one centimeter of the drinking
rim of such ware or strictly limit the lead
release rates from these areas. One
method to virtually eliminate consumer
exposure to toxic decorations is to put
the decoration under the glaze, i.e. to
glaze over top of the decoration. This Hand painted decoration
seals the decoration under a layer of
glaze and precludes attack from food or dishwasher solutions. However, a
major drawback of this approach is that often-sensitive decorations are
required to withstand the harsh high temperature glaze-firing environment in
contact with corrosive molten glaze. Special methods exist for fabricating
overglaze decorations with durable coatings and fluxes such that they
partially dissolve into the glaze when fired to combine the best features of
overglaze and underglaze methods. These approaches have collectively
become know as inglaze decoration.
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CHAPTER 2
Introduction
Foodware Safety
One of the most important contributions of ceramic technology has been
the development of vitreous technology that has ensured the safe use of
ceramic and glass foodware. Two general approaches to foodware safety
have followed nearly parallel paths during the past 100 years; the careful
formulation, control and processing of lead- and cadmium-containing glazes
to assure low migration levels, and the formulation of a new category of
glazes virtually free of the most toxic of the traditional constituents,
principally lead.
Lead Glazes
The use of lead in ceramic foodware has an extensive history. According
to written accounts and from the analysis of archeological artifacts, lead has
been used in foodware from ancient Egyptian times to the present. The broad
popularity of lead as a glass and glaze constituent stems from the numerous
processing and property characteristics it imparts to the vitreous state. The
addition of lead oxide to silicate glass or glaze compositions lowers the
fusion point, widens the processing range, reduces surface tension, and
permits greater flexibility in formulating a composition to achieve the desired
properties of low expansion, a smooth surface, and high brilliance. Lead
glasses and glazes are highly resistant to devitrification, have good chemical
durability, and have the ability to heal body defects such as blisters, pinholes,
drying cracks and other defects of the clay surface.
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Lead Glazes for Ceramic Foodware
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However, if lead glasses or glazes are improperly formulated or fired,
toxic amounts of heavy metals can be release via migration to food
substances in contact with the defective vitreous surface. The problem of
lead migration was recognized early in this century and an increasing level of
research was focused on fritting of lead oxides to reduce the lead availability
and on understanding the proper formulation of lead glazes to minimize
migration by interdiffusion. This effort culminated in the 1974 Geneva
Conference on Ceramic Foodware Safety at which state of the art technology
was presented and the groundwork was laid for broad international standards
on the test methods and permissible limits of lead and cadmium release from
foodware surfaces. At the close of the century, the ISO completed its third
issuance of international foodware standards that are the foundation for
regional standards and for safe worldwide production and trade of ceramic
products among developed countries.
Lead Free Glazes
An alternate approach to minimizing lead migration is the total avoidance
of lead in the glass or glaze composition. Initial studies on leadless glazes
pre-date the century, but significant formulation efforts commenced during
World War II when shortages of
lead
oxide
prompted
investigation of low-temperature
opaque glazes. Substitutions
were tested in which other fluxes
replaced part or all of the lead
oxide constituent. More recent
emphasis on leadless glass and
glaze development occurred in
the 1980's and 90's as
environmental,
occupational
safety,
and
lead
migration
HIGH GLOSS GLAZE ON CHINA TEA
regulations became stricter.
Three approaches to leadless compositions have been pursued. Direct
substitution of bismuth for lead is the most obvious and produces adequate
results. However, bismuth can impart a yellowish color under certain
circumstances, the supply of bismuth is limited, the price is high, and the
toxicity of bismuth itself may be an issue. A second group of leadless
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
compositions uses zinc and strontium to provide the necessary fluxing.
These glazes are glossy and fire well, but color development is poor. A third
approach is toward alkali borosilicate [ABS] formulations. These glazes rely
on alkali borate fluxing and a typical composition may contain approximately
10% B2O3 and 10% (Li, Na, K)2O by weight. The ABS glazes are becoming
widely used, particularly on bone china due to the high expansion of the
ware, but significant problems remain with its use.
Higher firing
temperatures are required to produce a smooth glaze surface, the leadless
glazes react less aggressively at the body interface, defect rates are higher,
and decoration is difficult. Continued development should result in increased
performance and acceptance of this system.
Overall, the two approaches to safe ceramic foodware outlined here have
produced outstanding results and have made a major contribution to world
health. These methods continue to be practiced and developed on a
worldwide basis with the desirable result that toxicity issues related to
ceramic and glass foodware are rapidly decreasing. Indeed the isolated issues
of lead poisoning associated with faulty ceramic ware have virtually
vanished, and when such instances do occur they result from a failure to
comply with good manufacturing practice
Glazes Function and Texture
Glazes on ceramic objects serve several purposes. A glaze seals the
surface of the body to reduce exposed porosity, an important feature with
regard to cleanability and safety for foodware surfaces. Food residues and
bacterial contamination can readily adhere to porous ceramics but not to
smooth, glazed surfaces. A glaze also adds hardness, environmental
durability and wear resistance to a body as well as aesthetic qualities.
Although the base ceramic ware is comprised of a mixture of glass and
crystals that vary over a range, such as represented by porcelain and
earthenware, glazes are nearly 100% glass. Varying amounts of crystals that
act as opacifying agents or colorants are sometimes added and will alter the
appearance of the glaze. The glass material in the glaze itself may also be
colored by dissolved metal oxides. More than one glaze may be applied,
especially if decorative glazes are being used. These various layers may be
fired at different temperatures. One or more of the layers may also be fired in
a single step along with the base ceramic ware. While this reduces the
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Lead Glazes for Ceramic Foodware
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number of firing steps and saves on costs, it offers less flexibility of glaze
material choice. Various categories of glazes will be subsequently discussed.
Clear Glazes
Glazes with no opacifiers can be used directly on a body or on an opaque
glaze or engobe. The clear glaze gives hardness, scratch resistance,
durability, and brilliance that an engobe cannot exhibit, or when a highly
opacifying glaze is not adequate.
The constituent oxides of a clear
glaze composition vary greatly, but
usually include silica, alkali and
alkaline earth oxides, alumina and
lead oxide. This silica is the glass
former, forming the basic structure
of the material. The alkali oxides
(usually Na2O or K2O) flux the
silica, i.e. lowering the melting point
to a temperature which is achieved
by common kilns, as well as
lowering the viscosity. The alkaline
CLEAR GLOSS GLAZE
earth oxides (usually CaO or MgO)
and alumina increase the environmental durability of the glass. Lead oxide
takes part in network formation, as well as improving flow properties and
altering optical properties.
Clear glaze can be colored while maintaining transparency using various
species that dissolve in the glass matrix. The firing conditions, such as using
a reducing environment, can affect the color that these species impart on the
glaze. Table I. lists some of the ions used to color leaded glaze.
Opacified, Matt, and Special Texture Glazes
To mask the body and give various appearances to the surface of an
object, many types of additives are used. Opacifiers are used to give the glaze
an intrinsic color and characteristic apart from the body. The result is most
often shiny due to the glass matrix around the opacifying additives, but to a
lesser degree than in the clear glazes. An opacified glaze may be covered
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Lead Glazes for Ceramic Foodware
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with a clear glaze in another firing in order to impart a brilliance that may not
be attainable by the opacifying layer.
The opacification can be caused by several different phenomena. The
most common is addition of crystalline material that does not melt during
firing, such as tin oxide. Light reflects off the boundaries between the
crystals and the glass, or refracts through the crystals at different angles,
creating a white appearance. The optical properties of the crystals, including
impurities, will determine if any color results from these reflections. A
similar optical situation occurs if the second material in the glaze is a gas,
either air or some other gas evolved from the constituent materials during
firing, such as carbon dioxide. Another less common situation occurs if two
types of glass phases are present, also creating reflection and refraction,
resulting in dispersed white light.
Other textures may be achieved using alternate additives. Matts impart a
partially diffuse reflective surface. These tend to offer less wear resistance
and more porosity. Other special textures are also possible using a wide
variety of materials, although the
more exotic types are used for nonfood surfaces. The mechanisms
discussed above occur in these
materials also, but usually on a
more heterogeneous level. For
example, very large bubbles,
crystals, or macroscopic areas of
different compositions create much
more striking visual effects. In
addition, the use of metallic
TEXTURE GLAZE ON LOW
particles will drastically change the
TEMPERATURE STONEWARE
effect on the incident light.
It is important to keep in mind that these unusual textures affect not only
the appearance of the object, but other properties as well. Large bubbles and
crystals affect the strength of the materials, often acting as stress
concentrators. The chemical and mechanical durability, as well as surface
porosity of such materials is also altered. Coating such a surface with a layer
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
of clear glaze may be used to reduce these problems, but may result in an
undesirable appearance.
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Lead Glazes for Ceramic Foodware
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CHAPTER 3
Lead Glazes and Their Development
Historical
Lead compounds were probably some of the earliest materials used for the
production of glaze coatings on earthenware. The incorporation of lead oxide
produces a molten glaze that has a low viscosity at low temperatures due to
its excellent fluxing properties. Furthermore the lead silicate that is formed
has a high refractive index, producing a brilliance that cannot be achieved by
alternative oxides. The result of these properties is that lead glazes can
exhibit smooth brilliant finishes and have wide latitude during firing.
In earlier times lead sulfide ore (galena) was dusted on to damp clayware
and fired to produce a crude form of lead glaze. This was later superseded by
the production of raw glazes using lead compounds such as litharge (PbO) or
red lead (PbS04) and white lead (2PbCO3. Pb(OH)2) suspended in water
together with clay. The dense oxides of lead are granular in form and tend to
settle out rapidly from suspension. However, white lead, the basic carbonate,
has a flaky structure that suspends well in water and consequently it was
widely used.
Function of lead in glazes
Lead oxide is a valuable component of many glasses and glazes. The
importance of lead oxide in such glasses has received wide discussion. Some
of the general characteristics of glazes, obtained from their lead oxide
content, include:
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Lead Glazes for Ceramic Foodware
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1.
Low Melting Range. PbO is one of the classic network modifiers,
which has had widespread and continuous use in glazes from early
times. The strong fluxing action of PbO allows the formulation of
glazes, which mature at relatively low temperatures in comparison
with their leadless counterparts.
2.
Wide Firing Range. PbO in the glaze reduces viscosity and allows
for satisfactory maturation over a wider firing range. Because of this
greater margin of safety with variations in firing, these glazes are
frequently referred to as being more foolproof.
3.
Low Surface Tension. PbO imparts low surface tension, which is
chiefly responsible for the smooth flow and generally high gloss of
these glazes. Low surface tension is the property that contributes to
the ability of lead glazes to heal over blisters, drying cracks, and
other defects in the glaze surface. Low interfacial tension contributes
to the good wetting and adherence of the glaze to the body. Low
surface tension coupled with the wide softening range of lead glazes
accounts for their superior maturing qualities.
4.
High Index Of Refraction. Brilliant glaze surfaces are attained due to
high index of refraction imparted by PbO.
5.
Resistance To Devitrification. The presence of PbO in the glaze
reduces any tendencies towards surface crystallization or
devitrification of the glaze.
The Use of Lead Frits
The main drawback to the use of lead is its high toxicity when absorbed
into the body. Unfortunately the oxides and the basic carbonate are all
soluble in the hydrochloric acid present in the stomach, thus providing an
easy route for assimilation of lead. Chronic lead toxicity was widely
recognized in the pottery industry from the last century, but it was not until
the latter part of the 19th century that efforts were started to try to eliminate
this widespread problem. Although lead poisoning occurred mainly in the
pottery works where the raw glazes were mixed and applied, it could also
occur in the general population if unacceptably high quantities of the toxic
metal were released from the glaze during normal household use.
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Since lead was an essential constituent of many glazes, steps were
originally taken to minimize the risk of assimilating lead compounds by
improving cleanliness and hygiene in the potteries. Following these changes,
development work was undertaken to produce lead compounds that would be
largely insoluble in stomach acids. Fritting to decrease lead solubility was
developed in the last decade of the 19th century and is still in use today. The
TRIBASIC LEAD SILICATE [LEFT] AND A LEAD SILICATE FRIT
raw lead compound, normally red lead is fused together with silica to
produce a silicate glass of low solubility. Obviously this process involves
some hazards in both the mixing and handling stages as well as from lead
fumes carried away with the combustion gases into the atmosphere.
However, with careful pollution control and strict hygiene regulations within
the factory it is possible to safely produce lead frits having a very low
solubility in stomach acids. In addition to the preparation of frits, recent raw
material practice for lead glazes has evolved to use other raw materials in
which the lead oxide is reacted with other constituents to reduce health risks
and also to assist in the reaction of the glaze. Tribasic lead silicate [3PbO SiO2] is one such raw material. Illustrations of tribasic lead silicate granules
and a typical frit powder are shown.
The Use of Boric Oxide in Lead Glazes
Since boric oxide is an essential component of many lead glazes but
cannot usually be incorporated in the lead frit since it would raise the lead
solubility of the frit to an unacceptable level, it is frequently included in the
form of a separate "borax frit". The borax frit will also normally contain
some other major oxides required in the final glaze. It is therefore possible to
standardize on a small number of lead frits without limiting the range of lead
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Lead Glazes for Ceramic Foodware
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glazes that may be produced. The lead silicate manufacturers can then
concentrate on the production of a small number of frits that will meet the
relevant regulations regarding lead release. A common lead frit of this type is
the so-called lead bisilicate containing 2.2% Al2O3 and having the following
molecular formula:
1.0 PbO - 0.074 Al2O3 1.82 SiO2
The use of this type of frit together with a borax frit and the usual mill
additions effectively eliminated the problem of severe lead poisoning in the
potteries. Lead silicate frits can be substituted for raw lead compounds from
knowledge of the PbO content and decreasing the level of silica added from
other sources to allow for the SiO2 present in the lead frit.
Applications for Lead Glazes
Lead cannot be used in glazes that are matured above about 1170°C due
to unacceptable losses by volatilization. This problem is more acute when
raw lead compounds are used to produce the glaze in comparison with low
solubility lead frits. For example, white lead can lose as much as 10% by
weight of its lead oxide content if fired at 1000°C for one hour. However,
fritted lead glazes show lower levels of loss by vaporization and also improve
the crazing resistance in comparison with raw lead glazes.
In addition to improving brilliance, the addition of lead to a glaze
formulation lowers the expansion coefficient in comparison with alkali
oxides and gives improved flow and elasticity to the finished glaze. The
following two glazes are typical of boron free compositions:
HIGH TEMPERATURE LEADED GLAZE MATURING AT ABOUT 1160°C
[units are moles of oxide]
K2O or
Na2O
CaO
PbO
0.2
0.4
0.4
Al2O3
0.2
SiO2
2.5
LEADED EARTHENWARE GLAZE MATURING AT ABOUT 1080°C
[units are moles of oxide]
K2O or
01
19
Lead Glazes for Ceramic Foodware
Na2O
CaO
PbO
0.2
0.7
An ILMC Handbook
Al2O3
0.15
SiO2
2.5
For the vast majority of glazes that are required to mature at lower
temperatures, boric oxide is added to produce a formulation of the type
shown below:
TYPICAL EARTHENWARE GLAZE
MATURING IN THE RANGE 960°C-1100°C
[units are moles of oxide]
K2O or
Na2O
CaO
PbO
< 0.4
< 0.5
0.2 - 0.5
Al2O3
0.15 - 0.4
SiO2
B2O3
2.0 - 5.0
0.4 - 1.0
The addition of boric oxide in the form of a borax frit permits a reduction
in the lead content of the glaze as well as decreasing the thermal expansion
and firing temperature. This type of earthenware glaze containing both lead
and borax can be extended to maturing temperatures well below 1000°C.
However, the way in which the glaze must be formulated for lower maturing
temperatures generally involves a penalty with respect to lead solubility.
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Lead Glazes for Ceramic Foodware
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CHAPTER 4
Lead Migration from Vitreous Surfaces
Factors Influencing Lead Migration
The factors affecting lead solubility from both frits and finished glazes
have been studied extensively since the time fritting was first introduced. Sir
Thomas Thorpe investigated the solubility of lead frits and in 1889 published
a paper, "The use of lead in the manufacture of pottery" in which he
demonstrated that the ratio of SiO2 to PbO was critical if optimum
insolubility is to be achieved. Thorpe examined the effect of the composition
of lead frits on their solubility in on 0.25% hydrochloric acid that was used to
simulate conditions in the stomach. He found that the lead solubility was
least in the silicate having the mol ratio of PbO:SiO2 of 0.5. Additional work
by Thorpe, and supplemented by subsequent work by J. W. Mellor concluded
that if the ratio of the moles of basic oxides plus alumina were no more than
half the number of moles of acidic compounds [primarily SiO2], then lead
release from the glass would be low.
Moles of basic oxides + Moles of alu min a
£ 0 .5
Moles of acidic oxides [Generally SiO2 ]
Additional efforts have been made during the last century by researchers
to further elucidate the relationship between the release of lead from leadcontaining silicate glasses by interdiffusion in aqueous solutions and various
experimental factors. The principal factors appear to be glass composition
and pH. Some investigators have characterized oxides as "good" or "bad"
based on statistical regression analysis of the general oxide contributions in
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
complex compositions, whereas other studies have examined possible
structural
mechanisms and proposed the existence of threshold
compositional levels for the onset of rapid lead interdiffusion. The structural
interpretation of such a threshold is based on the minimum number of nonbridging oxygen for each glass-forming atom to permit rapid lead
interdiffusion. Lehman and Greenhut identified an unusual effect of small
P2O5 additions to lead-containing silicate glasses. When small amounts of
P2O5 were added to lead silicate (1:1) glass, the apparent release rate of lead
ions into acidic solutions was greatly reduced. Microscopic observation of
the leached glass surface revealed the presence of small lead phosphosilicate crystals. The results of that study prompted a much larger study in
the beneficial effects of small phosphate additions were demonstrate for a
wide range of soda-lime lead silicate glasses.
Effect of Glass Structure
A more general effect that relates to the early work on lead glazes is the
effect of the ring structure of silicate glasses and the importance of
preventing a continuous chain of ring breakages from occurring in lead
glasses. It has been shown that six-member silicate rings predominate in
silicate glass structures typical of commercial glazes, as shown below.
Si
O
Na
O
O
Si
Si
O
O
Si
Si
O
O
Si
SIX-MEMBER SILICATE RING STRUCTURE
WITH ONE NON-BRIDGING OXYGEN LINKAGE
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
If each silicon is shared between four such rings and each oxygen is
shared between two such rings, then the total number of silicons in each ring
is 6 x 0.25 = 1.5 and the total number of oxygen in each ring is 6 x 0.5 = 3.0.
Thus each ring has the equivalent of 1.5 SiO2. To generate a broken linkage
in the ring structure one unit of Na2O, PbO or other basic oxide is required as
per the following relationship.
º Si - O - Si º + Na2O Þ 2[º Si - O- Na+]
This reaction breaks the ring structure of the glass and permits leaching
and interdiffusion of the modifier ions such as lead, alkalis and alkaline
earths. Therefore, a lead release threshold is expected when the mole ratio of
glass modifiers to glass formers is greater than 1.0/1.5 = 0.67. Under these
conditions each silicate ring will have at least one nonbriding oxygen unit -- a
necessary condition for charge-assisted diffusion of modifiers through the
network.
Lead Release from Silicate Glasses
7.00
Lead Release, 60 min, mg/cm2
6.00
5.00
4.00
PS
NPS
KNPS
CNPS
ACNPS
BCNPS
Linear (PS)
Linear (NPS)
Linear (CNPS)
3.00
2.00
1.00
0.00
0.50
0.60
0.70
0.80
0.90
1.00
1.10
Moles Modifier/Glass Former
LEAD RELEASE FROM SILICATE GLASSES WITH
DIFFERENT MOLE RATIO OF MODIFIER AND FORMERS.
23
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An ILMC Handbook
Studies have shown that such a general relationship exists, as shown in
the figure, Lead Release from Silicate Glazzes.
Role of Specific Oxides
In general terms it is possible to state that of the basic oxides normally
encountered in glazes the alkali oxides Na2O and K2O increase lead
solubility, MgO has little effect and CaO decreases solubility. Alumina is
very beneficial in reducing solubility whereas boric acid has the reverse
effect. Titania (TiO2) has also been found to have a marked effect on
lowering the solubility of lead frits. A level of 1 - 2% may be used in high
lead oxide frits although the coloring effect limits the level of titania that may
be incorporated.
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CHAPTER 5
Glaze Raw Materials
Many common raw materials are used to incorporate the various oxides
into glaze bodies. The primary concerns are safety when fired, material cost,
and required firing temperature. Non-lead containing raw materials in the
glaze industry include silica (quartz), various alkaline earth and alkali
compounds including oxides, hydroxides and carbonates, clay minerals
including kaolin and feldspars, other many other metal oxides and
compounds. This section describes the types of materials that can be used to
include the lead oxide component in glaze, and their properties.
Frits
The largest change in the use of lead oxide in glazes in the last quarter
century is the drastic increase in the use of frits, especially for small batches
and artware. Frits have many advantages, which almost always outweigh the
disadvantages. The major disadvantage is that in large quantities, frits may be
more expensive than lead oxide compounds. However, unless large volumes
of glaze are being used, frits are almost always much more beneficial and not
of significant cost.
Description
A frit is simply a pre-manufactured powdered glass of a fixed
composition. Powdered glass offers many benefits over crystalline oxide
compounds. First, the glass softens at a lower temperature than the oxide
compounds, facilitating firing. It acts as a compositionally independent flux
in the system. During firing the frit will melt and mix with the other
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Lead Glazes for Ceramic Foodware
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compounds much more easily than if it existed as discreet compounds.
Second, lead oxide already incorporated into a glass matrix with silica and
other materials is much less soluble than crystalline lead compounds,
decreasing possible routes of lead exposure to the body.
Types
Frits are available in a wide variety of compositions, including those
containing lead oxide. It is also possible to have a frit manufactured for a
certain glaze. This can reduce the need for inclusion of minor or even major
oxide components, simplifying the production process by eliminating
ordering, shipping, storage, weighing and mixing of such raw materials.
Lead Oxides
Lead Oxide -- PbO
Lead monoxide or litharge is the most important inorganic lead compound
and typical commercial powder produced by sublimation consists of particles
ranging from 0.25 - 0.50 mm for fine grades to 1.5 to 8 mm for coarser grades.
Lead monoxide occurs in two polymorphic forms, a tetragonal form a-PbO is
stable up to 489o C and the orthorhombic form b-PbO is stable above this
temperature (In some countries the designation for the alpha and beta forms
is reversed). The alpha form is red and the beta form is yellow, but
differences in color caused by the state of crystallization or by contamination
with Pb3O4 make color an unreliable indicator of crystal type. The density of
litharge is 9.35 g/cm3 for a-PbO and 9.7 g/cm3 for b-PbO.
The ceramic industry is the leading consumer of litharge. Glasses of high
lead content have greater density, lower thermal conductivity, higher index of
refraction, greater brilliance, and greater stability and toughness than
unleaded glass. The lead oxide content of commercial lead glasses generally
ranges from 15% to 58%. Glasses with high lead contents have electrical
capacities that compare favorably with those of mica. The unusual electrical
properties are caused by the replacement of the very mobile alkali ions by the
relatively immobile lead ion. Lead is widely used in optical, electrical, and
electronic glasses and in fine tableware.
Glazes and vitreous enamels are glassy coatings used to protect ceramic
bodies. Lead oxide is one of the major raw materials for glazes. It is such an
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Lead Glazes for Ceramic Foodware
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important component that glazes are often classified into leaded and leadless
types. The use of lead glazes is limited to about 1150 °C since above this
temperature lead compounds start to vaporize. The basic source of lead in a
glaze is the monoxide, but red lead, Pb3O4, is also used. The oxides are
converted into lead bisilicate frits to render the lead compounds insoluble.
Several lead frits are manufactured, differing in the ratio of lead oxide to
silica and alumina.
The advantages of lead in a glaze are numerous: wide range of
compatibility; ready fusibility; relative insolubility in water of the available
lead compounds; low viscosity and surface tension of its fused compounds;
low price, considering its fusing power; and good solubility with colored
oxides. Among its disadvantages are the high vapor pressure of lead
compounds, which causes problems during firing; scratchability and to a
lesser extent crazing of glazes; occasional attack of food juices or other
liquids on housewares; and the possible danger of lead poisoning.
Vitreous enamels for metals are often formulated with litharge, chiefly for
coating cast iron, but increasingly in aluminum for architectural applications.
Lead monoxide is the starting material in the manufacture of lead silicate and
lead carbonate.
Lead Orthoplumbate or Red Lead, Pb304
Of all the intermediate oxides only lead orthoplumbate, Pb3O4, is of
commercial importance. It is a brilliant red pigment marketed under the
names red lead in the United States and minium in Europe.
Red lead is the second most important lead pigment. It is used as an
inhibitor in surface coatings to prevent corrosion of metals. It is also used in
storage batteries and to a lesser extent in glass, lubricants, petroleum, and
rubber. Red lead is the raw material for lead dioxide. It is often used in the
manufacture of lead ferrite magnetic materials. Red lead is stable at ordinary
temperatures. When heated above 500 °C it decomposes to lead monoxide.
The reaction is reversible, and in practice red lead is made by controlled
oxidation of lead monoxide.
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Red lead is not attacked by acetic acid but is readily decomposed by nitric
acid and hydrochloric acid to yield the corresponding lead(II) salt and lead
dioxide.
Red lead is manufactured by heating lead monoxide in a reverberatory
furnace in the presence of air at 450-500 °C until the desired composition is
obtained. Orange mineral, a special brilliant orange grade, is made by
thermal oxidation under carefully controlled conditions. The rate of the
reaction is affected by the particle size of the litharge and can be increased by
operating under pressure. The rate increases in the presence of small amounts
of silver oxide and decreases when oxides of silicon, bismuth, zinc, or
antimony are present. Commercially produced red lead contains 70 to 99%
Pb3O4.
Lead Hydroxides and Carbonates
Crystalline Lead Oxide-Hydroxide
There appears to be only one crystalline lead oxide-hydroxide that has
been formulated as 3PbO.H2O. The compound is formed by hydrolysis of
lead acetate solutions or from reduced pressure evaporation of solutions of
tetragonal PbO in large volumes of carbon dioxide free water.
Pb(OH)2
Lead hydroxide, an intermediate in wet preparation of the monoxide, is an
amphoteric base. Pb(OH), is slightly soluble in acids and alkalis but
insoluble in acetic acid. Lead hydroxide reacts with carbon dioxide or
carbonates to form PbCO3. Basic lead carbonate, known as "white lead", is
discussed subsequently.
When heated in air the hydroxide is dehydrated to lead monoxide.
Dehydration begins at about 130 °C and is complete at 145 °C. In aqueous
solution lead hydroxide is an amphoteric base. Lead hydroxide can be
prepared by electrolysis of a lead salt or of an alkaline solution with a lead
anode. It can also be prepared by adding alkali to a solution of lead nitrate or
diacetate.
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An ILMC Handbook
2PbCO3 Pb(OH)2
This compound, the most important basic salt of lead, is one of the
pigments known as white lead. In recent usage all white pigments made from
lead are called white lead: basic lead carbonate is described as basic
carbonate of white lead, the basic sulfate as basic sulfate of white lead, and
the basic silicate as basic silicate of white lead.
Basic lead carbonate is stable up to 100 °C, loses water between 120-155
C. begins to lose carbon dioxide at about 190 °C. and decomposes at 400 °C.
o
EARTHENWARE BOWL GLAZED WITH CONE 05 LEAD FRIT GLAZE
In the presence of water and carbon dioxide it reverts to normal lead
carbonate. Its tendency to blacken when exposed to hydrogen sulfide detracts
from its value as a pigment.
Basic lead carbonate is manufactured by several methods. All processes
are based on the production of soluble lead acetate, which is then treated with
29
Lead Glazes for Ceramic Foodware
An ILMC Handbook
carbon dioxide to form white lead. Lead acetate is made from lead metal or
monoxide and acetic acid.
Lead Silicates
Lead metasilicate, which occurs in nature as the mineral alamosite, is the
most common silicate of lead. Lead pyrosilicate, 3PbO.2SiO2 is the mineral
barysilite. Lead orthosilicate, PbO.SiO2, and some other lead silicates
including 4PbO.SiO2 and possibly PbO.2SiO2 are also known.
As in the PbO-B2O3 system, PbO and SiO2 form glassy compounds over a
wide range of composition. Mixtures with up to 90% PbO stay glassy at
room temperature if cooling is rapid.
Lead silicates have a wide range of applications, including ceramics,
glasses, paints, rubbers, and other polymers. Commercial lead silicates are
generally not well-defined compounds but rather mixtures of the two oxides
in various ratios.
LEAD SILICATE PHYSICAL PROPERTIES
Compound
Common
name
Mineral
Name
Crystal
Structure
Melting
point, oC
Specific
Gravity
PbO.SiO2
Metasilicate
2PbO.SiO2
Orthosilicate
3PbO.2SiO2
Pyrosilicate
Alamosite
--
Barysilite
Monoclinic
Prism
765 - 770
Prism
Trigonal
740 -- 746
6.49
--
650
(decomp)
6.7
4PbO.SiO2
Basic
Silicate
-Several
forms
725
--
Lead silicate is an important ingredient of enamels and glazes, and is used
widely in flint and specialty glasses. Glasses containing lead bend light more
than ordinary glasses and can be cut into facets with a gemlike effect. Lead
silicate glass is opaque to ultraviolet radiation but transparent at other
wavelengths so that it can be used as an optical glass. Other uses include
electric and electronic bulbs, tubes, and other parts; radiation shielding; and
30
Lead Glazes for Ceramic Foodware
An ILMC Handbook
solder sealing glasses. Lead monoxide is the common source of lead in
glasses and ceramics, but lead silicates are also used.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
CHAPTER 6
Materials Handling
Safe Handling Practice
Every type of lead material can be handled and controlled with safety if
proper equipment is provided for the protection of the health of the industrial
worker. This is borne out by the many plants that have as their primary
function the processing and handling of lead and its compounds and do so
with entire success and safety. A clean, well-ventilated plant with efficient
material handling methods is essential for successful operations.
With hygienic plant controls, the general use of lead compounds in the
ceramic industry is available with all of the recognized advantages of these
versatile materials. Industrial health precautions are by no means confined to
the use of lead and its compounds. Silica, beryllium, cadmium, antimony,
selenium, tellurium and other elements and compounds present exposure
problems. While we are primarily discussing the handling of lead
compounds, it should be remembered that, basically, the same precautions
are necessary in plants where no lead is being used. Dust of any kind is a
health hazard.
Proper handling of lead compounds in the ceramic industry requires:
·
Proper plant hygiene,
·
Proper instruction and supervision of workers,
·
Regular monitoring by a health care professional.
32
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Hygiene and Medical Monitoring
Proper Plant Hygiene
A. Adequate pre-employment examination of all plant workers. This should
include previous working histories, and medical examinations.
B. Dispersion of dust should be minimized or eliminated if possible. All
operations that disperse dust should be controlled by closed systems of
local exhaust ventilation.
Lead compounds are packaged so that unloading, plant storage and
movement to the location where the packages area to be emptied present no
exposure problems. Dust hoods should be located where dry materials are
charged or discharged from processing equipment and should surround the
operation as completely as possible. Bins, elevators, chutes, covered
conveyors, covered mixers etc., can usually be made dust-free by drawing off
dust-laden air at critical points. By maintaining a slight negative pressure in
the closed equipment, some air may enter the system, but dust will not
escape. Relatively small air volumes are required to achieve dustless
operation for closed equipment. Exhausted dust-laden air should be properly
filtered before discharge outside the plant. Dust control and elimination are
important to all industries today and there are many reliable and qualified
manufacturers of equipment to do the job properly.
C. Only where local or general control is impossible, workers should be
provided with respirators specifically approved for this type of
protection.
D. Adequate washroom facilities should be provided. Hot water and soak
should be provided, along with disposable hand towels. Shower facilities
are recommended.
E. Proper locker room and shower facilities should be provided. Workers
should have separate lockers for street clothes and work clothes to
prevent contamination. Workers should shower prior to leaving the work
premises.
F. A suitable and separate place should be provided for workers to eat.
33
Lead Glazes for Ceramic Foodware
An ILMC Handbook
No food should be eaten until the worker has washed and no eating should
be allowed in the department or area where lead-containing materials are
used. Workers should not smoke on the job while handling these materials.
G. Work clothing should not be taken home. Lead compounds will easily
become attached to work clothing and commingling of work clothing
with home clothing exposes others in the household to lead. The
washing machine processes is inefficient. Washing work clothes with
other clothing spreads contaminants.
Proper Instruction and Supervision
The purpose, correct use, and maintenance of respirators should be
explained to the workers and enforced. Workers must be provided with
respirators that fit the individual. Respirators assigned to one individual
should be regarded as personal as a toothbrush and should not be
interchanged. The use of respirators is no substitute for adequate plant
engineering.
Regular Medical Monitoring
Most practicing physicians rarely have occasion to treat cases of heavy
metal overexposure. Therefore, the appropriate health care professional
should be informed of any such materials being handled by a worker. The
health care professional will then be in a position to study the problem and
outline a regular safety program. Monitoring of lead in the air, on surfaces, or
regular blood tests may be deemed appropriate by health care personnel.
There are both governmental and private agencies in many cities
competent to evaluate and advise on engineering and medical control of lead.
Material Safety Data Sheets (MSDS)
Lead compounds need to be handled with care to prevent accidental
inhalation or ingestion of lead. Lead compounds vary considerably in the
biological availability of lead. The frits and compounds in which lead is
combined with other components, most notably silica, are the most stable in
acidic environments and in general are the easiest to work with from a safety
perspective. Lead oxides such as PbO and Pb3O4 are intermediate in
durability and white lead, 2PbCO3 Pb(OH)2, is the compound most readily
34
Lead Glazes for Ceramic Foodware
An ILMC Handbook
dissolved in stomach acids, of the commonly used compounds, and must be
handled carefully.
Materials Safety Data Sheets are available from all chemical suppliers for
all compounds sold commercially, and virtually all MSDS sheets are now
available over the internet. A simple search strategy of typing MSDS
followed by the compounds name usually produces good results. The MSDS
contain an abstract of useful information regarding the handling, storage, and
disposal of the respective compound, as well as critical health and safety
data. The general outline of the MSDS is as follows:
Section 1: Identification
Section 2: Hazardous Composition/Ingredients
Section 3: Hazardous Identification
Section 4: First Aid Measures
Section 5: Fire Fighting Measures
Section 6: Accidental Release Measures
Section 7: Handling And Storage
Section 8: Personal Protective Measures
Section 9: Chemical And Physical Properties
Section 10: Stability And Reactivity
Section 11: Toxicological Information
Section 12: Ecological Information
Section 13: Disposal Considerations
Section 14: Transportation Information
Section 15: Regulatory Information
Section 16: Labeling Information
35
Lead Glazes for Ceramic Foodware
An ILMC Handbook
MSDS for seven lead compounds commonly used in the preparation of
glazes for dinnerware are given in Appendix D. The seven compounds are:
·
·
·
·
·
·
·
Lead Bisilicate
Litharge [PbO]
Lead Monosilicate
Lead Monoxide
Red Lead [Pb3O4]
Tribasic Lead Silicate
White Lead [lead carbonate hydroxide]
36
Lead Glazes for Ceramic Foodware
An ILMC Handbook
CHAPTER 7
Glaze Compositions and
Lead Migration Behavior
Summary of Experimental Results
A. Tests made on a large number of Cone 3-5 [~1160o - 1190o C]
production, clear lead glazes show lead release values of less than 0.5
mg/ml [part per million, ppm] and in a number of cases less than 0.1
ppm. These values are important since 0.5 mg/ml reflects the lowest
permissible limit for lead release from ceramic ware according to the
current ISO standards, and 0.1 mg/ml represents the lowest significant
values that can be measured with flame atomic absorption
spectroscopy, the method of choice for manufacturing control
measurements world-wide. Thus the Cone 3-5 clear glazes, which
constitute the large part of dinnerware glazes, show low lead release
as has been found in a number of other laboratories during the past
three decades.
B. Data taken on typical Cone 5 clear glazes as used for institutional
dinnerware also showed lead release values within ISO compliance
limits.
C. A number of typical commercial stains, representing those most
commonly used along with the appropriate clear base glazed, were
tested. None of these affected the lead release of the base glaze
adversely, showing these have been well designed.
37
Lead Glazes for Ceramic Foodware
An ILMC Handbook
D. Direct additions of various coloring oxides have shown the
detrimental effects of copper oxide especially for the lower
temperature glazes. This confirms earlier knowledge that led to the
abandonment of the low temperature copper green glaze years ago.
As is often noted in the literature, copper oxide should not be used
either as a direct addition or component of a stain used in a lead glaze.
It should also be noted that of more practical significance is the
normal practice involving the use of prepared compositions or stains,
which include the coloring oxides but are designed to be compatible
with specific base glazes and their maturation.
E. The additions of various opacifying oxides (zirconium silicate, tin
oxide, or titanium oxide) to lead glazes, in appropriate amounts to
attain the desired opacity, do not affect the lead release adversely and
for some glazes (showing higher lead release) such additions prove
beneficial. Ions of high charge and small size, introduced as their
oxides, fit into the interstices of the silicate structure and in many
instances result in a strong contraction of the surrounding unsaturated
oxygen ions, strengthen the structure and reduce lead release.
F. A study of variations in the alkali oxides and alkaline earth oxides in a
typical Cone 4 glaze did not show any significant differences in lead
release for the concentration used. An increase in lead release was
noted with increase in boric oxide in the glaze. Additions of zinc
oxide did not affect the lead solubility of the base glaze adversely.
Essentially the same results were noted when these glazes were fired
at Cone 01 [~1130o C]. The acid resistance of this typical Cone 4
lead glaze and the modifications studied is good. As noted below
changes of this type may be reflected in the lead release from lower
temperature glazes.
G. In a low temperature (Cone 07) lead silicate (PbO·1.3SiO2) base glaze
small single alkali oxide additions increase the lead release with Li20
increasing it least, Na20 being intermediate, and K20 increasing the
lead release most. These data show increasing lead release with
increasing ionic radii of the alkali oxide added. A mixture of any two
of the three alkali oxides (totaling 0.1 molecular equivalent) resulted
in lower lead release than the same molar addition of any of the single
38
Lead Glazes for Ceramic Foodware
An ILMC Handbook
alkali oxides. The addition of the three alkali oxides (totaling 0.1
molecular equivalent) gave the lowest lead release. Hence the use of
mixed alkali oxides would is preferred over single alkalis with respect
to lead release.
H. The alkali earth oxide additions in the same low temperature base
glaze gave similar trends to the alkali oxide additions in respect to
lead release, showing (with exception of CaO in the data taken) an
increase in lead release with increase of ionic radii of the oxide added.
The best combinations of alkaline earth oxides, for the same total
molar concentration, included those having smaller ionic radii, e.g.,
MgO-CaO or MgO-SrO or MgO-CaO-SrO.
I.
A designed experiment on a Cone 03 low temperature lead silicate
base glaze (PbO·1.5SiO2) involved varying additions of A1203, B203,
and Sr02. Higher levels of A1203 definitely reduce the lead release
whereas increasing B203 increases lead release. An attempt was made
to correlate the acid resistance with calculated structural factors and
glaze hardness (as an indication of the strength of the structure). The
same experiment repeated with a higher Cone 1 glost fire gave
different results thought to be due principally to the greater
thermochemical action of this low temperature glaze on the body at
the higher temperature. Zr02 additions were effective in increasing
acid resistance at the higher firing temperature.
J. Depletion tests involving the repeated testing of the same glaze
surface is of interest from the standpoint of continued service. A
typical Cone 4-5 dinnerware glaze showed a very low initial value of
0.10 ppm and decreased to 0.06 ppm after eight cycles.
Tests on Cone 3-5 Clear Production Glazes
A number of glazed cup specimens were tested which represent current
commercial production, i.e., Cone 3-5 clear glazes now being used. Three
glazed cups were selected at random from a number of glazed specimens in
each case representing a specific glaze. The data are given below:
Plant Code
Cone
A-l*
3**
39
Lead PPM
(Av. for 3 cups)
0.06
Lead Glazes for Ceramic Foodware
A-2
A-3
B-1
B-2
B-3
B-4
C-1
D-1
D-2
D-3
D-4
D-5
D-6
D-7
D-8
E-1
E-2
F-1
G-1
G-2
H-1
I-1
J-1
J-2
J-3
An ILMC Handbook
3
3
4-5
4-5
4-5
4-5
1
4-5
4-5
4-5
4-5
4-5
4-5
4-5
4-5
4-5
4-5
4-5
4-5
4-5
3
4-5
4
4
4
Average
Standard Dev.
Coef. of Var.
0.39
0.33
0.04
0.07
0.10
0.07
0.08
0.04
0.02
0.16
0.08
0.07
0.11
0.06
0.08
0.27
0.10
0.13
0.45
0.20
0.23
0.10
0.07
0.15
0.11
0.137
0.111
81%
*1, 2, 3 etc. designate different glazes of subsequent tests on other
samples of the same type glaze.
These data show lead release values of less than 0.5 parts per million for
the Cone 3-5 clear glazes and corroborate a large body of data extending as
far back as to work carried out at the National Bureau of Standards reported
in 1939 that shows that clear glazes fired in this range produce low levels of
40
Lead Glazes for Ceramic Foodware
An ILMC Handbook
lead release. Indeed most work in past decades has concentrated on lead
migration from colored glazes in organic acids. In the latter research most
attention was given to several low temperature colored glazes (Circa Cone
05) and further reference is made to these in the discussion of the results of
testing similar glazes in the present investigation. The low lead release
characteristic of the Cone 3-5 clear glazes is seen further in this investigation
from measurements on the base glazes to which various coloring oxide
additions were made to evaluate the effects of these additions.
The experience of many laboratories many years has shown that the Cone
3-5 clear glazes, which constitute the large part of lead dinnerware glazes,
show very low lead release (less than 0.5 part per million). Since no difficulty
has been experienced with such glazes as regards lead migration, the
principal effort of recent research has focused on the effects of various oxide
additions, lower temperature glazes, and production parameters.
Cone 5 Clear Glazes for Institutional Dinnerware
An additional area of commercial interest is the glaze used for
institutional dinnerware. Institutional dinnerware is traditionally a thicker
and stronger dinnerware intended for the more intense use of hotels and other
commercial or institutional users. The glost fire for such ware is traditionally
higher than for other forms of dinnerware and Cone 5 is representative. A
representative base glaze that was the average of five different commercial
compositions was selected for this study and the oxide formula is given
below.
Base Glaze
0.066 K2O
3. 369 SiO2
0.340 A12O3
0.179 Na2O
0.261 PbO
0.314 B2O3
0.494 CaO
All of the above base glaze composition was fritted except the necessary
molecular equivalent of A12O3 and SiO2 to provide a ten percent (10%) clay
mill addition. This glaze was then applied on hotel china bisque cups and
fired to Cone 5 in a commercial glost fire. The lead release from this glaze
and from commercial glazes on cups from two hotel china producers, using
the standard 24 h 4% acetic acid test, is given below:
Lead PPM
41
Lead Glazes for Ceramic Foodware
Base Glaze
Commercial HC Glaze 1
Commercial HC Glaze 2
An ILMC Handbook
Individual Data
0.18 - 0.14
0.31 - 0.24 - 0.27
0.12 - 0.08 - 0.05
Average
0.16
0.27
0.08
As might have been expected from earlier data, the lead release from this
clear Cone 5 glaze and from the two commercial hotel china glazes was
within current ISO permissible limits.
Effect of Coloring Oxides on Lead Release
The objective of these studies was to investigate the effect of coloring
oxide additions on the lead release of glazes since considerable literature data
have showed that certain pigment additions can promote lead migration in the
glazes [copper the most notorious example] whereas others tend to stabilize
the glaze. The normal practice involves the use of prepared pigment
compositions or stains, which include the coloring oxides but are designed to
be compatible with the glaze and its maturation.
Cu, Cr, and Co in a Clear Cone 4 Glaze
This glaze employed two frits, one leaded and one leadless, and had the
following composition:
Cone 4 Glaze
0.013 K2O
0.182 Na2O
0.572 CaO
0.233 PbO
0.290 A12O3
0.360 B2O3
2.971 SiO2
Glaze Batch
Frit A
Frit B
Whiting
Clay (Ajax
PC)
Flint
(Supersil)
51.5
25.1
4. 0
4.7
14.7
100.0
42
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Frit A
0.02 K2O
0.29 Na2O
0.69 CaO
0.27 A12O3
0.57 B2O3
2.49 SiO2
0.25 A12O3
1.92 SiO2
Frit B
1.00 PbO
Frit B represents the cone deformation eutectic for the PbO-Al2O3-SiO2
system and has high acid resistance in powder or dust form.
The above glaze and modifications of this glaze with various mill
additions of coloring oxides were applied on bisque cups from three plants B,
C and D. The coloring oxides and amounts used (as percent by weight mill
additions on the basis of the dry glaze batch) are given in the table below.
Each value for lead release given is the average for three cups. The test
specimens were all fired to approximately five (5) hours to Cone 4 in a gasfired laboratory kiln.
Lead PPM
Bisque cups used from
Coloring
Oxide
Base Glaze
CuO
CuO
Cr2O3
Cr2O3
Co3O4
Co3O4
Oxide
Addition
Weight
Percent
–
0.1
2.0
1.0
3.0
0.01
0.1
Plant B
Plant C
Plant D
0.07
0.08
1.58
0.07
0.04
0.04
0.01
0.10
0.05
1.19
0.07
0.03
0.08
0.01
0.03
0.03
1.14
0.04
0.03
0.11
0.11
With the exception of the modification of a two percent (2%) by weight
mill addition of CuO to the base glaze, the lead release values for the base
glaze and all the other modifications are too low to draw any conclusions. In
large part all these values are less than one part in ten million.
43
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Fe and Mn in a Clear Cone 4 Glaze
The same Cone 4 glaze using two frits was used in evaluating the effects
of ten percent (10%) by weight mill additions of Fe2O3, MnO2 and equal
parts of these two oxides on the lead release in comparison with that for the
base glaze. These glazes were applied on bisque cups from Plant C. These
were fired in five (5) hours to Cone 4 in a gas-fired laboratory kiln. The
results were as given below:
Coloring Oxide
Base Glaze
Fe2O3
MnO2
Fe2O3-MnO2
Oxide Addition
Weight Percent
–
10
10
5-5
Lead PPM
0.10
0.37
0.97
1.49
These coloring oxides caused significant increase in lead release when
compared to that from the base glaze. The mixture of the two oxides showed
greater influence than either of the single oxides for the same total amount of
oxides introduced. It is not obvious why Fe2O3 and/or MnO2 have an adverse
effect on the acid. resistance of a lead frit. The fact that Fe and Mn can each
readily assume different valences might be an important factor in promoting
their mobility in these glazes.
Fe, Mn and Zircon in a Clear, Cone 02-4 Glaze
Base Glaze
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.321 A12O3
0.360 B2O3
3.064 SiO2
Glaze Batch
Frit D
Clay
90
10
Frit D
0.09 K20
0.09 Na2O
0.19 A12O3
2.80 SiO2
44
Lead Glazes for Ceramic Foodware
0.58 CaO
0.24 PbO
An ILMC Handbook
0.36 B2O3
Mill Additions, Weight Percent
BG
–
–
–
Fe2O3
MnO2
ZrO2·SiO2
BG-1
4.5
9.0
0.5
BG-2
9.0
4.5
0.5
BG-3
5.0
5.0
2.0
The above glazes were applied on bisque cups from Plant C and fired in
4.5 hours to Cone 01 and in a second fire to Cone 4 in a gas-fired laboratory
kiln. All of the glazed specimens showed good, bright glazes for both firing
treatments. In general, the appearance of the glazes was slightly better for the
higher cone temperature especially for BG-3. The average values of lead
release from these glazes are given below:
Glaze
BG
BG-1
BG-2
BG-3
Lead Migration, ppm
Cone 01
Cone 4
0.04
0.16
0.81
0.07
0.60
0.10
0.15
These oxide additions show no detrimental effects on lead release of this
type glaze for the normal Cone 4 firing. At the lower Cone 01 firing, the lead
release was measurably greater than that from the base glaze with the
possible exception of the BG-3 modification. This modification had less of
the coloring oxides and more of the zircon additive; the latter would be
expected to increase the acid resistance. The satisfactory behavior of BG-1
and BG-2 at the normal Cone 4 firing is of interest from the standpoint of the
so-called Rockingham glazes.
Cu Fe, Mn and Zircon in a Clear, Cone 02-4 Glaze
The same base glaze was used as above with mill additions of Fe2O3, and
MnO2 to note also the effect of CuO as follows:
Mill Additions, Weight Percent
BG-4
BG-5
45
BG-6
BG-7
Lead Glazes for Ceramic Foodware
Fe2O3
MnO2
CuO
5.0
5.0
–
An ILMC Handbook
5.0
5.0
0.1
5.0
5.0
0.25
5.0
5.0
0.5
These glazes were applied on bisque cups from Plant C and given similar
firings to Cone 01 and Cone 04. All of the glazed specimens were good in
appearance. The average values for lead release were as given below:
Glaze
BG-4
BG-5
BG-6
BG-7
Lead PPM
Cone 01
Cone 4
0.09
0.07
0.06
0.08
0.77
0.09
1.45
0.10
The effect of CuO additions at levels up to 0.5 weight percent were not
detrimental for the normal Cone 4 firing of this type glass, but increased
migration rates were observed for lower temperature firing at Cone 01.
Hence further tests were made using a lower temperature base glaze.
Cu, Cr and Co in a Low Temperature Cone 05 Glaze
The glaze used in the work is the Cone 05 glaze used on high talc-clay
bodies typically found in the artware and pottery industries.
Base Glaze
0.071 K2O
0.113 Na2O
0.455 PbO
0.361 CaF2
0.279 Al2O3
0.675 B2O3
2.726 SiO2
0.035 ZrO2
Glaze Batch
Frit C
Clay
90
10
Frit C
0.071 K2O
0.113 Na2O
0.455 PbO
0.361 CaF2
0.131 Al2O3
0.675 B2O3
2.429 SiO2
0.035 ZrO2
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
The above glaze and modifications of this glaze with various mill
additions of coloring oxides as shown below were applied on bisque cups
from Plant C. Each value for lead release is the average for three cups tested.
The specimens were all fired in 4.5 hours to 1024o C in a gas-fired laboratory
kiln.
Coloring
Oxide
Base Glaze
CuO
CuO
Cr2O3
Cr2O3
Co3O4
Co3O4
Oxide
Addition
Weight
Percent
–
0.1
2.0
1.0
3.0
0.01
0.1
Lead Release
ppm
1.20
3.10
12.00
4.75
3.31
6.90
1.17
The effects of the coloring oxide mill additions on the lead release from
this low temperature glaze (maturing at 1024o C.) were marked. The lead
release increased ten times by the two percent (2%) by weight mill addition
of CuO over that for the base glaze.
This glaze was designed for the talc-clay type of artware and tile bodies. It
is not used for dinnerware bodies. However, it was selected for this
investigation because its maturing temperature is near the lower end of the
temperature range under study. Furthermore, the coloring oxides are
normally added as part of prepared stain compositions.
Copper-containing stains have had use also in artware glazes. However,
they have not been used commercially for colored dinnerware glazes during
the past several decades and were used only to a limited extent in earlier
years. The industry has been aware of problems associated with such glazes
for many years.
Effect of Commercial Stains on Lead Release
In this work segment commercial glaze stains were added to an
appropriate clear base glazes and the effect of these additions on lead
migration were assessed. These studies represented a wide range of
47
Lead Glazes for Ceramic Foodware
An ILMC Handbook
commonly used, typical commercial stains introduced in appropriate
concentrations in compatible base glazes. Tests were conducted on these as
described below:
Pb-Sb Yellow Stain In Cone 08-04 Glaze
Base Glaze
0.071 K2O
0.113 Na2O
0.455 PbO
0.361 CaF2
0.279 A12O3
0.675 B2O3
2. 726 SiO2
0.035 ZrO2
Glaze Batch
(Including Stain Addition)
Frit C
Clay
Pb-Sb Yellow
Stain, (GS313)
90
10
5
Frit C
0.071 K2O
0.113 Na2O
0.455 PbO
0.361 CaF2
0.131 A12O3
0.675 B2O3
2.429 SiO2
0.035 ZrO2
The above base glaze and the same glaze with a five percent (5%) by
weight mill addition of the commercial Pb-Sb Yellow Stain were applied on
bisque cups from Plant C and fired to Cone 06 down. The average lead
release from three cup specimens of each were as follows:
Lead PPM
Base Glaze 0.11
Base Glaze + Five Percent (5%) Pb-Sb Stain 0.04
The Pb-Sb stain addition did not affect the lead release from the base
glaze adversely.
48
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Cr-Al Pink Stain in Cone -06-02 Glaze
Base Glaze
0.01
0.13
0.28
0.58
K2O
Na2O
ZnO
PbO
0.296 A1203
0.210 B203
2.241 SiO2
0.192 ZrO2
Glaze Batch
(Including Stain Addition)
Frit F
Clay
Zirconium
Silicate
Alumina
Cr-Al Pink Stain
(GS-515)
85
10
10
5
5
Frit F
0.01 K2O
0.13 Na2O
0.28 ZnO
0.58 PbO
0.14 Al2O3
0.21 B2O3
1.78 SiO2
The above base glaze and the same glaze with a five percent (5%) by
weight mill addition of the commercial Cr-Al pink stain were applied on
bisque cups from Plant C and fired in 4 hours to Cone 06 down.
Lead PPM
Base Glaze 1.52
Base Glaze + Five Percent (5%) Cr-Al Stain 0.62
The lead release from the above, low temperature fritted lead glaze was
markedly reduced by the five percent (5%) by weight mill addition of the
commercial Cr-Al pink stain.
49
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Sn-Sb Gray Stain in Cone 02-4 Glaze
Base Glaze
0.09 K20
0.09 Na2O
0.58 CaO
0.24 PbO
0.326 A12O3
0.360 B203
3.269 Si02
0.196 ZrO2
0.072 SnO2
Glaze Batch
(Including Stain Addition)
Frit D
Clay
Zirconium
Silicate
Tin Oxide
Sn-Sb Gray
Stain (GS-868)
87
10
10
3
5
Frit D
0.09 K2O
0.09 Na2O
2.80 SiO2
0.24 PbO
0.19 Al2O3
0.58 CaO
0.36 B2O3
The base glaze and the same glaze with a five percent (57c) by weight
mill addition of the commercial Sn-Sb gray stain were applied on bisque cups
from Plant C. The glazed cups were fired in a gas-fired laboratory kiln in four
(4) hours to Cone 1 down. The average lead release for three cups of each is
given below:
Lead PPM
Base Glaze 0.03
Base Glaze + Five Percent (5%) Sn-Sb Stain 0.05
50
Lead Glazes for Ceramic Foodware
An ILMC Handbook
The Sn-Sb stain mill addition did not affect the lead release from the
above base glaze adversely. The difference between 3 and 5 ppm is not
statistically significant at the 95% confidence level.
Co-Cr-Fe Black Stain in Cone 1 Glaze
Base Glaze
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.321 Al2O3
0.360 B2O3
3.064 SiO2
Glaze Batch
(Including Stain)
Frit D
Clay
Co-Cr-Fe Black
Stain (GS-815)
90
10
8
Frit D
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.19 Al2O3
0.36 B2O3
2.80 SiO2
The above base glaze and the same glaze with an eight percent (8%) by
weight mill addition of the commercial Co-Cr-Fe black stain were applied on
bisque cups from Plant C. These specimens were fired in a gas-fired
laboratory kiln in four (4) hours to Cone 1 down. The lead release data are as
follows.
Base Glaze
Base Glaze + Eight Percent
(8%) Co-Cr-Fe Stain
Lead PPM
0.02
0.03
The mill addition of the Co-Cr-Fe stain to the base glaze did not result in
any significant increase in lead release.
51
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Cr-Al Pink Stain in Cone 02-4 Glaze
Base Glaze
0.07 K2O
0.18 Na2O
0.29 CaO
0.32 ZnO
0.14 PbO
0.338 A12O3
0.260 B2O3
2.856 SiO2
0.177 ZrO2
Glaze Batch
(Including Stain Addition)
Frit E
Clay
Zirconium
Silicate
Alumina
Cr-Al Pink Stain
(GS-515)
85
10
10
5
5
Frit E
0.07 K2O
0.18 Na2O
0.29 CaO
0.32 ZnO
0.14 PbO
0.20 A12O3
0.26 B2O3
2.43 SiO2
The above base glaze and the same glaze with a five percent by weight
mill addition of the Cr-Al pink stain were applied on bisque cups from Plant
C. These were fired in a gas-fired laboratory kiln to Cone 1 in four hours.
Lead PPM
Base Glaze 0.03
Base Glaze + Five Percent (5%) Cr-Al Stain 0.05
The Cr-Al stain mill addition did not affect the lead release from the base
glaze adversely.
52
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Various Commercial Stains in a Cone 02-4 Glaze
Base Glaze
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.321 A1203
0.360 B203
3.253 SiO2
0.190 ZrO2
Glaze Batch
(Including Various Stains)
Frit D
Clay
Zirconium
Silicate
Stain
90
10
10
5
Frit D
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.19 Al2O3
0.36 B2O3
2.80 SiO2
The stains as listed in the table below were each used in five percent (5%)
amounts as mill additions to the base glaze. These glazes were applied on
bisque cups from Plant C. The test cups were fired in four (4) hours to Cone
1 down in a gas-fired laboratory kiln. The lead release data for the base glaze
and the various modifications are given below:
Base Glaze
+ 5% Co-Al Blue (GS-1)
+ 5% Co-Cr Green (GS-100)
+ 5% Cu Turquoise (GS104)
+ 5% V-Zr Blue (GS-119)
+ 5% Sn-V Yellow (GS-309)
+ 5% Pr-Zr Yellow (GS322)
53
Lead PPM
0.11
0.03
0.09
0.05
0.05
0.03
0.04
Lead Glazes for Ceramic Foodware
An ILMC Handbook
+ 5% V-Zr Yellow (GS-328)
+ 5% Cr-Sn Maroon (GS404)
+ 5% Cr-Sn Pink (GS-510)
+ 5% Fe-Zr Pink (GS-521)
+ 5% Cr-Fe-Zr Brown (GS607)
+ 5% Zr Gray (GS-874)
0.04
0.01
0.04
0.03
0.03
0.04
None of the various commercial stain mill additions to the base glaze
effected the lead release of the modified glazes adversely. This is of interest
since these stains represented those most commonly used at present.
Effect of Opacifying Oxides on Lead Release
The objective of this study segment was to explore the effects of
commonly used opacifying oxides in different concentrations as mill
additions on the lead release of the glazes.
Standard Glaze Fired to Cone 4 and 01.
Base Glaze
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.321 A12O3
0.360 B2O3
3.064 SiO2
Glaze Batch
(Not including opacifiers)
Frit D
90%
Clay 10
Frit D
0.09
0.09
0.58
0.24
K2O
Na2O
CaO
PbO
0.19 A12O3
0.36 B2O3
2.80 SiO2
54
Lead Glazes for Ceramic Foodware
An ILMC Handbook
The base glaze and various modifications to include opacifying oxide mill
additions are given below along with the lead solubility data. The glazes were
applied on bisque cups from Plant C and glost-fired on a 4.5 hour schedule to
Cone 4 and a second firing to Cone 01 in a gas-fired laboratory kiln.
Opacifying
Oxide
Base Glaze
SnO2
SnO2
ZrO2·SiO2
ZrO2·SiO2
ZrO2·SiO2
TiO2
TiO2
TiO2
Weight
Percent Mill
Addition
5
10
5
10
15
0.5
2.5
5.0
Lead PPM
Cone 4
Cone 01
0.07
0.14
0.05
0.12
0.08
0.10
0.02
0.13
0.03
0.13
0.09
0.17
0.10
0.13
0.21
0.16
0.04
0.08
The glazes were all good in appearance with the exception of the
yellowish coloration of the glazes with the higher levels of TiO2. The
yellowish coloration was greater at the lower cone temperature. In all cases
and at both cone temperatures the opacity increased (as would be expected)
with greater amounts of any of the three opacifiers.
The lead release values were in all cases too low to draw any conclusions
in respect to type and amount of opacifying oxide mill addition. However, it
can be concluded that these additions to the base glaze do not affect the lead
solubility of the base glaze adversely. They may in cases of base glazes
having higher lead release prove beneficial.
High Lead Glaze Fired to Cone 4 and 01.
A second study at the cone 01 and cone 4 firing levels examined the effect
of the opacifying oxides in a higher PbO, softer glaze as given below.
Base Glaze
0.071 K2O
0.113 Na2O
0.455 PbO
0.279 Al2O3
0.675 B2O3
2.726 SiO2
0.035 ZrO2
55
Lead Glazes for Ceramic Foodware
An ILMC Handbook
0.361CaF2
This lower temperature glaze contained 0.035 ZrO2 as part of the frit. The
glaze batch included ninety percent (90%) of the above glaze as Frit C and a
ten percent (10%) clay mill addition. The additions of opacifiers in kind and
amount were the same as used in Example 1 and are shown below along with
the lead release data.
The glazes were applied on bisque cups from Plant C. These specimens
were fired to Cone 01 and Cone 4. The glazes were fairly good in
appearance, showed the same general trends in respect to opacification and
the yellowing of the higher levels of TiO2 additions. Results are given in the
following table.
Opacifying
Oxide
Base Glaze
SnO2
SnO2
ZrO2·SiO2
ZrO2·SiO2
ZrO2·SiO2
TiO2
TiO2
TiO2
Weight Percent
Mill Addition
(Contained
0.035 ZrO2 in
Frit.
No mill addition
other than clay)
5
10
5
10
15
0.5
2.5
5.0
Lead PPM
Cone 01
Cone 4
0.14
0.13
0.24
0.10
0.09
0.13
0. 08
0.14
0.23
0.05
0.20
0.11
0.03
0.16
0.12
0.11
0.09
Again the lead release values were too low to form any conclusions in
respect to the kind and amount of opacifier additions other than they do not
affect the lead release of the base glaze adversely.
Effect of Variations in Alkali, Alkaline Earth, Boric and Zinc Oxides and
Beryl in a Cone 4 Glaze
Base Glaze
0.09 K2O
56
Lead Glazes for Ceramic Foodware
0.09 Na2O
0.58 CaO
0.24 PbO
0.321 Al2O3
0.360 B2O3
An ILMC Handbook
3.064 SiO2
Glaze Batch
Frit D
Clay
90
10
In all cases the glazes were applied on bisque cups from Plant C and fired
to both Cone 1 and Cone 4 on a 4.5 hour heating schedule.
Effect of Alkali Oxides
The alkali variation consisted of altering the 0.18 molecular equivalents of
alkali oxide in the base glaze, with the remaining constituents held constant:
0.58 CaO
0. 19 Al203
0.24 PbO
0. 36 B203
2.80 SiO2
The frits prepared had the following alkali oxide content:
Molecular Equivalents
Frit
A
B
C
D
E
F
Glaze Using
Frit
A
B
C
K20
0.18
–
–
0.09
–
0.06
Na2O
–
0.18
–
–
0.09
0.06
Li2O
–
–
0.18
0.09
0.09
0.06
Cone 01
Lead release,
ppm
Cone 4
0.35
0.22
0.09
0.12
0.10
0.08
57
Lead Glazes for Ceramic Foodware
D
E
F
An ILMC Handbook
0.10
0.07
0.05
0.16
0.11
0.08
All of these glazes were good in appearance at both cone temperatures.
These alkali oxide modifications in this typical Cone 4 lead glaze do not
show any significant differences in lead release. The values for glazes A and
B were somewhat higher when fired four cones lower at Cone 01. The acid
resistance of this glaze and its various modifications is very good.
Effect of Alkaline Earth Oxides
Various alkaline earth oxides were substituted in part for the 0.58
molecular equivalents of CaO in the base glaze. The frits were made with the
following portion of the base glaze held constant:
0.09 K2O
0.09 Na2O
0.24 PbO
0.19 A12O3
0.36 B2O3
2.80 SiO2
The frits prepared had the following alkaline earth oxide content:
Frit
G
H
I
J
K
L
M
N
O
P
Glaze Using
Frit
G
CaO
0.48
0.38
0.48
0.38
0.48
0.38
0.38
0.38
0.38
0.37
Molecular Equivalents
MgO
BaO
0.01
–
0.20
–
–
0.01
–
0.20
–
–
–
–
0.10
0.10
0.10
–
–
0.10
0.07
0.07
Cone 01
0.17
58
SrO
–
–
–
–
0.01
0.20
–
0.10
0.10
0.07
Lead Release, ppm
Cone 4
0.22
Lead Glazes for Ceramic Foodware
An ILMC Handbook
H
I
J
K
L
M
N
O
P
0.06
0.08
0.04
0.04
0.01
0.14
0.06
0.09
0.06
0.10
0.15
0.11
0.12
0.13
0.10
0.12
0.08
0.08
All of the glazes were good in appearance for both firing treatments.
The lead release in all cases was very low so it is difficult to draw
conclusions on the relative effects of these different combinations of alkaline
earth oxides in this typical Cone 4 glaze.
Effect of Boric Oxide
The same base glaze was used as above with three levels of B203 as
follows: 0.18 eq., 0.36 eq., and 0.54 eq. The intermediate level was the
normal level for this glaze. The constant portion for the three glazes, made
from ninety percent frit (90%) and ten percent (10%) clay, was as follows:
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.321 Al2O3
3.064 Si02
The glazes were applied on bisque cups from Plant D and fired in five (5)
hours to Cone 46.
The results were as follows:
Glaze
Eq. B203
In Frit
B-1
0.18
B-2
0.36
Lead
Release,
ppm
0.0400.0430.019
0.0850.08259
Average
0.034
0.080
Lead Glazes for Ceramic Foodware
B-3
An ILMC Handbook
0.072
0.0920.0690.236
0.54
0.132
The average values show an increase in lead release with increasing B2O3
in the glaze frit. This is consistent with earlier findings.
Effect of Zinc Oxide
Three levels of ZnO were used in the same base glaze as follows: 0.00 eq.
, 0.05 eq. and 0.10 eq. ZnO. The empirical molecular formulas for the three
glazes, prepared from ninety percent (90%) frit and ten percent (10%) clay,
were as follows:
ZnO
K2O
Na2O
CaO
PbO
A12O3
B2O3
SiO2
Z-1
0.00
0.09
0.09
0.58
0.24
0.32
0.36
3.06
Z-2
0.05
0.085
0.085
0.55
0.23
0.32
0.36
3.06
Z-3
0.10
0.08
0.08
0.52
0.22
0.32
0.36
3.06
The glazes were applied on bisque cups from Plant D and fired on a 4.5
hour heating schedule to Cone 26. The data were as follows:
Glaze
Eq. ZnO
In Frit
Z-1
0.00
Z-2
0.05
Z-3
0.10
Lead
Release
PPM
0.05-0.010.04
0.08-0.010.03
0.08-0.070.04
Average
0.03
0.04
0.06
All glazes were good, bright, glossy glazes. There appeared to be an
increase in lead release with increase in ZnO, however, these values are very
60
Lead Glazes for Ceramic Foodware
An ILMC Handbook
small. It would be better to state that ZnO, within the concentration range in
which it would be used, has little or no adverse effects on lead release.
Two additional glazes were prepared introducing 0.10 eq. and 0.15 eq. of
ZnO in the frit at the expense of the other R2O/RO members other than PbO.
The latter was held constant at 0. 24 eq. as in the base glaze.
Z-4
0.10
0.08
0.08
0.50
0.24
0.19
0.36
2.80
ZnO
K2O
Na2O
CaO
PbO
A12O3
B2O3
SiO2
Z-5
0.15
0.07
0.07
0.47
0.24
0.19
0.36
2.80
These glazes, fired to Cone 4, showed average lead release values of 0.13
and 0.11 ppm respectively. Again the addition of these two levels of ZnO did
not affect the lead solubility of the base glaze adversely.
Effect of Beryl
Three glazes were prepared, using ninety percent (9O%) frit and ten
percent (10%) clay, in which the frit contained 0.00 eq., 0.05 eq. and 0.10 eq.
of BeO respectively. The empirical molecular compositions of these glazes
were:
BeO
K2O
Na2O
CaO
PbO
A12O3
B2O3
SiO2
BE-1
0.00
0.09
0.09
0.58
0.24
0.32
0.36
3.06
BE-2
0.05
0.085
0.085
0.55
0.23
0.32
0.36
3.06
61
BE-3
0.10
0.08
0.08
0.52
0.22
0.32
0.36
3.06
Lead Glazes for Ceramic Foodware
An ILMC Handbook
These glazes were applied on bisque cups from Plant D and fired on a 4.5
hour heating schedule to Cone 2. The lead release data were:
Glaze
Eq. BeO
In Frit
BE-1
0.00
BE-2
0.05
BE-3
0.10
Lead
Release
PPM
0.05-0.010.04
0.12-0.150.14
0.02-0.010.02
Average
0.03
0.14
0.02
All three glazes were clear, bright, and glossy glazes when fired at Cone
2.
Effect of Base Glaze Variations: Lead Silicate (PbO·1.3SiO2)
This is a typical base glaze as was used formerly for tangerine or uranium
red glazes. Various modifications were made of the lead silicate base glaze
(PbO·1.3SiO2) as follows:
(a) Introduction of 0.1 molecular equivalent alkali oxides at the expense of
PbO, simply as Na2O, K2O and Li2O, as combinations of two of these
oxides, and as all three alkali oxides.
(b) Introduction of 0.1 molecular equivalent alkaline earth oxides at the
expense of PbO, singly as CaO, BaO, MgO and SrO, as various
combinations of two and three of these oxides, and as all four oxides.
(c) Introduction of opacifying oxides, TiO2, ZrO2 and SnO2 in small amounts.
All of the compositions discussed below and designated as glazes 1 to 29
were fritted entirely and water-quenched. The frits were poured at circa 1065o
C. after 1.3 to 1.5 hours smelting periods. Slips with satisfactory spraying
consistency were obtained by ball milling to pass a 200 mesh [75 mm] sieve
with additions of methocel. The glazes were sprayed on bisque cups from
Plant C. The specimens were glost-fired in a gas-fired laboratory kiln on a
4.5 hour heating schedule to Cone 07.
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Lead Glazes for Ceramic Foodware
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Additions of Alkali Oxides to Base Glaze
Glaze
1
2
3
4
5
6
7
8
Composition of Glazes, Molecular Equivalents
Li2O
Na2O
K20
PbO
1.0
0.1
0.9
0.1
0.9
0.1
0.9
0.05
0.05
0.9
0.033
0.033
0.033
0.9
0.05
0.05
0.9
0.05
0.05
0.9
Glaze
1
2
3
4
5
6
7
8
% Melted Oxide Composition
Na2O
K20
2.17
3.27
1.09
0.72
1.09
1.65
1.08
1.64
Li2O
1.06
0.53
0.35
0.52
-
PbO
72.01
71.25
70.45
69.66
70.85
70.46
70.45
70.05
SiO2
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
SiO2
27.99
27.69
27.38
27.07
27.53
27.38
27.38
27.23
In the case of all specimens, the fired glazes were good in appearance.
The lead release data are given below:
Glaze
1
2
3
4
5
6
7
Lead PPM
Individual Data Average
0.34 - 0.47 - 0.41
0.41
3.54 - 0.56 - 0.50
1.53
4.30 - 4.45
4.38
4.10 - 4.24 - 6.80
5.05
1.04 - 0.90 - 0.72
0.89
0.98 - 0.38 - 0.78
0.71
1.76 - 1.08 - 2.16
1.67
63
Lead Glazes for Ceramic Foodware
8
An ILMC Handbook
1.80 - 1.54 - 1.90
1.75
For a constant alkali level of 0.1 molecular equivalent in this base lead
silicate glaze, it is seen that all three single alkali oxide additions increase the
lead release over that for the base lead silicate glaze with Li2O increasing it
least, Na2O being intermediate and K2O increasing the lead release most. The
effects of these alkali oxide additions on the structure and lead release should
depend on their radii and field strengths and also on the sizes of the
interstices that exist in the glass structure. The data show increasing lead
release with increasing ionic radii of the alkali oxides (Li2O-0.60, Na2O-0.95
and K2O-1.33).
Consistent with earlier findings, it is seen that the mixture of any two of
the three alkali oxides totaling 0.1 molecular equivalent resulted in lower
lead release values than the same amount of any of the single oxide additions.
The two binary additions containing K2O also showed higher lead release
than the binary of the other two oxides. The addition of the three alkali
oxides gave the lowest average lead release value for a given total alkali
oxide addition to such a base glaze, the use of all three alkali oxides would be
preferred over any two in respect to lead release.
Additions of Alkaline Earth Oxides to Base Glaze
The modifications of the base glaze by additions of alkaline earth oxides
are shown below for glazes 9 through 23 inclusive:
Composition of Glaze, Molecular Equivalents
Glaze
9
10
11
12
13
14
15
16
17
18
MgO
0.1
0.05
0.05
0.05
0.033
0.033
0.033
CaO
0.1
0.05
0.033
0.033
-
BaO
0.1
0.05
0.033
0.033
64
SrO
0.1
0.05
0.033
0.033
PbO
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
SiO2
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
Lead Glazes for Ceramic Foodware
19
20
21
22
23
0.025
-
0.033
0.025
0.05
0.05
-
An ILMC Handbook
0.033
0.025
0.05
0.05
0.033
0.025
0.05
0.05
0.9
0.9
0.9
0.9
0.9
1.3
1.3
1.3
1.3
1.3
% Melted Oxide Composition
Glaze
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
MgO
1.42
0.71
0.71
0.70
0.46
0.46
0.46
0.35
-
CaO
1.87
0.99
0.65
0.65
0.64
0.49
0.97
0.97
-
BaO
5.21
2.65
1.76
1.75
1.75
1.33
2.65
2.63
SrO
3.58
1.81
1.20
1.19
1.18
0.90
1.81
1.78
PbO
70.99
70.59
69.43
68.26
70.79
70.20
69.60
69.95
70.35
69.56
69.44
69.80
69.41
70.10
68.84
SiO2
27.59
27.44
26.99
26.53
27.51
27.23
27.05
27.18
27.34
27.04
26.99
27.33
26.97
27.12
26.75
Again the compositions were entirely fritted. The glazes were sprayed on
cups from Plant C and were fired in a gas-fired laboratory kiln on a 4.5 hour
heating schedule to Cone 076. All of these glazes were good in appearance.
The lead release data were:
Lead PPM
Glaze
9
10
11
12
Individual Data
2.86 - 3.08 - 1.86
0.84 - 0.94 - 1.08
3.85 - 1.45 - 7.55
5.60 - 5.40 - 6.70
65
Average
2.60
0.95
4.28
5.90
Lead Glazes for Ceramic Foodware
13
14
15
16
17
18
19
20
21
22
23
An ILMC Handbook
0.90 - 0.30
0.40 - 0.62
2.72 - 2.46
0.39 - 0.85
1.68 - 0.80
2.03 - 1.60
2.14 - 1.26
2.24 - 1.40
1.32 - 1.76
0.67 - 0.82
0.36 - 0.66
- 0.45
- 0.51
- 3.36
- 0.59
- 1.04
- 1.56
- 1.32
- 1.80
- 2.42
- 0.94
- 2.20
0.72
0.51
2.85
0.78
1.17
1.75
1.57
1.81
1.83
0.81
1.07
The alkaline earth oxide additions gave similar trends to the alkali oxide
additions in respect to lead release. With 0.1 molar (molecular) equivalent
additions of single oxides show an increase in lead release in the order: MgO,
SrO, BaO or with increasing oxide ionic radii. The value for the 0.1 molar
addition of CaO was anomalous in this experiment.
Additions of alkaline earths in equivalent molar pairs of MgO-CaO,
MgO-SrO, CaO-SrO, CaO-BaO and SrO-BaO totaling 0.1 molar equivalent
show a lead release below that of MgO above with MgO-BaO slightly above
that of Mg0 above.
Additions of alkaline earths in equivalent molar triplets totaling 0.1 molar
equivalents again show lead release values falling below that of MgO along,
but above that of MgO-CaO and MgO-SrO.
Additions of equi-molar equivalent quadruple of the four alkaline earths
totaling 0.1 molar equivalents falls below that of MgO alone, but above the
values of the alkaline earth triplets.
From these data, it may be inferred that alkaline earth lead silicate glasses
of best chemical durability with respect to lead release are to be obtained
with oxide additions of MgO-CaO or MgO-SrO and triplet additions of
MgO-CaO-SrO, that is, combinations of alkaline earth oxides having the
smaller ionic radii.
66
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Additions of Opacifying Oxides
Composition of Glaze, Molecular Equivalents
Glaze
24
25
26
27
28
29
PbO
1.0
1.0
1.0
1.0
1.0
1.0
SiO2
1.3
1.3
1.3
1.3
1.3
1.3
TiO2
0.03
0.06
-
ZrO2
0.03
0.06
-
SnO2
0.03
0.06
% Melted Oxide Composition
Glaze
24
25
26
27
28
29
PbO
73.49
72.91
73.18
71.88
72.98
71.91
TiO2
0.79
1.57
-
SiO2
25.72
25.52
25.02
24.58
25.56
25.17
ZrO2
1.80
3.54
-
SnO2
1.46
2.92
The glazes were prepared as frits and applied on bisque cups from Plant
C. The specimens were glost-fired in a gas-fired laboratory kiln on a 4.5 hour
heating schedule to Cone 07. The glazes were good in appearance.
The lead release data were:
Glaze
24
25
26
27
28
29
Individual Data
0.45 - 0.47 - 0.61
0.39 - 0.47 - 0.51
0.17 - 0.30 - 0.38
0.30 - 0.23 - 0.32
0.67 - 0.68 - 0.19
0.29 - 0.19 - 0.49
Average
0.51
0.46
0.28
0.28
0.51
0.32
These data are to be compared with the value for the base glaze of 0.41
PPM lead release (similarly applied and fired). The ZrO2 additions (both
levels) gave a somewhat lower lead release. Ions of high charge and small
67
Lead Glazes for Ceramic Foodware
An ILMC Handbook
size introduced as their oxides, e.g., TiO2 or ZrO2 may cause breakage of
linkages. These cations of high field strength, when they fit into the
interstices may cause a strong contraction of the unsaturated oxygen ions
surrounding them and lead to a tight binding and an overall strengthening of
the structure.
Effect of Base Glaze Variations: Lead Silicate (PbO·1.5SiO2)
Latin Square Experiment
A Latin square experiment was used to show effects of three levels of
additions of Al2O3, B2O3 and ZrO2 to a base glaze of PbO·1.5SiO2. The
levels of the additions selected were as given below:
Levels
0
1
2
Molar Equivalents
(Al2O3) (B2O3) (ZrO2)
A
B
Z
0.00
0.00
0.00
0.15
0.15
0.02
0.30
0.30
0.04
The molar formulas for the nine selected members are as follows:
A0B0Z0
A0B1Z1
A1B2Z2
A1B0Z2
AlB1Z2
A1B2Z0
A2B0Z2
A2B1Z0
A2B2Z1
PbO
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
SiO2
1.50
1.50
1.50
1.50
1.50
1.50
1.50
1.50
1.50
A1203
0.00
0.00
0.00
0.15
0.15
0.15
0.30
0.30
0.30
B203
0.00
0.15
0.30
0.00
0.15
0.30
0.00
0.15
0.30
ZrO2
0.00
0.02
0.04
0.02
0.04
0.00
0.04
0.00
0.02
The compositions were fritted entirely with the exception of 0.15
equivalents of Al203 introduced as clay for the Al and A2 members. The A0
glazes, containing no A12O3 addition to the batch, were suspended using a
gum-Methocel solution. The ZrO2 was introduced into the frit batches as
milled zircon (ZrO2·SiO2). The frit batches were mixed thoroughly, then dripfritted (through a small orifice in the crucible of 1/8 inch diameter) at
68
Lead Glazes for Ceramic Foodware
An ILMC Handbook
temperatures of 1050o - 1165o C. The glaze batches were ball milled to pass
200 mesh. The glazes were applied on bisque cups from Plant C. The
specimens were fired in a gas-fired laboratory kiln on a 4. 5 hour heating
schedule to Cone 03. The glazes were all good, bright glazes with the
exception of A2B0Z2 that was poorer in appearance. This glaze had high
levels of additions of Al2O3 and ZrO2 and no B2O3.
The lead release data for the nine members, using the standard test, were
as follows:
A0B0Z0
A0B1Z1
A1B2Z2
A1B0Z2
AlB1Z2
A1B2Z0
A2B0Z2
A2B1Z0
A2B2Z1
Individual Data
0.23 - 0.19 - 0.17
0.82 - 0.75 - 0.95
1.34 - 0.99 - 0.61
0.43 - 0.20 - 0.25
0.22 - 0.28 - 0.13
0.20 - 0.17 - 0. 24
0.31 - 0.25 - 0.26
0.32 - 0.33 - 0.35
0.33 - 0.25 - 0.22
Average
0.20
0.84
0.93
0.29
0.21
0.20
0.27
0.33
0.27
The individual data, with one exception, showed reasonably good
agreement.
Knoop Hardness
Knoop hardness measurements were made on these glazes using a 1000
gram load. The objective was to relate this property with the strength of the
structure and the R and P factors. These factors are calculated as follows:
R=
Sum Of Oxygens In Molecular Formula
Sum Of Si, Al and B
P=
Sum Of Oxygens In Molecular Formula
Sum Of Network Modifiers
R is a measure of the number of single-bonded oxygen. P is a measure of
the oxygens available per network-modifying ion. The R values are less than
2.5 and the P values are greater than 3.9 for normal glasses. In general, the
structure becomes stronger and more resistant to acid attack as R is reduced.
B2O3-containing glazes differ from aluminosilicate glazes. Al3+ always has 4
69
Lead Glazes for Ceramic Foodware
An ILMC Handbook
coordination, while B3+ may have 3 or 4 coordination. It has been suggested
that in normal glazes all the B2O3 and A12O3 accept oxygen from the
modifier oxides, the Al2O3 forming A1O4 tetrahedra, and the B2O3 broken BO-B bonds at the glaze-maturing temperature. When the glaze is cooled
slowly it is likely that BO4 tetrahedra form. Glazes with very high B2O3
content must contain BO3 triangles since there is insufficient oxygen brought
in by the modifiers to break the B-O-B bonds; these glazes thus may have
poor durability. Although increasing B3+ ions will reduce the R factor as
rapidly as Al3+ ions, the resultant structure will not he as strong as if A12O3
had been added instead of B2O3; in fact the addition of B2O3 most probably
will give a structure weaker than the parent lead silicate glaze and thus less
durable.
These structural factors as calculated for the nine glazes are given in the
following table along with average values for Knoop hardness and lead
release (all 27 combinations are listed with data taken only on the nine
selected members tested):
A0B0Z0
A0B0Z1
A0B0Z2
A0B1Z0
A0B1Z1
A0B1Z2
A0B2Z0
A0B2Z1
A0B2Z2
A1B0Z0
A1B0Z1
A1B0Z2
A1B1Z0
A1B1Z1
A1B1Z2
A1B2Z0
A1B2Z1
R
Factor
2.67
P Factor
KHN1000
4-.00
389
Lead
PPM
0.20
2.49
4.40
417
0.84
2.37
4.79
406
0.98
2.49
4.40
417
0.29
2.37
2.31
4.79
5.35
419
41.4
0.21
0.20
70
Lead Glazes for Ceramic Foodware
A1B2Z2
A2B0Z0
A2B0Z1
A2B0Z2
A2B1Z0
A2B1Z1
A2B1Z2
A2B2Z0
A2B2Z1
A2B2Z2
An ILMC Handbook
2.37
2.31
4.79
5.35
413
424
0.27
0.33
2.16
5.72
415
0.27
The Latin square design permits extracting information from a minimum
number of samples but it assumed no interactions between the variables
being studied. This approach can be most helpful in determining direction of
future studies and elimination of unnecessary testing.
The average values for the different levels of the three oxides in the nine
members were:
A0
A1
A2
B0
B1
B2
Z0
Z1
Z2
Lead PPM
0.67
0.23
0.29
0.25
0.46
0.48
0.24
0.47
0.49
KHN1000
404
417
417
406
420
412
409
416
413
R Factor
2.51
2.39
2.28
2.51
2.39
2.28
2.43
2.38
2.37
P Factor
4.40
4.85
5.29
4.40
4.85
5.29
4.90
4.84
4.79
The data would suggest that A1B2Z0 (not tested) member would be the
most acid-resistant of the 27 members. Higher levels of Al2O3 definitely
reduce the lead release whereas increasing B2O3 levels increase it. For this
type of glaze and the relatively low firing temperature, ZrO2 does not appear
to improve the acid resistance.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Analysis of variance for the individual lead release data shows the
variance in respect to the A levels to be highly significant whereas variance
in respect to the B and Z levels are significant. In contrast the levels of A, B
and Z are insignificant as a source of variance for the individual hardness
data. Therefore, only the lead release data are considered in respect to the
structural factors.
The R factors decrease with higher levels of A and B and also for Z (to a
much lesser extent). The P factors increase markedly for higher levels of A
and B and decrease slightly for higher levels of A. The lead release data
confirm what might be expected for the A12O3 and B2O3 additions. It would
be of interest to try slower cooling of the glazes containing higher levels of
B2O3 to form more BO4 tetrahedra and possibly attain greater strength and
durability. Firing the same glazes to Cone 1 or 2 might also be informative in
respect to the performance of ZrO2. Also other experiments might be
designed with greater compositional variation to attain larger differences in
the measured properties as well as the calculated structural factors.
Effects of Al2O3, B2O3 and ZrO2
The same Latin square experiment, designed to show the effects of three
levels of additions of A12O3, B2O3 and ZrO2 to a base glaze of PbO·1.5Si02,
was repeated except that a higher glost firing temperature was used. Again
the glazes were applied on cups from Plant C. (The previous firing was to
Cone 036). The specimens in this experiment were fired in a gas-fired
laboratory kiln on a 4.5 hour heating schedule to Cone 16. The glazes were all
good, bright, glossy glazes in appearance.
The lead release data for the nine members, using the standard test, were
as follows.
A0B0Z0
A0B1Z1
A0B2Z2
A1B0Z1
A1B1Z2
A1B2Z0
Lead PPM
Individual Data
0.13 - 0.20 - 0.25
0.59 - 0.29 - 0.40
0.08 - 0.06 - 0.07
0.18 - 0.11 - 0.19
0.18 - 0.19 - 0.25
0.09 - 0.07 - 0.08
72
Average
0.21
0.43
0.07
0.16
0.21
0.08
Lead Glazes for Ceramic Foodware
A2B0Z2
A2B1Z0
A2B2Z1
An ILMC Handbook
0.27 - 0.27 - 0.23
0.54 - 0.60 - 0.45
0.18 - 0.31 - 0.17
0.26
0.53
0.22
The individual data showed fairly good agreement. The average values for
the different levels of the three oxides in the nine members were:
Lead PPM
A0 - 0.23
B0 - 0.21
Z0 - 0.27
Al - 0.15
B1 - 0.39
Z1 - 0.27
A2 - 0.34
B2 - 0.12
Z2 - 0.18
Thus, of the 27 possible combinations of A0A1A2, B0B1B2, Z0Z1Z2, the
above data indicate that A1B2Z2would be the most acid resistant although this
combination was not one of the glazes tested. Of the glazes tested, A0B2Z2
appeared to be the most acid resistant. These two compositions would seem
to indicate that for the higher firing temperatures, the low level of Al2O3 and
the high levels of B2O3 and ZrO2 are favorable from the standpoint of acid
resistance. Analysis of variance of these lead release data again indicates that
changes in the levels of A12O3, B2O3 and ZrO2 all have significant effects on
the resulting lead release of the glazes.
The effects of the oxides are not in agreement for the two firing
temperatures and in part with what might have been expected from structural
considerations. The Latin square designed experiment assumes no
interactions in the glaze system, even though these several compositions had
been tested independently instead of as thin glaze applications on a body. The
effect of body solution from this corrosive lead silicate glaze, especially at
the higher temperatures, is also not considered in this analysis. These are
most likely the principal reasons for the lack in agreement on the effects of
the levels of the oxides at the two firing temperatures.
If the lead silicate base glaze and the modifications as used here had been
tested as fine powders, relatively high lead release values would be expected.
It is important to note that these compositions applied as glazes and fired at
relatively high temperatures (for the compositions involved) gave very low
lead release values, when fired both at Cone 03 and Cone 1.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Repeated Extractions on the Same Glaze Surface
Depletion tests involving the repeated testing of the same cup specimen is
pertinent from the standpoint of continued service.
A cup from Plant B, which was glazed with a clear, Cone 4-5 glaze, was
so tested. This was selected since this glaze is typical of the type used widely
in the dinnerware industry and represents the great bulk of all dinnerware
glaze surfaces. Using the standard test, the lead release data for repeated
testing of the same cup were as follows:
Repeated Tests on Clear, Cone 4-5 Glaze
Test Number
1
2
3
4
5
6
7
8
Lead PPM
0.10
0.09
0.10
0.09
0.05
0.06
0.05
0.06
The lead release values are all very low; the difference between the high
and low value is 5 ppm. Repeated leaching of the glaze appears to give
decreasing lead release as the lead is depleted from the surface.
Repeated Tests on Low Temperature Cone 05 Glaze
This glaze was selected since it showed higher lead release due to the
presence of a two weight percent mill addition of CuO. The glaze batch was:
Frit C
Clay
CuO
90
10
2
Frit C
0.071 K2O
0.113 Na2O
0.455 PbO
0.131 A12O3
0.675 B2O3
2.429 SiO2
0.035 ZrO2
74
Lead Glazes for Ceramic Foodware
An ILMC Handbook
0.361 CaF2
This glaze was applied on bisque cups front Plant C to 1875ºF. in a gasfired laboratory kiln. The lead release data for repeated extractions on the
same cup are given below:
Test Number
1
2
3
4
5
6
7
8
Lead PPM
0.78
0.36
0.46
0.32
0.37
0.24
0.13
0.12
The progressive decrease of lead release from the same glaze surface
(same cup specimen) on continuing testing is noted. The glaze surface
involved here was good in appearance and of light green color. Similar
trends for low temperature, copper-containing glazes were previously
observed on glazes that incorporated copper-containing stains.
Repeated Tests on Low Temperature Uranium Red Glaze
A Cone 03-04 uranium red glaze was tested which gave the following
values for the three cup specimens tested: 1.10-2.14-2.24; Average 1.83 PPM
Lead. The individual cups giving the high and low values were subjected to
repeated extractions using the standard tests. The results were:
Test No.
1
2
3
4
5
6
7
8
Lead PPM
Cup LV
Cup HV
1.10
2.24
0.84
0.46
0.55
0.64
0.65
0.70
0.55
1.04
1.04
1.22
1.14
1.27
1.50
1.56
75
Lead Glazes for Ceramic Foodware
9
10
11
12
13
14
15
16
17
18
19
20
21
An ILMC Handbook
1.37
1.28
1.41
1.58
1.53
1.92
1.80
1.74
2.18
1.66
1.82
1.62
1.62
As will be noted for both of these cup specimens, there is an initial
decrease in lead release after the first extraction, - then the lead release
progressively increased with repeated extractions. After eight repeated
extractions, the lead release from the glaze on Cup LV exceeded the initial
amount and this became progressively worse on continued testing up to
seventeen tests after- which the lead release falls off again.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
CHAPTER 8
Effect of Glaze Processing Variables
Introduction
In addition to the composition of glazes the way in which they are applied
and fired has a significant effect on the appearance and properties of many
glazes. Therefore, a study was undertaken to assess the effect of various
production parameters on the lead release of a range of glazes as measured by
the standard 4% acetic acid, 24 h test.
Production parameters varied in this study included:
·
The application of the glaze and thickness;
·
The characterization of the bisque body and the interaction of the
glaze and bisque
·
The glost-firing parameters such as time, temperature and
atmosphere.
While the manufacturing technologies for these parameters are well
developed, the relationship between these parameters and the lead release
performance is not well documented in the literature. The following
summarizing points can be drawn from the data presented in the balance of
this section.
In general, a trend was noted for increased lead release with increased
glaze thickness. The lead release values for the typical Cone 4-5 dinnerware
glaze, applied in different thicknesses, were too low to draw any conclusions.
77
Lead Glazes for Ceramic Foodware
An ILMC Handbook
In contrast, tests on two Cone 076 low temperature glazes showed a marked
increase in lead release with increasing glass thickness.
Tests on different production bisque cups from three plants, for similar
glaze applications and firing treatments, suggested that variation in bisque
properties would be reflected when the lead release values are sufficiently
high to have experimental significance. The lead release values for the typical
Cone 4-5 dinnerware glaze, applied on the different bisque, were too low to
show any trends. However, when a low temperature (1875ºF) glaze was used
along with modifications of this glaze (CuO and other coloring oxide mill
addition) the variation in the bisque properties as affecting the glaze
maturation was reflected in the trends on lead release values.
Variations in the time and temperature of firing of the typical Cone 4-5
dinnerware glaze again gave lead release values too low to consider that any
significant trends were shown. Factorial designed experiments were run
using a Cone 02-4 glaze and different levels of firing time, temperature, and
glaze thickness. In these tests the time and temperature were the most
important factors. The lead release was reduced by longer firing times and
higher temperatures for this glaze-body system. In another designed
experiment a low temperature glaze composition (Cone 07) was fired at
higher than normal cone temperatures (Cone 02-5) for different time and
glaze thicknesses. The importance of the time factor was emphasized here,
permitting more body solution and improved acid resistance.
For well designed glazes, the lead release from the glazed surface was not
increased substantially by firing considerably lower than the normal maturing
temperatures.
Relationship Between Lead Release and Glaze Thickness
The data below are for production glazes, which were applied and fired at
the plant. The test specimens received heavy, medium, and light applications
of the respective glazes. The average thickness of the glaze was measured for
three cup specimens of each glaze and each weight of application. All four of
these glazes and their light to heavy applications were bright, glossy and of
good texture. Data for the four production glazes are given below:
Cone 3 Clear Glaze #1
78
Lead Glazes for Ceramic Foodware
Application
Heavy
Medium
Light
An ILMC Handbook
Thickness
(in.)
0.006
0.005
0.004
Lead ppm
0.44
0.39
0.10
Thickness
(in.)
0.005
0.004
0.003
Lead ppm
0.82
0.65
0.59
Cone 3 Clear Glaze #2
Application
Heavy
Medium
Light
Cone 01 All-Fritted Clear Glaze #1
Application
Medium
Light
Thickness
(in.)
0.005
0.004
Lead ppm
0.15
0.09
Cone 01 All-Fritted Clear Glaze #2
Application
Heavy
Medium
Light
Thickness
(in.)
0.008
0.004
0.004
Lead ppm
0.60
0.31
0.35
The above data show a trend towards increased lead release with
increased glaze thickness.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Effect of Glaze Thickness of a Cone 4 Fritted Lead Glaze
Glaze
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.321 A12O3
0.360 B2O3
3.064 SiO2
The glaze was prepared from ninety percent (90%) Frit D and ten percent
(10%) clay mill addition from the above glaze formula. This base glaze and
various modifications involving different mill additions of opacifying oxides
(previously described) were used here with different thickness of application.
In these tests the base glaze and modifications were applied on standard
bisque cups from Plant C using light, medium and heavy applications, approximating 1.5 mils, 2.8 mils, and 3.5 mils respectively fired thickness.
The specimens were fired on a 4.5 hour heating schedule to Cone 4 down in a
gas-fired laboratory kiln.
BG
BG-1
BG-2
BG-3
BG-4
BG-5
BG-6
BG-7
BG-8
Light
1.5 mils
Lead ppm
Medium
2.8 mils
0.07
0.13
0.03
0.17
-0.12
0.15
0.03
0.04
0.14
0.12
0.10
0.13
0.13
0.17
0.13
0.16
0.08
Heavy
Glaze
3.5 mils
0.01
0.02
0.04
-0.05
0.08
0.03
0.06
0.01
Again the lead release values are too low to draw any conclusions on the
relation of lead release and glaze thickness.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Effect of Glaze Thickness in Cone 07 Glazes
The scatter in lead release data for assumedly similar specimens is
generally found to be greater when the acid resistance is relatively poor.
Although the variation from specimen to specimen would depend upon a
number of factors, in the following tests an attempt was made to relate the
thickness of the glaze application to lead release. All the specimens were
fired on the same 4.5 hour heating schedule to Cone 07. They involved
different thickness of applications on the similar bisque cups from Plant C.
The actual fired thickness was measured using a microscope with a filar
head. The glaze compositions were selected to have poor acid resistance and
were not of a type as would be used commercially. The compositions were:
Li2O
Glaze
A
B
0.1
-
A
B
1.06
-
K2O
PbO
Molecular Equivalents
0.9
0.1
0.9
% Melted Oxide Composition
71.25
3.27
69.66
SiO2
1.3
1.3
27.69
27.07
Sample
Fired Thickness
of Glaze (mils)
Lead ppm
A-1
A-2
A-3
2.71
3.03
5.24
0.5
0.6
3.5
B-1
B-2
B-3
B-4
B-5
B-6
B-7
2.77
2.98
3.09
3.10
3.22
4.84
6.66
1.9
3.7
4.1
4.2
6.8
12.4
16.5
81
Lead Glazes for Ceramic Foodware
An ILMC Handbook
From the above data, it can be concluded that for such compositions in
this R2O-PbO-SiO2 system, the fired glaze thickness is a significant factor in
respect to lead release.
Effect of Different Bisque on Lead Release
Some of the base glazes and modifications by mill additions of either
coloring oxides or opacifying oxides were applied on bisque cups received
from three different plants. The glaze application and firing treatment were
held as constant as possible so that any differences in lead release might
relate to variation in bisque properties and interfacial action. Several
examples of the results of such tests are given below:
Clear Cone 4 Glaze with Coloring Oxides
The Cone 4 glaze used here had the following composition:
0.013 K2O
0.182 Na2O
0.572 CaO
0.233 PbO
0.290 A12O3
0.360 B2O3
2.971 SiO2
Glaze Batch
Frit A
Frit B
Whiting
Clay
Flint
51.5
25.1
4.0
4.7
14.7
The compositions of the two frits are as follows:
Frit A
0.02 K2O
0.29 Na2O
0.69 CaO
0.27 A12O3
0.57 B2O3
2.49 SiO2
0.25 A12O3
1.92 SiO2
Frit B
1.00 PbO
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Lead Glazes for Ceramic Foodware
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The test specimens were all fired approximately five (5) hours to Cone 4
in a gas-fired laboratory kiln. Each value for lead release reported is the
average for three cup specimens.
LEAD RELEASE DATA IN PPM FOR
GLAZE APPLIED TO BISQUE CUPS FROM THE INDICATED PLANT
Glaze
BG
BG-1
BG-2
BG-3
BG-4
BG-5
BG-6
Plant B
0.07
0.08
1.58
0.07
0.04
0.04
0.01
Plant C
0.10
0.05
1.19
0.07
0.03
0.08
0.01
Plant D
0.03
0.03
1.14
0.04
0.03
0.11
0.11
With the exception of BG-2, which was a modification of the base glaze
(BG) to include a two percent (2%) by weight mill addition of CuO, all the
lead release values were too low to draw any conclusions regarding effects of
different bisques.
Lower Temperature Glaze with Coloring Oxides
Glaze
0.071 K2O
0.113 Na2O
0.455 PbO
0.361 CaF2
0.279 A12O3
0.675 B2O3
2.726 SiO2
0.035 ZrO2
Glaze Batch
Frit C
Clay
90
10
Frit C
0.071 K2O
0.113 Na2O
0.455 PbO
0.361 CaF2
0.131 A12O3
0.675 B2O3
2.429 SiO2
0.035 ZrO2
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
The above glaze and modifications of this glaze with various mill
additions of coloring oxides were applied on bisque cups from Plants B, C
and D. Each value for lead release is the average for three cups tested. The
test specimens were all fired in 4.5 hours to 1025o C. in a gas-fired laboratory
kiln. The data are given below:
LEAD RELEASE DATA IN PPM FOR
GLAZE APPLIED TO BISQUE CUPS FROM THE INDICATED PLANT
Glaze
BG-LT
BG-LT -1
BG-LT -2
BG-LT -3
BG-LT -4
BG-LT -5
BG-LT -6
Plant B
0.34
0.80
12.50
2.25
1.24
5.25
0.42
Plant C
1.20
3.10
12.00
4.75
3.31
6.90
1.17
Plant D
2.55
2.55
13.40
6.85
5.80
4.15
1.28
The effects of the coloring oxide mill additions on the lead release from
this low temperature glaze were marked (previous note for these glazes on
Plant C bisque only). With some exception, the same glaze applied on Plant
B bisque gave the lowest value and on Plant D bisque the higher value with
the Plant C bisque being intermediate. This suggests that the bisque
properties and resultant body-glaze interaction would be reflected when the
lead release values are sufficiently high to have experimental significance.
As previously noted this type glaze was designed for talc-clay type tile
bodies and is not used for glazing dinnerware bodies.
Clear Cone 4 Glaze with Opacifying Oxides
Base Glaze
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.321 A12O3
0.360 B2O3
3.064 SiO2
Glaze Batch
(Not Including Opacifier)
84
Lead Glazes for Ceramic Foodware
Frit D
Clay
An ILMC Handbook
90
10
Frit D
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
0.19 A12O3
0.36 B2O3
2.80 SiO2
The above base glaze and modifications of this glaze with various mill
additions of opacifying oxides were applied on bisque cups from Plant B, C
and D. The glaze cups were glost-fired to Cone 46 and a 4.5 hour heating
schedule in a gas-fired laboratory kiln.
LEAD RELEASE DATA IN PPM FOR
GLAZE APPLIED TO BISQUE CUPS FROM THE INDICATED PLANT
Glaze
Plant B
Plant C
Plant D
BG
0.05
0.14
0.04
BG-1
0.04
0.12
0.01
BG-2
0.03
0.10
0.06
BG-3
0.08
0.13
0.05
BG-4
0.09
0.13
0.01
BG-5
0.04
0.17
0.02
BG-6
0.04
0.13
0.02
BG-7
0.04
0.16
0.01
BG-8
0.09
0.08
0.01
All the glazes were good in appearance. Because of differences in the
fired color of the bisques from the three different plants any differences in
opacity were difficult to evaluate. All of the lead release values were too low
to draw any conclusions regarding effects of the different bisques.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
General Effects of Varying Firing Time and Temperature
The same Cone 4 glaze using a single lead frit and various modifications
of mill additions of opacifying oxides were given additional firings to other
Cone temperatures but in each case using a 4.5 hour heating schedule. In all
cases the glazes were applied on bisque cups from Plant C using the same
application procedure.
Base Glaze
0.09 K2O
0.09 Na2O
0.58 CaO
0.24 PbO
Glaze
BG
BG-l
BG-2
BG-3
BG-4
BG-5
BG-6
BG-7
BG-8
0.321 A12O3
0.360 B2O3
Cone 02
0.06
0.07
0.08
0.07
0.07
0.11
0.14
0.13
0.06
3.064 SiO2
Lead Release, ppm
Cone 1
Cone 3
0.09
0.01
0.01
0.06
0.02
0.01
0.09
0.01
0.01
0.02
0.16
0.02
0.06
0.01
0.14
0.04
0.04
0.01
Cone 4
0.14
0.12
0.10
0.13
0.13
0.17
0.13
0.16
0.08
Cone 5
0.04
0.01
0.09
0.06
0.11
0.04
0.09
0.06
0.05
The glazes were all fairly good in appearance. The modifications,
involving mill additions of opacifiers, showed greater opacification at the
lower cone temperature for the same level and kind of opacifying oxide. At
all temperatures the opacification increased with increasing levels of each of
the three opacifiers as would be expected. The high level of TiO2 gave most
of the yellowing shown for this oxide addition and this was greater with the
lower cone temperature firings.
Again for all the glazes, fired to the five different cone temperatures on
the same heating schedule, the lead release values were too low to consider
that any significant trends were shown.
86
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An ILMC Handbook
The same glazes as used above were applied on bisque cups from Plant C
and fired to Cone 46 on slower and faster heating cycles than that used
previously. The heating schedules were 2.5 hours, 4.5 hours and 6.5 hours in
all cases to Cone 4.
LEAD RELEASE DATA IN PPM
Glaze
2.5
Hours
4.5
Hours
6.5
Hours
BG
0.04
0.14
0.27
BG-1
0.04
0.12
0.13
BG-2
0.02
0.10
0.03
BG-3
0.06
0.13
0.17
BG-4
0.04
0.13
0.07
BG-5
0.07
0.17
0.08
BG-6
0.04
0.13
0.13
BG-7
0.05
0.16
0.14
BG-8
0.04
0.08
0.14
Again the glazes were all fairly good in appearance. Opacification was
greatest for the fastest firing schedule. The high levels of TiO2 additions
showed greatest yellowing for the faster firing. The lead release values,
although they might appear to be favored by the fastest firing, are too low in
magnitude to form any conclusions. For this reason and also because
commercial firings generally involve longer firing times, the statistically
designed experiment reported later involved greater variation in these firing
parameters.
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Lead Glazes for Ceramic Foodware
An ILMC Handbook
Effect of Glaze Thickness, Firing Time and Temperature: Commercial
Glazes
Factorial Experiment Design
A factorial designed experiment was used to study the effect of
production parameters on the lead release from a typical lead glaze using the
standard tests. In such an experiment each variable is varied at all levels of all
other variables. The variables or factors were:
·
Glaze thickness at three levels
·
Firing time at three levels
·
Firing temperature at three levels
·
Three-fold replication
In this work the same glaze composition and same body (bisque cups
from Plant C) were used throughout the tests.
The glaze was a standard commercial Cone 02-4 composition,
incorporated a single lead frit, as follows.
Glaze
0.09 K2O
0.09 Na2O
0.53 CaO
0.24 PbO
0.321 A12O3
0.360 B2O3
3.064 SiO2
Glaze Batch
Frit D
Clay
90
10
The sample cups used were the standard bisque cups as obtained from
Plant C.
The production parameters and the levels of each were as follows:
Experimental Factor
Fired Glaze thickness,
mm
50
88
Levels
125
200
Lead Glazes for Ceramic Foodware
Firing temperature, oC
Firing time, hours
An ILMC Handbook
1100
4
1140
10
1175
17
Three-fold replication was used throughout the experiment.
Results
The data are given on the following page. Statistical analysis of the data
suggests that time and temperature are the more important factors influencing
lead release from this glaze and thickness does not have a major effect.
However, it should be noted that thickness was the most difficult parameter
to control in this experiment. Lead re- lease was reduced at longer times and
higher temperatures of firing for this glaze body system. Since the lead
release values are all very low, an experiment will be designed with a high
solubility glaze.
89
EFFECT OF GLAZE THICKNESS, FIRING TIME AND TEMPERATURE:
COMMERCIAL GLAZES
Time, h
50 mm
Temperature, oC
1100
1140
1175
LEAD ppm
125 mm
Temperature, oC
1100
1140
1175
200 mm
Temperature, oC
1100
1140
1175
4h
0.287
0.138
0.290
0.080
0.069
0.079
0.141
0.044
0.050
0.127
0.112
0.128
0.064
0.101
0.059
0.050
0.046
0.031
0.141
0.132
0.136
0.059
0.070
0.084
0.026
0.160
0.204
Av.
Range
0.238
0.152
0.076
0.011
0.078
0.097
0.122
0.016
0.075
0.042
0.042
0.019
0.136
0.009
0.071
0.025
0.130
0.173
10 h
0.060
0.012
0.002
0.044
0.034
0.040
0.031
0.033
0.017
0.002
0.010
0.028
0.014
0.022
0.040
0.040
0.020
0.014
0.021
0.016
0.021
0.044
0.025
0.047
0.034
0.029
0.029
Av.
Range
0.025
0.058
0.039
0.010
0.029
0.021
0.013
0.026
0.025
0.026
0.025
0.026
0.019
0.005
0.039
0.021
0.031
0.005
EFFECT OF GLAZE THICKNESS, FIRING TIME AND TEMPERATURE:
COMMERCIAL GLAZES
[CONTINUED]
Time, h
17 h
Av.
Range
50 mm
Temperature, oC
1100
1140
1175
0.000
0.047
0.088
0.000
0.026
0.004
0.000
0.006
0.000
LEAD ppm
125 mm
Temperature, oC
1100
1140
1175
0.050
0.030
0.005
0.005
0.014
0.014
0.085
0.018
0.019
200 mm
Temperature, oC
1100
1140
1175
0.030
0.023
0.019
0.013
0.023
0.000
0.042
0.026
0.016
0.000
0.000
0.047
0.080
0.020
0.029
0.026
0.041
0.031
0.038
0.021
0.016
0.013
0.014
0.024
0.003
0.012
0.019
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Effect of Glaze Thickness, Firing Time and Temperature: Laboratory
Fritted Glazes
The previous factorial experiment used a commercial glaze in studying
the effect of production parameters on lead release. In the present case a
selected lead frit composition was investigated using the standard test. The
variables or factors again were:
·
Glaze thickness at three levels
·
Firing time at three levels
·
Firing temperature at three levels
·
Three-fold replication
The glaze used in this experiment had the following molecular
composition:
0.9 PbO
0.1 K2O
0.30 B2O3
1.50 SiO2
The glaze slip consisted of the fritted glaze composition suspended in a
Methocel solution. Bisque cups from Plant C were used throughout the tests.
The production parameter matrix was similar to the prior example but with a
few differences in the firing times.
Experimental Factor
Fired Glaze thickness,
mm
Firing temperature, oC
Firing time, hours
50
Levels
125
200
1100
4
1140
8
1175
12
The data are given on the following page. Summarizing the results, from
the statistical point of view, the primary parameters affecting the lead release
of this glaze-body combination is time with some effect due to temperature,
some effects due to time and temperature, and with thickness (in this
experiment) having a relatively insignificant effect.
The frit used in this experiment would be relatively poor in acid resistance
if tested as a fine powder. Applied as a glaze on the bisque from 50 - 125 mm
in fired thickness, and fired in from 4 to 12 hours to Cone 02 to Cone 5, the
92
Lead Glazes for Ceramic Foodware
An ILMC Handbook
glazes do have good acid resistance. This reflects the body solution into the
glaze providing a higher Al2O3 and SiO2 content. It would be expected that
such a glaze fired at too low a temperature, which did not permit such body
solution into the glaze providing a higher Al2O3 and SiO2 content, would be
much poorer in acid resistance. The same glaze applied on bisque cups from
Plant C and fired to Cone 01 showed extremely high lead release. Yet this
glaze, fired at this low temperature, was relatively good in appearance. This
is an extremely important consideration even though such a composition
would not be used as a dinnerware glaze. As shown in the following section,
satisfactory and well-designed glazes can be fired considerably lower than
their normal maturing temperature without significant increase in lead
release.
93
Lead Glazes for Ceramic Foodware
An ILMC Handbook
EFFECT OF GLAZE THICKNESS, FIRING TIME AND TEMPERATURE:
LABORATORY FRITTED GLAZES
Lead Release Levels (ppm)
Levels of Thickness
50 mm
Temperature, oC
1100 1140 1175
0.06
0.08
0.06
0.07
0.03
0.01
0.010
0.07
0.04
.
0.20
0.15
0.12
0.07
0.05
0.04
0.01
0.05
0.05
125 mm
Temperature, oC
1100 1140 1175
0.04
0.07
0.06
0.11
0.02
0.06
0.00. 0.17
0.05
1
0.20
0.10
0.29
0.07
0.03
0.10
0.07
0.06
0.11
200 mm
Temperature, oC
1100 1140 1175
0.05
0.03
0.06
0.05
0.05
0.15
0.09
0.12
0.13
0.19
0.06
0.04
0.25
0.08
0.07
0.34
0.11
0.07
8h
0.04
0.02
0.02
0.03
0.05
0.04
0.05
0.07
0.06
0.01
0.01
0.00
0.02
0.01
0.06
0.03
0.04
0.02
0.03
0.03
0.03
Sum
Avg.
Range
0.08
0.03
0.02
0.12
0.04
0.02
0.18
0.06
0.02
0.02
0.01
0.01
0.11
0.21
0.09
0.09
0.04
0.07
0.03
0.03
0.01
0.03
0.05
0.02
[continued on next page]
0.09
0.03
0.00
4h
Sum
Avg.
Range
0.04
0.03
0.04
0.06
0.06
0.09
94
Totals
0.59
0.20
0.12
0.50
0.16
0.18
0.75
0.25
0.33
0.19
0.07
0.03
0.32
0.11
0.05
0.48
0.16
0.05
Lead Glazes for Ceramic Foodware
An ILMC Handbook
EFFECT OF GLAZE THICKNESS, FIRING TIME AND TEMPERATURE:
LABORATORY FRITTED GLAZES
[continued from prior page]
Lead Release Levels (ppm)
12 h
Sum
Avg.
Range
Totals
Sum
Avg.
Range
Levels of Thickness
50 mm
Temperature, oC
1100 1140 1175
0.01
0.00
0.05
0.04
0.01
0.03
0.03
0.01
0.02
125 mm
Temperature, oC
1100 1140 1175
0.04
0.06
0.05
0.040 0.02
0.02
.
0.06
0.03
0.03
0.09
0.03
0.03
0.02
0.01
0.01
0.10
0.03
0.03
0.14
0.05
0.02
0.11
0.04
0.04
0.10
0.03
0.03
0.22
0.07
0.05
0.03
0.01
0.03
0.03
0.01
0.02
0.45
0.15
0.10
0.16
0.06
0.08
0.23
0.07
0.08
0.37
0.13
0.06
0.29
0.10
0.08
0.40
0.13
0.10
0.36
0.13
0.10
0.32
0.11
0.11
0.60
0.20
0.17
0.50
0.16
0.14
0.37
0.12
0.12
0.46
0.15
0.09
1.23
0.42
0.30
0.98
0.33
0.31
1.46
0.48
0.36
95
200 mm
Temperature, oC
1100 1140 1175
0.05
0.03
0.00
0.10
0.00
0.02
0.07
0.00
0.01
Totals
Lead Glazes for Ceramic Foodware
An ILMC Handbook
Effect of Under-Firing on Lead Release
Under firing tests were first run on the Pb-349 based glaze and later also
on some commercial glazes. The tests on the Pb-349 based glaze are first
reported. Again the Pb-349 frit was used in ninety percent (90%) amount
along with ten percent (10%) clay in preparing the glaze. A large number of
bisque cups from Plant D were glazed and fired on a five (5) hour heating
schedule to Cone 4. Then other fires were made at Cones 1, 01, 03, 05, 07,
and an additional low temperature firing to 815o C. The lead release data for
the various cups fired to the various temperatures are given below:
Firing Cone or
Temperature
46
16
016
036
056
076
Lead Release
ppm
0.115-0.0720.184
0.106-0.1150.060
0.061-0.1050.077
0.284-0.151
0.065-0.092
0.057
Average
0.124
0.094
0.081
0.218
0.079
0.057
This suggests that the underfiring of a glaze composed essentially of a
‘well-balanced’ frit does not show increase in lead release on underfiring.
Properties of Underfired Production Glazes
Five commercial glazes from Plants B, C, I, K and L were applied on
bisque cups from the respective plants. These were fired on a 4.5 hour
heating schedule to Cone 026 or approximately 6 cones below their normal
maturing temperature.
Plant
B
C
Lead Release
ppm
0.36-0.08-0.16
0.10-0.05-0.07
96
Average
0.20
0.07
Lead Glazes for Ceramic Foodware
1
K
L
An ILMC Handbook
0.08-0.02-0.05
0.65-1.28-0.95
0.17-0.01-0.11
0.05
0.96
0.10
All glazes were bright and glossy. Glazes B, I and L were clear, colorless
glazes. Glaze C was a light blue glaze. Glaze K was a brown, Rockingham
type glaze. Four of the five commercial glazes showed very low values and
the fifth was less than one ppm, when these commercial glazes were fired 6
cones lower than their normal maturing temperatures.
97
Lead Glazes for Ceramic Foodware
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CHAPTER 9
Decoration of Dinnerware
The decoration of porcelain,
stoneware, earthenware and
bone china typically involves
the use of ceramic colors, i.e.,
high-temperature
resistant
colored silicate materials. A
wide palette of colors has been
produced for the ceramics
industry based in part on
traditional methods but more
recently on research and
development of specialized
CONE 05 EARTHENWARE
compounds. Early application
methods consisted of brushing but more recent technology has introduced
lithographic, screen, and offset printing. Decoration is an essential part of the
aesthetic and technical design of tableware and other utensils, and plays an
important role in product differentiation.
Requirements for Ceramic Colors
The requirements that ceramic colors need to meet depend on the desired
properties of the finisher design and the process employer to produce this
design. The design itself is selected according to two criteria: aesthetic and
useful properties. To satisfy the aesthetic demands, the color should be as
opaque as possible, glossy and pleasing to the eye. Richness of color is also
98
Lead Glazes for Ceramic Foodware
An ILMC Handbook
desirable and the design drawn up by the artist must be reproduced as
faithfully as possible. End-user properties, such as dishwasher durability,
scratch resistance, resistance to food and beverages, taste neutrality, and low
toxicity, are important as well. Certain tableware items must also be
heatproof and thermal shock resistant.
Composition and Preparation of Ceramic Colors and Inks
Overview
The color inks used today consist of a pigment, a frit (glassy binder to
ensure adhesion of the pigment to the ware surface) and an organic medium
to enhance applicability. Usually the pigment and grit are ground together to
form a color powder. These powders have a typical particle diameter of 1 20 mm and a relatively high density of approximately 3 - 4 g/cm3. These
color powders can be easily dispersed in organic media to produce liquid ink
suitable for further processing.
Composition and Preparation of Pigments
Many pigments are available, but very few meet the exacting
requirements of ceramic coatings. Not only must they exhibit a pure and
intensive color tone, they must withstand firing temperatures without
deterioration, be inert to the chemical attack of the hot glass melt and retain
their color tone under oxidizing or reducing atmospheres. Consequently,
only a few classes of pigments are suitable for coloring ceramic ware. These
are mainly crystalline compounds such as spinels, rutiles, and varieties of
corundum or crystals with lattice defects occupied by certain ions. The
compositions differ depending on the required hue and shade.
To produce a green color, chrome oxide (Cr2O3) can be used, but the
result is often not a pure saturated green color. Chrome/cobalt or
aluminum/zinc spinels are preferred and can be used to produce a variety of
green shades.
For blue, zirconium vanadium blue or different types of zircon silicate as
well as Co-Al spinels are available to make up sky blue shades. The famous
and beautiful intensive blue formed from cobalt requires higher temperatures
(approximately 1200°C) to give the best results.
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Yellow shades are a bit difficult. Naples yellow can be produced with a
lead antimonide but this compound is only stable to 900°C. Titanium nickel
yellow (a rutile) and the bright cadmium sulfide can be used but these
compounds also have limited thermal stability. Praseodymium doped zircon
pigments are the one choice available for high temperature yellows.
The production of red pigments continues to be a challenge for the
ceramist. Most red pigments are either toxic, unstable at high temperatures,
or produce reds of low purity. The deep red cadmium selenium sulfide
pigment is prized for its pure red color, but it is not thermally stable at
elevated temperatures and it is highly toxic. Recent developments have
ameliorated many of these problems by encapsulating the color center phase
in zircon, thus stabilizing the pigment with respect to bleaching and toxic
exposure. A colloidal dispersion of gold in glasses and glazes produces
colors ranging from ruby red to deep purple with low consumer toxicity.
Unfortunately, the temperature stability of gold purple is not excellent.
Another possibility is the tin/chrome/calcium-silicate pigments, which
produce a pinkish red shade but are quick to react to unsuitable frits. Iron
oxide has been well known since antiquity to produce reddish colors when
properly compounded and fired, although most of these colors tend towards
rust or brownish colors.
In contrast to the spectral colors, black and white pigments are rather
simple to produce. Black is obtained with chrome/iron pigments, which may
be adjusted with the addition of cobalt oxide to give blue, or cuprous
manganese or nickel oxide to produce truly neutral black. White pigments
are readily produced from zirconium or tin oxide.
FIRING TEMPERATURES FOR DECORATION OF
THE VARIOUS TYPES OF CERAMIC WARE
[TEMPERATURES IN CELSIUS]
Underglaze
Ceramic
Ware
Earthenware
Stoneware
Porcelain
Decorating Process
In-glaze
Overglaze
750-820
1350 - 1450
Ion Colored
Glaze
1000 - 1040
1200 - 1260
1150 - 1250
100
800 - 840
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750 - 820
In addition to the direct use of pigments, other coloring methods are
available. The most significant of these alternative is ion coloring, which
utilizes the color centers developed by elements or compounds dissolved in a
glaze or glass to produce attractive colors. Good examples of ion coloring
are the cobalt blues and the iron palette, which depending on the stage of
oxidation produces colors ranging from blue-green through yellowish-brown.
Frits, Composition and Manufacture
A frit is a powdered glass that was melted to a certain composition for
specific use in glazes. In decorating, the frit is combined with the pigment
and serves to adhere the pigments to the substrate at the processing
temperature. Frits are glassy powders with a melting range between 700 and
1200°C depending on the application temperature. Thermal expansion is an
important property of the frit. The thermal expansion coefficient of the frit
must be closely matched to that of the pigment and the substrate, otherwise
cracking and flaking may result.
Most frits are lead borosilicate glasses
to which other elements are added to
satisfy property and processing
requirements.
For
example,
aluminum is added to improve
resistance, while alkali metals are
added to reduce the viscosity to
permit easy spreading along the ware
surface at low firing temperatures.
BAND DECORATION
Recent developments have focused
on lead-free glazes and decorations to avoid the issue to lead in the
workplace and lead migration during use. Two main approaches are used to
eliminate lead from glazes: use of an alkali borosilicate glass system in
which lead is replaced by a combination of non-lead fluxes, principally
alkalis and borates, but also including transition elements such as zinc; and
bismuth borosilicate glass systems in which bismuth, an element chemically
similar to lead, replaces the lead. However, with respect to end-use
conditions, it is quite easy with proper formulation and processing to apply
and fire glazes and decorations with high lead contents that meet FDA and
ISO lead release specifications for foodware surfaces.
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Frits are usually manufactured in rotary tube furnaces, sometimes also in
tanks. A batch of powdery raw materials is placed into the furnace, melted
according to a certain time and temperature schedule, and then quenched in
water. The resulting frit granulate is ground with the pigment to the required
fineness.
Composition and Preparation of Media
The third important component in the production of a ceramic color ink is
the medium, an organic polymer mixed with plasticizers and other agents to
act as a vehicle for the pigment and frit. When mixed with the decoration
color powder, the medium forms an easy-to-apply ink, which ensures
accurate reproduction and subsequent fast dying. The medium must then burn
off without residue during fining. The media are categorized according to the
different drying processes such as evaporation of solvents or stiffening of
melted thermoplastic media. In chemical film formation, the liquids react to
form solid network structures. Evaporation of the solvent is the most
common drying method.
The type of polymer used determines the choice of solvents. Common
solvents are aromatic compounds, esters and ketones. Usually combinations
of solvents are added to adjust the solvent power and set the required drying
time. For screen printing it should be noted that the often desired fast drying
could lead to a thickening of the ink on the screen. as the more volatile
solvents evaporate and viscosity increases. Here the best possible
compromise must be sought.
Application of Ceramic Decoration
Overview
The original artwork is reproduced by painting or printing the image onto
the ware surface. The easiest ways of applying the colors are painting
processes, like banding and spraying. These are generally only able to
produce a relatively simple, plain surface covering. Should the desired
decoration be more complex, besides hand painting, other possibilities are
printing processes like screen-printing or offset printing. Indirect screen
printing is the most important decoration technique used in the ceramic
industry today.
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Preparation of Printing Materials
The designer s original artwork is either photographed by a reproduction
camera or read by a scanner. After this each color to be printed is copied onto
a screen or offset plate. To produce a motif requiring more than one color, a
screen or offset plate must be prepared for each individual color. This can
result in up to 20 screens or correspondingly 20 color printing passes. To
keep costs within acceptable limits, it is important to keep the number of
Decoration
Application Process
Direct
Indirect
Screen Printing
Offset Printing
Wet Printing With
Decals
Dry
Pad Printing from
Engraving
Spraying
Screen Printing
Transfer Screen
Printing
Wet
Banding
Heat Release
Transfer
Hand Painting
DECORATION APPLICATION FLOW CHART
printing passes required to produce the design to a minimum. Overprinting
the basic colors of yellow, cyan, magenta and black to make the full spectrum
of colors can be achieved to a certain extent. However, in practice many
more colors are sometimes used to achieve detailed results, much like in the
commercial paper printing process. Despite these limitations, current
decorating practice can achieve good quality decorations with only four to
nine colors instead of twenty or more colors commonly used several decades
ago.
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Preparations of Printing Inks
For the preparation of printing inks, the color powder must be dispersed in
the medium as homogeneously as possible and without the formation of
agglomerates. For this, the color powder is carefully dried and mixed with
the medium before being dispersed with special equipment, usually a triple
roll mill. This mixture is adjusted to required printing viscosity by the
addition of a further medium.
Application
The most important printing process is screen-printing. In screen printing
the color ink is pressed through a finely meshed screen by means of a
squeegee. This process allows a thick layer of color to be applied. The screen
is usually made of steel or polyester cloth. The screen gauze is clamped in a
frame and a light-sensitive coating is applied. Those areas where no color is
required, i.e. non-image areas, are covered by a screen stencil or a polymer
film coating to prevent exposure in the subsequent lithography process.
Then, the design is transferred to the screen by lithography and the exposed
area is rendered water soluble and washed out. During the printing process,
the washed out areas are filled with color ink and transferred to the ware. For
screen printing, both flat-bed and cylinder printing machines are available.
The latter allows greater precision and higher printing speeds and as such are
more popular. For the screen gauze, steel and synthetic materials with
different thread counts are used depending on what layer thickness is
required. Current practice consists of either printing directly onto the ware
(direct screen-printing) or on an intermediate vehicle like paper or silicon
(indirect screen-printing).
Of the two processes, indirect screen printing onto paper (decal
production) is the most efficient and popular process in the ceramics
industry. This is because indirect screen-printing enables a large number of
colors to be printed cost-effectively. The finished decal can be applied to any
three-dimensional object such as a cup or a coffee pot, while until recently
the printing of such items by direct printing was extremely difficult or
impossible. Of course, the decal process involves several extra steps
compared to the direct process, and recent developments in total transfer
direct printing have brought it to a comparable level with the decal process
for some applications.
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Another decorative printing process is transfer screen-printing. This
process requires thermoplastic screen-printing inks. In this process, an image
is screen printed hot so that the inks are liquid. A pad picks up this
decoration and transfers it to the cold ware where the ink sets immediately
and adheres to the ware surface. This process historically was limited to
simple decorations of only moderate quality. Recently this process has
become quite popular due to the increased quality of the process and its low
cost.
Banding and spraying still hold certain significance as coating processes.
Many plates and cups are decorated with colored edges. These are obtained
by pressing rollers or a brush soaked in the color ink onto a rotating ceramic
ware thus transferring the color to ware surface. If a plate or jug is to be
completely covered with a color then spraying is the preferred method. The
inks should be considerably thinner in consistency than for other application
processes.
Firing
After the ware has been decorated, it is fired. This process can be roughly
divided into four stages. The first stage is the burning out of the organic
components. This usually takes place between 300 and 500°C. During the
second stage the ceramic colored powder sinters onto the ware surface. In
the third stage complete melting occurs to form a glossy coating. Cooling
follows as the final stage in the process.
There are three ways in which a decoration can be situated with respect to
the glazing of ware: overglaze, inglaze and underglaze. Overglaze
decorations for porcelain are applied directly over the glost glaze and the
decorations are fired between 800 and 840°C for two to eight hours. The
resultant colors are not usually dishwasher durable, but the pallet of pigments
that can survive this temperature is quite large and for that reason overglaze
decoration is popular with designers.
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Thermal Stability of Overglaze Colors
Temperature, C
700
750
800
850
900
950
100
Fe2O3
Cr2O3-SnO2
Sb2O3-PbO
CdS
UO2
CoO-Cr2O3
CuO
CoO
Cr2O3 - V2O3
To ensure good dishwasher durability the trend is toward inglaze firing.
This high-temperature fast firing is conducted at temperatures between 1150
and 1250°C for one and a half-hour. It is known as inglaze firing because at
these temperatures the ceramic colors sink into and react with the glaze. In
this way the decoration is less exposed to attack by cleaning detergents.
Moreover, the high-melting frits used in this process are considerably more
resistant than the low-melting onglaze fluxes.
Underglaze decoration is much more limited than the other two processes
since the decoration must survive the full fury of the glost fire, thus severely
limiting the palette and line sharpness possible by this method. In underglaze
decoration, the decoration is applied to the ware surface before the glaze is
applied on top. Naturally, this also results in excellent dishwasher durability
as the ceramic colors are completely protected from chemical attack.
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Colors Imparted By Selected Ions In
Lead Silicate and In Alkali Alumino Silicate
Glasses And Glazes
Ion
In Lead Glass/Glaze
Ag++
Au++
Co++
Cr+6
Fe++
Cu++
Pale Green
Faint Violet (Colloidal)
Intense Blue
Yellow Orange
Blue
Intense Green
Fe+++
Mn+++
Mo++
Ni++
Pd++
Yellow/Green
Purple
Faint Green
Yellow Green
Gray Black (Colloidal)
Pt
Ti++
V4+
V6+
W6+
Gray Black (Colloidal)
Faint Yellow
Faint Yellow
Intense Orange
Pale Yellow
Selenium
Cadmium selenide
Red
Orange
In Alkali Alumino
Silicate Glass/Glaze
Purple
Green
Coral Red
Olive Brown
Black
Red
Summary
A wide range of ceramic colors is available to the designer of ceramic
ware, although the firing temperature of the ware considerably limits the
palette. More varied colors are available for low temperature overglaze
decoration of porcelain or general decoration of cone 06 earthenware. Recent
technological advanced in decoration chemistry has expanded the palette of
colors and reduced toxicity of certain pigments by encapsulating the
pigments in zircon or other stable crystals. The inclusion pigments are a
good example of this development. Furthermore, low lead and lead-free
decorations have made substantial advancements in recent years.
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Technology has also advanced in the area of applying decorations by
various printing processes. The computerization of traditional paper printing
methods has also improved the ceramic decoration printing processes. The
traditional screen printing technology, either direct or decal, has been
substantially improved so that many decorations can be printed with only
four to nine colors and line sharpness and registration are also improved.
New media have been developed wherein former drying times of 2 h have
been slashed to just a few seconds. Offset printing is being continually
perfected to give a more opaque covering. In certain sectors new application
processes like heat release will gradually replace traditional manual
application techniques.
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Appendix A:
Supplemental Discussion of Early Work
Fritting Approaches to Control Lead Solubility
The use of lead in glazes was investigated at great length in England circa
1900 by Thorpe and his associates. Thorpe studied the effect of composition
of lead frits on solubility in 0.25% hydrochloric acid; this was taken as
equivalent to the strength of the acid in the gastric juices at body temperature.
An average value for the gastric juices may be 0.17% but the 0.25%
concentration was selected to compensate for the acid being at body
temperature. Thorpe found that the lead bisilicate (PbO·2SiO2) was least
soluble of lead silicate glasses tested. Thorpe recommended the following
empirical relationship to predict the solubility of the lead frit:
Thorpe’s Ratio
Mols Of Basic Oxides + Mols Alumina 223
´
Mols Of Acidic Oxides
60
For low solubility the above ratio should not exceed 2. Mellor (27) later
stated the molecular relationship as:
RO + Al 2 O 3
= 0.5 (Max )
RO 2
If the ratio is equal to or less than 0.5, the frit will have a satisfactory low
solubility. These ratios would hold for the compositional systems involved in
this work.
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The British Government Home Office Circular of December 14, 1899,
stated that no glaze composition containing lead or a compound of lead
(other than galena) shall be regarded as satisfying the requirements as to
insolubility, which yields to a dilute solution of hydrochloric acid more than
four percent (4%) of the dry weight of a soluble lead compound calculated as
PbO when determined as given below (Thorpe’s two percent (2%) solubility
limit was raised to four percent (4%) as a result of arbitration).
A weighed quantity of dried material (particle size not specified) is to be
continuously shaken for one (1) hour, at the common temperature with 1000
times its weight of an aqueous solution of hydrochloric acid containing
0.25% HCl. This solution is thereafter allowed to stand for one (1) hour, and
to be passed through a filter. The lead salt contained in an aliquot part of the
clear filtrate is then to be precipitated as lead sulfide and weighed as lead
sulfate. It is interesting to note that the lead bisilicate, approximating sixty
five percent (65%) PbO and thirty five percent (35%) SiO2 has sufficient
insolubility to enable the mill mixture of the ordinary glaze to pass readily
this Government test.
Rix in 1902 summed up the experience up to that time and stated that the
lead should be fritted and that the lead frit should be insoluble in dilute
hydrochloric acid. Rix stated that lead in the fritted form could be substituted
for the raw glaze without change in the glaze composition and without
change in the required firing temperature; the fluidity, of course, is affected
by the character of the lead frit. The early potters realized that the use of
fritted lead assisted in the immunity of the worker and frits were developed
with low solubility. Rix also noted that production losses were greater when
leadless glazes were substituted owing to the greater care required in the
manipulation and firing of these glazes.
Koerner in 1906 confirmed Thorpe’s conclusions. He showed that
increasing the silica in lead silicate frits reduced the solubility. He also found
that the introduction of alumina greatly reduced the solubility of lead
monosilicate.
Bartel in 1918, in an extensive investigation of lead solubility of fritted
glazes, also evaluated lead silicates from PbO·1.5SiO2 to PbO, 4SiO2 with
increments of 0.25 SiO2. The most acid resistant composition in this system
PbO·Si02 was found to be PbO·2.5SiO2. Bartel tested many lead frits
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prepared from red lead, the carbonates of Na, K, Ca, Ba and Mg, ZnO,
Zettlitz kaolin and pure sand. The test used a fine powder of the frit (through
a given size sieve with no residue), exposure to four percent (4%) acetic acid
for one-half (1/2) hour, adding a measured amount of K2Cr2O7 to precipitate
any Pb as PbCrO4, then determining the excess K2Cr2O7 by an chemical
process. Reference was made to the difference between such finely ground
frit material and the layer of glaze on the ware, and also to the influence of
other oxides in the glaze on the lead solubility.
Petrick in 1920 noted that when alkalis or borax are used in lead glaze
frits they make the same soluble. Lead frits containing no alkalis or borax, on
the other hand, are insoluble. By making two frits, one containing all of the
lead oxide with no alkalis or boric acid, and another containing all of the
alkalis and boric acid, the amount of soluble lead oxide is greatly reduced.
Petrick recalculated a number of commercial glaze frits preparing a high lead
and high alkali frit in the place of the alkali-lead frit.
Frit - Number VII
0.774 PbO 0.055 CaO
0.044 K2O 0.134 A12O3
2.374 SiO2
0.127 Na2O
Frit - Number XII
0.75 PbO
0.20 CaO
0. 12 A12O3
2. 4 SiO2
0.05 K2O
Frit Number VII, of which dilute HCl dissolves 1.0% lead oxide, was
used for the preparation of low melting glazes and frit Number XII, of which
dilute HCl dissolves 0.55-0.60% lead oxide, was used for the preparation of
the higher maturing glazes. Using combinations of these frits the soluble lead
oxide was decreased from 31.7% to 0.76% in one glaze, from 23.05% to
0.3% in the second, and in another from 20.72% to 0.25%. Pandermit
(natural calcium borate) may be used in the place of the leadless frit. By
preparing a tin glaze by the use of the two frits given above, the percentage
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of soluble lead oxide was reduced from 4.18 to 1.03%. The presence of CaO
reduces the solubility of frits. The amount of lead oxides soluble in dilute
acid according to Thorpe’s method should be less than 1.00%.
Vargin and Fradkova in 1934 reported on decreasing noxious effects of
lead glazes by an understanding of the influence of the frit constituents on the
acid stability of the frit. The influence of SiO2, A12O3, B203, PbO and CaO
on tire acid stability of frits used for preparing lead glazes was studied.
Harkort in 1934 studied the acid solubility of lead frits using a four
percent (4%) acetic solution as the agent and a controlled size of the frit
powder. The soluble lead was determined by the molybdate method. He
showed with this test that the lead monosilicate powder released lead in large
amounts, and that by introduction of alumina in the frit a practically complete
lead “fastness” was attained. Alumina was demonstrated to be more powerful
in this respect than is silica, calcia, or magnesia. Titania was also found to
exert a marked effect in lowering the solubility of lead frits and glazes. The
addition of one percent (1%) to two percent (2%) titania is very beneficial but
the coloring effect limits its use.
Harkort used the following frit in demonstrating how additions of boric
oxide raises the solubility of lead frits:
0.2 CaO
0.1 Al2O3
1.0 SiO2
0.8 PbO
To avoid increasing the solubility of the lead, it was suggested that boric
oxide be introduced as a borax frit. Alkali oxides are also detrimental. The
needed amount of alumina to attain acid resistance does not increase the
maturing temperature.
Rieke and Mields in 1935 reported on acid resistance of compositions in
the PbO-SiO2, K2O-PbO-SiO 2, and Na2O-PbO-SiO2 systems. The resistance
of these frits to four percent (4%) acetic acid was determined by the
electrolytic separation as PbO2. The tests were conducted on that portion of
the powdered flux which passed through a German standard sieve DIN
Number 20 and remained on Number 40.
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It was found that a uniform increase or decrease of PbO in the binary
system, PbO·SiO2. does not give a corresponding rise or fall in the lead
solubility but shows alternately rapid rise in solubility, followed by a sharp
rise in resistance. Lowest lead solubility occurred in the region corresponding
to PbO·2SiO2 and PbO·4SiO2. With a constant alkali content the lead
solubility decreased with falling PbO content, totally insoluble frits being
obtained with ten percent (10%) to fifteen percent (15%) Na2O content and
thirty percent (30%) to twenty five percent (25%) PbO content. Frits with
five percent (5%) alkali content showed the greatest resistance with forty five
percent (45%) to fifty percent (50%) PbO, yet further increases in the PbO
content produced only slight increases in solubility. In the constant-alkali
series, the solubility increased sharply with increasing PbO content from a
point which varies in the two alkali-lead-silicate systems. Frits with high
alkali contents showed this sharp rise with lower PbO contents more than low
alkali frits. In the constant PbO content frits, the high lead fluxes showed a
rapid rise in solubility with increasing alkali content. With a decrease in the
PbO content the frits became more stable, even with increasing alkali
content. The lowest PbO content series showed increasing stability with
alkali additions up to ten percent (10%). Plotting the solubility of the frits of
both series in the two 3-component systems, it is seen that the region of
stable frits (lead solubility below one percent (1%)) in the Na2O series with
low Na2O content, beginning with forty five percent (45%) SiO2, extends at
fifty percent (50%) SiO2 over the whole range of composition investigated.
The totally insoluble frits were found in the range of lowest PbO content (2530%) and medium alkali content (10-15%).
Koenig, in work sponsored by the United States Potters Association,
worked primarily on the development of highly acid resistant lead frits. The
use of a highly acid-resistant lead frit in a fritted glaze was considered to be
an important precautionary measure in handling the material in the plant.
The lead solubility of a lead frit in dilute acid is closely related to its
chemical composition. For example, alumina strongly increases the acid
resistance of the lead frit while the presence of alkalis and especially boric
oxide reduces it. The detrimental effects of boric oxide are clearly marked.
Lead borate shows complete solution of its lead content under test conditions
used in this work. Lead borosilicates are highly soluble. A high boric oxide
content is chiefly responsible for the poor acid resistivity of many
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commercial lead frits examined. The action of alkalis in decreasing the acid
resistance is also pronounced, although it apparently is not as great as that of
boric oxide. Increasing the alumina content is effective in compensating for
high lead solubility resulting from boric oxide and alkalis in the frit. The
same result can be achieved by increasing the silica content, although silica is
not as powerful in this respect as is alumina. Zinc oxide, barium oxide, and
calcium oxide are other glaze constituents effective in decreasing the lead
solubility in the order given, with the latter having the greatest beneficial
effect. Other effective oxides in increasing acid resistance include those of
beryllium, titanium and zirconium.
The lead solubility in weak acid from lead bisilicate is progressively
reduced by the addition of alumina. Of a series of lead aluminosilicates
studied, the deformation eutectic composition (1 PbO 0.254 A12O3 1.91
SiO2) was recommended. This frit is about six times as acid-resistant as the
lead bisilicate and is more readily prepared.
The solubility of the lead content of the frit in weak acid was shown to
increase with decreasing particle size so the extent of grinding should be
optimized to provide the desired overall glaze properties.
Highly acid resistant lead frits can and should be employed in dinnerware glaze formulation. Two recommended methods are as follows:
(1) The use of a single lead frit. This requires a high fritted content in
the glaze, since the boric oxide and alkalis introduced in the lead fruit must
be compensated for by larger amounts of alumina, silica etc. The trend
towards fritting the entire glaze composition provides for highly acid
resistant lead frits (using 8-12% clay in the mill batch) or less than one
percent colloidal material (fast-fired glazes) to suspend the frit.
(2) The use of two frits. The lead frit has the maximum acid resistance
attainable from the glaze constituents. The boric oxide and alkalis are
introduced in a second (leadless) frit. (At the time this work was done this
approach was most highly recommended. Since then the trend has been
towards glazes with higher fritted content, circa ninety percent (90%) frit).
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Solubility Tests On Frit Powders
Harkort used a method analogous to that applied to glasses to test the lead
solubility of frits. The frit was sized and the fraction between 144 and 400
mesh was washed with alcohol to remove all dust. A thirty-minute test with
boiling four percent (4%) acetic acid was employed. The soluble lead was
determined by the molybdate method. Koenig used a similar test in research
sponsored by the United States Potters Association.
Currier developed a standard method for determining leachable lead in
lead frits. The procedure is sufficiently simple for use by control laboratories
whereby duplicate results can be obtained. It is based on leaching the lead at
40º ± 0.5ºC from frit -180, +200 mesh particles) with 0.137 N HCl and
precipitating the lead as PbCrO4. The following data reported by Currier are
of interest:
Frit
Designation
PbO
(%)
Al2O3
(%)
SiO2
(%)
Leachability
(%)
A
65.0
2.0
33.0
B-1
61.3
7.1
31.6
B-2
61.4
3.0
35.6
C-1
64.8
0.12
34.9
C-2
64.9
1.17
33.8
C-3
64.8
3.34
32.1
C-4
64.8
5.47
30.1
C-6
64.7
1.97
33.2
1.58
1.58
0.39
0.38
0.29
0.30
1.68
1.68
1.86
1.88
0.64
0.64
0.85
0.85
0.95
0.94
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C-7
64.4
0.99
34.5
C-8
60.15
6.36
32.87
D-3
17.2
8.25
5.17 (B2O3)
1.38
1.38
0.55
0.55
0.32
0.32
*Percent of PbO in original sample.
Bennett and Vaughan reported on various chemical methods for the
determination of lead in solution derived from attack of hydrochloric acid on
lead frits. The accuracy of the chromate method was found to be as good as
that of the sulfide-sulfate method, provided that certain interfering ions are
absent. The chromate method has the advantage of simplicity and increased
rapidity, complete determinations being carried out in twenty-four (24) hours.
The method is not so reliable as the sulfate method if other ions are in the
solution from which the lead is to be precipitated. but this is not likely to be
the case where solubilities of lead bisilicate frits are being determined. If,
however, other uses are found for the method, care must be taken to ensure
the absence of interfering ions.
Later Bennett presented details on a method for determination of lead
solubility, by precipitation of the lead as sulfide, followed by determination
as chromate. The sulfide-chromate method was claimed to be a useful
technique for determining the lead solubility of glazes containing elements
liable to cause contamination of the lead chromate precipitate, when the
straight chromate method is used. It is more rapid than the sulfide-sulfate
method, and is capable of eliminating the same interfering elements. When
using the straight chromate method the chief cause of contamination is SiO2,
which is precipitated in a gelatinous form. Much smaller quantities of Al, Fe
and Ti are precipitated. When Ba is added to the glaze in a soluble form,
BaCrO4 is precipitated with the PbCrO4.
Lenk reported on additions to essentially a lead mono- silicate frit to
markedly improve its acid resistance when subjected to the British Home
Office Test (boiling 0.25% HCl). A fourteen percent (14%) addition of the
following frit to the frit batch reduced the solubility to one percent (1%):
0.915 Na2O 0.824 SiO2
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0.085 CaO 0.895 TiO2
0.313 F2
The addition had little effect On the softening temperature of the frit and
it increased the maturing temperature Of typical glazes by not more than
30ºC. without adversely affecting the other properties. The simultaneous
addition of A12O3 to the frit batch further decreased the solubility to 0.3%.
Some of the earlier solubility tests did not specify the particle size of the
frit powder being tested. The importance of doing this was soon realized.
Data were presented in reports of several investigations (2, 44 et al.) which
showed the increase in solubility with increasing fineness of the lead frit
particles. Aside from the intrinsic solubility of the specific frit composition,
the degree of fineness of the frit powder was clearly the principal factor of
the various test parameters. The rapid increase in lead solubility with
decreasing particle size, not only required that this be controlled in testing,
but also emphasized the necessity of not reducing the size beyond the
necessary minimum in practice. The following data (44) are for the solubility
of the same frit ground to different degrees of fineness:
Ground frit
Particle size range
A
% Composition
B
C
D
Under 3 microns
diameter
3-6
6-12
12-24
24-36
36-48
48-96
Over 96
4.1
4.2
7.8
10.3
1.6
2.6
4.0
0.8
7.1
14.8
65.0
2.1
2.2
4.6
6.4
3.9
46.5
30.1
9.1
7.9
9.0
11.2
12.5
26.8
15.7
22.1
10.4
18.7
24.5
10.0
3.7
0.3
Solubility %
1.1
1.8
3.4
5.5
Podmore made an experimental and theoretical comparison of the
relationship between lead solubility and the specific surface of the lead frit.
The purpose was to be able to compare solubilities of frits which has been
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ground to different degrees of fineness. The relationship suggested, the socalled Podmore Factor was:
Lead Solubility x 100
Specific Exposed Surface
Smith also studied the relationship between solubility and specific surface
for lead bisilicate frits. It was shown that during the solubility test the dilute
acid must penetrate the frit particles to some depth, and that the removal of
lead is not simply a surface effect. An equation relating the solubility and
specific surface was derived.
Norris et al. reported on the physical aspects of solubility determination
and the relative importance of various test conditions. The relationship
between the lead extracted and the “acid-contract” time was found to be
essentially logarithmic for periods of a few hours. After this the rate of
extraction proceeds at a rate greater than logarithmic. The steepness of the
curves at periods of the order of 1-2 hours is such that the filtration time must
be small and reasonably constant. The relationship between solubility and
temperature was found to be linear over the range tested for normal frits, the
coefficients in a number of cases being of such a value that fairly close
temperature control is necessary for the accurate determination of solubility.
The behavior of the “coated” frits tested were different, the coefficient
increasing with temperature (British Patent Number 625,474 claims reduction
of solubility of high lead frit particles by coating with a layer of silica).
Norris and Bennett (44) reported the following data for the effect of
temperature on solubility of eight (8) frits:
Temp., ºF
35
50
65
80
95
110
A
0.4
0.3
0.4
0.4
0.6
0.9
B
5.2
6.2
6.8
7.8
8.7
9.3
C
8.2
8.9
9.7
10.3
11.0
11.8
Solubility % PbO
D
E
F
1.9
10.9
2.4
7.5
12.9
8.7
15.1
4.5
9.4
16.8
5.8
10.4 19.2
7.1
11.5 21.4
G
0.6
0.6
0.6
0.6
1.0
1.4
H
1.1
1.7
2.8
3.7
6.0
7.5
Norris et al. stated that when proper control of sampling total acid-contact
time, and temperature is exercised, the variability associated with the
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physical aspects of this (British) method of test is roughly equal to that of the
chemical estimation of the lead extracted. For solubilities of the order of five
percent (5%) and below, the standard deviation for the whole solubility
determination usually lies between 0.1 and 0.5%.
Lead Extraction From Glazed Surfaces
The literature contains voluminous references to the chemical durability
and solubility tests on glazes and other glass surfaces. In large part this
involved resistance to attack by water, acid and alkaline solutions. J. W.
Mellor presented an extensive review of the earlier work up to 1934 on the
durability of glazes.
The work of Geller and Creamer at the National Bureau of Standards,
reported in 1939, is of special interest in that it was comprehensive and it
involved the assistance of the United States Potters Association and the Food
and Drug Administration. Various tests were considered which used as
reactants, various fruit juices, citric acid and vinegar. One of these tests,
using five percent (5%) acetic acid or white distilled vinegar and
precipitation of lead by H2S, had extensive use prior to the development of
the Dithizone Test. The results indicate that glazes of one color only, among
those tested, had poor acid resistance. Two other glazes from one
manufacturer, however, were found to be marginal, and a third may cause
trouble if indicated corrective measures, such as a preliminary acid wash,
should be neglected by the manufacturer. For other glazes tested in no case
did they exceed one-half (1/2) part per million (intermediate shades of
yellow, brown, blue and gray). Except for the maroon glaze of one
manufacturer, the only specimens to give up more than two (2) parts per
million of Pb were the tangerine (reds) and the greens. Some of the dark
blues and yellows showed solubilities equal to, or in excess of, 1 part per
million.
Weyl and Rudow reported on the use of the Dithizone Test to determine
the lead solubility of earthenware glazes. The glazes were tested with four
percent (4%) acetic acid. The effect of repeated tests on both a high-lead and
a low-lead glaze was noted. A sharp fall in the quantity of lead release by the
glaze was observed after the first treatment with acetic acid.
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Relation of Glaze Structure to Durability
Increasing attention has been given to glass structural theory and its
application to glazes. A symposium on “Structure And Properties Of Glazes”
reported in the 1956 Transactions of the British Ceramic Society is of great
interest. Glaze properties, including durability, are considered on the basis of
structural considerations.
Moor in showing how the properties of the glassy matrix (glaze) depend
on its structure, reviewed the structural effects of introducing alkali oxide,
and alkaline earth oxides in silicate-, borate-, and borosilicate glasses and
also the structural role of alumina. Of main concern in this reference is the
effect of various oxides on the structural features and resistance to attack by
water (durability) as noted in part below.
The alkali ions are held comparatively weakly in the interstices of silicate
glasses, and at ordinary temperatures the energy of their thermal vibrations
may be sufficient to enable them to escape and to diffuse through the
structure. If they reach the surface they are trapped by moisture, to form the
hydroxide, and cannot return into the glass. The risk of deterioration
increases with increased alkali content. With low alkali contents the alkali
ions can constrain the structure sufficiently to form interstices in which they
are closely surrounded by the number of oxygens corresponding to the most
stable state of coordination. The effects of alkalis on deterioration depend on
their field strengths and on their ionic radii, but also on the sizes of the
interstices which can be formed or exist in the glass structure. The Li+ ion
enclosed by four oxygens is held quite strongly but in a larger interstice is
less strongly held and, being small, it can readily find its way through the rest
of the structure. The Na+ ion enclosed by six oxygens is held moderately
strongly, and it is conveniently housed in interstices which naturally occur in
the silica network. But if it is freed it can also pass through the network fairly
easily. The K+ ion is held fairly strongly when surrounded by eight to ten
oxygens but it is too large to pass through the interstices and cannot diffuse
nearly as rapidly as either the Na+ or Li+ ions. When mixtures of the alkalis
are present, the effects depend on the proportions of the alkalis in relation to
the proportions of the interstices of different sizes which can exist
simultaneously in the structure.
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Replacement of some alkali by boric oxide gives improved resistance to
attack by water due to fewer alkali ions being present and also, if the amount
of boron is small, it will all become four coordinated with oxygen. The BO4
tetrahedra are linked direct to the SiO4 tetrahedra, as structure-building units,
and the alkali ions which have donated the necessary oxygen are held close to
the BO4 tetrahedra, in interstices surrounded entirely by “double- bonded”
oxygens. The absence of any “single-bonded” O- ions in the groups of
oxygen surrounding the alkali ions which have donated their oxygen to the
four coordinated boron strengthens or stiffens the structure as a whole. It is
important, in considering the effect of boric oxide, to ensure that there is
enough alkali (or alkaline earth) for at least one-fifth of the boron to be four
coordinated; also the total molecular B2O3 content should, preferably, be not
greater than one-eighth of the molecular amount of SiO2 present. Boron
present in excess of the amount represented by the two requirements has an
increasingly harmful effect on the durability because the BO3-SiO2 linkages
are very weak.
Replacement of some alkali, or even some SiO2, by A12O3 improves the
resistance to attack by water. The effect is due to the formation of A1O4
tetrahedra, linked by “double-bonded” oxygens to SiO4 tetrahedra, the alkali
or alkaline earth ions which have donated the necessary oxygens being held
in close associations with the A1O4 groups. The alkali ions are thus less free
than they would be in the absence of A12O3; also, each A12O3 molecule
introduced causes one “gap” in the structure to be closed.
Divalent ions have marked effects on the durability as compared with the
binary alkali silicates and borates, but increase beyond about ten percent
(10%) has only slight further effect. Zinc oxide, by virtue of its ability to
form ZnO4 tetrahedra, each closely associated with two alkali ions, causes
these alkali ions to be held more strongly than they would be in ordinary
interstices surrounded entirely by SiO4 groups. Each ZnO4 tetrahedron causes
one gap to be closed, and in this way, as well as in holding the alkali ions
more strongly, contributes considerably to improve resistance to attack by
water.
Lead oxide behaves differently from the other divalent oxides. In a PbOSiO2 glass the proportion of PbO can be increased to correspond with 2.5
PbO·SiO2, in which the SiO4 tetrahedra cannot possibly be all linked together
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by the oxygens at their corners. The Pb2+ ions must, therefore, be capable of
forming “bridges” between the SiO4 groups to an extent which is far greater
than the bridging action of other divalent ions. It has been suggested that, in
silicate glasses, the Pb+ ion may form groups of the PbO4 type or, possibly,
with the Pb2+ ion surrounded eight oxygen ions.
Moore (53) concluded that the maximum durability corresponds
primarily, with the most compact and most strongly bonded structure, and
this type of structure usually gives a maximum softening point and a
minimum coefficient of thermal expansion.
Bloor (54) also discussed the structure of glazes in relation to certain
properties, e.g., fusibility, surface tension, viscosity, thermal expansion, etc.
Glazes were divided into two groups, those containing SiO2 and A12O3 but
no B2O3 and those containing all three. The reason for this is that Si4+ and
A13+ always have 4 coordination in glasses, while B3+ may have 3 and 4
coordination. For each glaze two factors were calculated:
R=
Sum Of Oxygens in Molecular Formula
Sum of Si and Al (and B in second group)
P=
Sum Of Oxygens in Molecular Formula
Sum of Network - Modifiers
R is a measure of the number of single-bonded oxygens. P is a measure of
the oxygens available per network-modifying ion. Normal glasses are said to
be those whose R values are less than 2.5 and whose P values are greater than
3.9. For the glazes studied by Bloor (54) containing Si4+ and Al3+ (no B3+ )
the limits found for R and P were 2.64 to 1.96 and 3.5 to 21.6 respectively.
The value R = 2. 64 and the high value for P were noted for high-lead glazes.
In these cases part of the lead may be present as a network-former and the
remainder as a network-modifier, or such exceptions may be due to the
stabilizing effect of the Pb2+ ion. Plotting P values versus glaze maturing
temperatures indicates that the effect of the number of network modifying
ions outweighs the effect of the type of network-modifying ion on the
fusibility of a glaze (which incidentally has been a long established
observation in glaze technology).
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The B2O3-containing glazes differ from the aluminosilicate glazes. It was
suggested that in normal glazes all the B2O3 and Al2O3 accept oxygens from
the modifier oxides, the Al2O3 forming A1O4 tetrahedra, and the B2O3 broken
B-O-B bonds, at the glaze-maturing temperature. When the glaze is cooled
slowly it is likely that B04 tetrahedra form. Glazes with very high B2O3
content (R<2) must contain BO3 triangles since there is insufficient oxygen
brought in by the modifiers to break the B-0-B bonds. These glazes would be
very soft, and may well have poor durability. R is generally less and P more
for the B2O3-containing glazes, which permit reduction in the number of
network-modifying ions in the glaze.
Smith (42) in 1949 examined the structure of a lead bisilicate type glass
from the basis of then current theory on glass structure; the latter was
reviewed in this reference.
The molecular formula for the commercial lead bisilicate as then in use
was given as:
0.9737 PbO
0.075 A12O3
0. 0156 CaO
0. 018 Fe2O3
1.802 SiO2
0.0107 K2O
For the above composition R = 2.51 and P = 4.67. Although P is greater
than 3.9, R is not less than 2.5; this lead bisilicate, therefore has a structure
composed of a spatial conglomerate of two-dimensional and one-dimensional
networks. In order to make the structure more resistant to attack, R must be
reduced to below 2.5. This can be done by the addition of glass-forming ions
- the practical ones available being B3+, Si4+ and Al 3+.
Structures bonded in three directions only are obviously inferior in
strength to those bonded in four directions, and hence - even though B3+ ions
will reduce R as rapidly as Al3+ and more rapidly than Si4+, the resultant
structure will definitely not be as strong as it would had SiO2 and Al2O3 been
added instead of B2O3; in fact, the addition of B2O3 will most probably give a
structure weaker than the original lead bisilicate. To increase the strength of
the bisilicate structure it is necessary to add glass-forming ions with
coordination numbers of 4. These are Si4+ and Al3+. It can be shown that
Al2O3 reduces the R value of the frit much more rapidly than SiO2. The
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addition of A12O3 in small quantities (even of the order of five percent (5%)
to the above frit brings its R value well inside the limit of R less than 2.5.
This gives a structure with a special conglomerate of three-dimensional and
two-dimensional networks and will be thus far more resistant to attack.
Ions of small charge and large size when introduced as their oxides, cause
breakages of the silicon-oxygen linkages, the extra O2- ions thus may be
taken up, and the large weakly-charged cations filling the interstices. The
introduction of these ions will therefore weaken the frit structure. Ions of
high charge and small size, e.g., Ti4+, Zr4+, when introduced as their oxides,
also cause breakage of linkages. These cations of strong field, when they fit
into the interstices, however, cause a strong contraction of the unsaturated
oxygen ions surrounding them and lead to a tight binding. This results in an
overall strengthening of the structure, up to a point, but if this limit is
exceeded devitrification will occur. When it acts as a network modifying
cation Al3+ will fall into this category. Thus the best method of increasing the
strength of the frit structure is to add alumina. This rapidly reduces the R
value of the structure, and absorbs the oxygen ions introduced by the PbO
into the network. It is also possible to increase the strength of the frit by
adding Si4+ ions and then to replace these extra Si4 ions isomorphously with
the same number of Al3+ ions, and add Pb2+ ions to maintain electroneutrality.
The R value of the structure will remain the same and so will its strength
since the slight expansion of the framework, caused by the replacement of
Si4+ by A13+ will be counterbalanced by the contraction caused by the Pb2+
ions also introduced. It may be that before the structure of maximum strength
(with a fixed PbO content) is reached (either by the replacement of SiO2 by
A12O3, or by merely adding A12O3, to the frit structure) it will be
impracticable to continue this strengthening because of the detrimental effect
on one or more of the physical properties of the frit. Smith (78) in a later
paper stated it is to be expected that additions of alumina to lead bisilicate, up
to the limited composition of twenty-one percent (21%) alumina and seventynine percent (79%) lead borosilicate, will steadily increase the strength of the
structure and hence its resistance to chemical attack. This is not due to any
chemical reaction which takes place, but to the structural change brought
about by the introduction of the Al 3+ ions.
Ainsworth (55) proposed a diamond pyramid indentation test for
investigating the structure of glazes based on a surface measurement. This
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provides a direct measurement of the strength property of the glaze. The
measurement is sensitive to composition changes and should be capable of
detecting changes of about one percent (1%) in alkali content, for example,
over wide ranges of composition. The effects of all the oxides investigated
were explained by considering three factors, the charge on the added cation,
its size, and the way in which it goes into the structure.
Fajans discussed the role of Pb2+ in glasses. The Pb2+ ion does not possess
the small size and high charge usually ascribed to network-forming cations.
The interesting behavior of Pb2+, which can form a glass of composition
2PbO·SiO2, is attributed to its highly polarizable nature. The tetragonal
structure of PbO in which four of the eight O2- surrounding one Pb2+ (18 + 2
outer electrons) are much closer to the latter than are the other four O2- is
connected with the high polarizability of Pb2+. If one assumes that this lack of
symmetry applies to the analogous situation of Pb2+ surrounded by O2- in a
silicate, it is possible to understand the glass forming ability of Pb2+.
Tindall and Franklin, in the A. T. Green Book, discussed the durability of
glass in relation to structure and composition - and, of special interest here the behavior of overglaze colors. It was noted that the behavior of each ion in
a glass is dependent on its immediate surroundings, so that the relationship
between glass composition and chemical durability is a complicated one,
particularly for the complex mixtures that form practical glasses and enamels.
Owing to volatilization and the influence of surface forces, the composition
of a glass surface will differ from that of the interior, as it will tend to contain
a higher proportion of those cations that lower the surface energy, e.g., Pb2+,
B3+. Even if the glass were homogeneous the conditions of the ions in the
surface layer would differ from those in the interior, for the fields of the latter
are completely screened by the surrounding ions, whereas those at the surface
are incompletely screened at one side. Although the effect is reduced by a
greater degree of polarization of the surface ions, considerable residual forces
remain unsatisfied and act as absorption centers for which reactions are
initiated. Normally these residual surface forces screen them- selves by
absorbing moisture and grease from the atmosphere, cleansing materials etc.
The behavior of overglaze colors was studied by the British Ceramic
Research Association. In view of the complicated relationship between the
temperature, the concentration and nature of the attacking solution, and the
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glass composition which determine the rate of chemical attack, it was
decided to investigate the overglaze colors using test conditions
corresponding as closely as possible to actual usage conditions. The
overglaze colors normally involve lead borosilicate or alkali-lead-borosilicate
glasses containing appreciable amounts of coloring materials. The effect of
inorganic detergents was in general in conformity with predictions that could
be made on the basis of glass structural theories. The results obtained with
organic washing agents were not predictable from structural theories. The
results with weak organic agent and conditions of attack have their own
individual relationship for glasses of a particular composition and these
relationships may differ for different types of glass. The leadless glazes were
on the whole much more susceptible to acid attack than were the lead glazes.
The influence of added constituents in a lead borosilicate glass did not
correspond to their influence in other glasses which have been investigated
and reported. Small amounts of B2O3 lowered the chemical resistivity and
A12O3 did not appear to have a beneficial effect, but a proportion of Na2O
actually improved the performance of many lead-based compositions, even if
it replaced SiO2-TiO2 gave no apparent improvement, but ZrO2 caused a
noticeable improvement its effect being increased by the presence of TiO2.
Since these overglaze colors contain a large amount of highly polarizable
lead ions, it was suggested that more study of this type glass is needed which
would give more insight into the nature of the glass structure.
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Appendix B:
Tests For Lead Extracted
From Glazed Surfaces
Tests For Lead Extracted From Glazed Surfaces
ASTM C738 - 94 (Reapproved 1999)
Standard Test Method for Lead and Cadmium Extracted from Glazed
Ceramic Surfaces.
1. Scope
1.1 This test method covers the precise determination of lead and
cadmium extracted by acetic acid from glazed ceramic surfaces. The
procedure of extraction may be expected to accelerate the release of lead
from the glaze and to serve, therefore, as a severe test that is unlikely to be
matched under the actual conditions of usage of such ceramic ware. This test
method is specific for lead and cadmium.
1.2 The values stated in SI (metric) units are to be regarded as the
standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if
any, associated with its use. It is the responsibility of the user of this standard
to establish appropriate safety and health practices and determine the
applicability of regulatory limitations prior to use.
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2. Summary of Test Method
2.1 Lead and cadmium are extracted from the test article by leaching with
4 % acetic acid for 24 h at 20 to 24°C (68 to 75°F) and are measured by
flame atomic absorption spectroscopy.
3. Interferences
3.1 There are no interferences when instrumental background correction
and light sources specific for lead and cadmium are used.
4. Apparatus
4.1 Atomic Absorption Spectrometer equipped with light sources (hollow
cathode or electrodeless discharge lamps) specific for lead and cadmium,
instrumental background correction, and a 4-in. ( I 02-mm) single slot or
Boling burner head. Digital concentration readout may be used. Use
air-acetylene flame, instrumental background correction, and operating conditions recommended by instrument manufacturer. Using these conditions,
characteristic concentration (concentration that gives 0.0044 absorbance)
should be approximately (+20 %) 0.2 and 0.45 ppm for Pb measured at 217.0
and 283.3 nm, respectively. Characteristic concentration should be approximately (+20 %) 0.02 ppm for Cd.
NOTE 1: 1 ppm = 1 mg/mL.
4.2 Lead Lamp, set at 283.3 or 217.0 nm.
4.3 Cadmium Lamp, set at 228.8 nm.
4.4 Glassware of chemically resistant borosilicate glass, to make reagents
and solutions. Clean by rinsing with dilute nitric acid (10 % by volume)
followed by copious quantities of water.
5. Reagents
5.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests.
Unless otherwise indicated, it is intended that all reagents shall conform to
the specifications of the Committee on Analytical Reagents of the American
Chemical Society, where such specifications are available.2 Other grades
may be used provided it is first ascertained that the reagent is of sufficiently
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high purity to permit its use without lessening the accuracy of the
determination.
5.2 Purity of Water—Unless otherwise indicated, references to water shall
be understood to mean distilled water.
5.3 Acetic Acid (4 % by Volume)—Mix 1 volume of glacial acetic acid
with 24 volumes of water.
5.4 Detergent Wash—Use detergent designed for washing household
dishes by hand. Mix with lukewarm tap water according to product
instructions.
5.5 Lead Nitrate Solution (1000-ppm Pb)—Dissolve 1.598 g of lead
nitrate (Pb(NO3)2) in 4 % acetic acid and dilute to I L with 4 % acetic acid.
Commercially available standard lead solutions may also be used.
5.6 Hydrochloric Acid (1% by weight)—Mix I volume of concentrated
hydrochloric acid (HCl, sp gr 1.19) with 37 volumes of water.
5.7 Cadmium Solution (1000-ppm Cd)—Dissolve 0.9273 g of anhydrous
cadmium sulfate in approximately 250 mL of 1 % HC1 (see 5.6) and dilute to
500 mL with 1% HCI. Commercially available standard cadmium solutions
may also be used.
6. Procedure
NOTE 2: Take a method control through entire procedure. Use a
laboratory beaker with dimensions similar to ware being tested.
6.1 Preparation of Sample—Take, at random, six identical units and the
method control vessel and clean with detergent wash. Then rinse with tap
water followed by distilled water. Dry, and fill each unit with 4 % acetic acid
to within approximately 6 to 7 mm (A in.) of overflowing. (Distance shall be
measured along the surface of the item tested, not in the vertical direction.)
Record the volume of acid required for each unit in the sample (Note 3).
Cover each unit with fully opaque glass plate (so that extraction is carried out
in total darkness) o prevent evaporation of solution, avoiding contact between
cover and surface of leaching solution. (If opaque glass is not available,
cover glass with aluminum foil or other material to prevent exposure to
light.) Let stand for 24 h at room temperature (20 to 24°C (68 to 75°F)).
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NOTE 3: If the sample unit is extremely shallow, or if it has an irregular
brim, the analyst should be aware of evaporation of leaching solution. If such
a loss is anticipated, record the headspace upon filling the vessel to 6 to 7
mm (/ in.) of the brim. Adjust to the same headspace with 4 % acetic acid
after the 24-h leaching. Stir the solution and proceed with the determination.
6.2 Preparation of Standards:
6.2.1 Lead Standards—Dilute lead nitrate solution (see 5.5) with acetic
acid (see 5.3) to obtain working standards having final concentrations of 0.0-,
1-, 2-, 3-, 5-, and 10-ppm Pb.
6.2.2 Cadmium Standards—Dilute cadmium stock solution (see 5.7) with
acetic acid (5.3) to obtain working standards having final concentrations of
0.0-, 0.1-, 0.2-, 0.3-, 0.5-, and l.0-ppm Cd.
6.3 Determination of Lead by Atomic Absorption—Stir the leaching
solution and remove a portion by pipetting into a clean flask. Use lead lamp
(4.2) and concomitantly measure absorbance of lead working standards
(6.2.1) and leach solutions. Dilute with 4 % acetic acid if leach solutions
contain over 10-ppm Pb. Concentrate leach solutions containing less than
l-ppm Pb by accurately transferring a minimum of 50.0 mL of solution to a
250-mL beaker and evaporating almost to dryness on a steam bath. Add 1
mL of HC1, then evaporate to dryness. Dissolve the residue in 4 % acetic
acid by adding exactly 0.1 volume of solution taken for concentration, cover
with watch glass, and swirl to complete dissolution. Calculate lead
concentration (ppm Pb) of leach solution by comparison to standard curve.
6.4 Determination of Cadmium by Atomic Absorption— Proceed as in 6.3
using the cadmium lamp (4.3) and standards (6.2.2). Dilute with 4 % acetic
acid if leach solutions contain over l ppm Cd. Concentrate leach solutions
containing less than 0.l-ppm Cd as in 6.3.
7. Report
7.1 Report the type of units tested, the volume of acid used, and the lead
and cadmium leached in parts per million for each unit tested.
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8. Precision and Bias
8.1 Precision—In an analysis of variance study from eight laboratories,
the standard deviation between laboratories was 0.06 mg/L for lead and 0.007
mg/L for cadmium. The within laboratory precision had a standard deviation
of 0.04 mg/L for lead and 0.004 mg/L for cadmium. The standard deviation
for interaction between laboratories and samples is 0.06 mg/L for lead and
0.010 mg/L for cadmium. Reproducibility is defined as the square root of the
sum of the three component variances. The reproducibility was 0.10 mg/L for
lead and 0.013 mg/L for cadmium.
8.2 Bias—The bias of this test method is further limited by the ability to
obtain representative samples of the statistical universe being sampled. An
analysis of large populations (100 to 500) has shown that the lead and
cadmium release data conformed to a Pearson III distribution with a
coefficient of variation between 30 and 140 %, typically 60 %.
ISO Standard 6486: Ceramic Ware, Glass-Ceramic Ware, and Glass
Dinnerware In Contact With Food -- Release of Lead and Cadmium
Part 1: Method
Introduction
Lead and cadmium release from ceramic and glass ware surfaces is an
issue which requires effective means of control to ensure the protection of the
population against possible hazards arising from the use of improperly
formulated and/or processed ceramic, glass-ceramic, and glass dinnerware
used for the preparation, serving and storage of food and beverages. As a
secondary consideration, different requirements from country to country for
the control of the release of toxic materials from the surfaces of ceramic ware
present non-tariff barriers to international trade in these commodities.
Accordingly, there is a need to maintain internationally accepted methods of
testing ware for lead and cadmium release, and to define permissible limits
for the release of these toxic heavy metals.
The limits for lead and cadmium release specified in this standard are not
intended to be regarded as the maximum amount of these metals to which
exposure can be considered safe. They are levels which are consistent with
good manufacturing practice in the respective industries, harmonize
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regulatory levels in principal world markets, and reflect a general objective of
reducing overall exposure to these metals.
1 Scope
This part of ISO 6486 specifies a test method for the release of lead and
cadmium from ceramic ware, glass-ceramic ware, and glass dinnerware
intended to be used in contact with food, but excluding porcelain enamel
articles.
This part of ISO 6486 is applicable to ceramic ware, glass-ceramic ware,
and glass dinnerware which is intended to be used for the preparation,
cooking, serving and storage of food and beverages, excluding articles used
in food manufacturing industries or those in which food is sold.
2 Normative References
ISO 1042 : 1983,
ISO 3585 : 1991,
ISO 3696 : 1987,
ISO 385-2 : 1984,
ISO 4788 : 1980,
ISO 648 : 1977,
ISO/DIS 8655-2
ISO/DIS 8655-4
Laboratory glassware - One-mark volumetric flasks
Borosilicate glass -- properties
Water for analytical laboratory use - Specifications
and test methods.
Laboratory glassware - Burettes - Part 2: Burettes for
which no waiting time is specified.
Laboratory glass ware - Graduated measuring
cylinders
Laboratory glassware - One-mark pipettes
Piston and/or plunger operated volumetric apparatus
(POVA) - Part 2: Operating considerations
Piston and/or plunger operated volumetric apparatus
(POVA) - Part 4: Specifications
3 Definitions
For the purpose of this part of ISO 6486, the following definitions apply.
atomic absorption spectrometry (AAS): spectroanalytical method for
qualitative determination and quantitative evaluation of element
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concentrations. The technique determines these concentrations by measuring
the atomic absorption of free atoms.
atomic absorption: absorption of electromagnetic radiation by free atoms
in the gas phase. A line spectrum is obtained which is specific for the
absorbing atoms.
bracketing technique: Analytical method consisting of bracketing the
measured absorption or machine reading of the sample between two
measurements made on calibration solutions of neighboring concentrations
within the optimum working range.
calibration function: Function relating atomic absorption instrument
readings, either in absorption or in other machine units, to the concentration
of lead or cadmium which generated the instrument reading.
ceramic ware: Ceramic articles which are intended to be used in contact
with foodstuffs, for example foodware made of china, porcelain and
earthenware, whether glazed or not.
cooking ware: Foodware, specifically intended to be heated in the course
of preparation of food and drinks by conventional thermal methods and by
microwaves.
dinnerware: Articles specially intended for the serving of food on the
table, including plates, dishes and salad bowls, but excluding volumetric
ware typically used for beverages, such as goblets and decanters.
direct method of determination: Analytical method consisting of inserting
the measured absorption or machine reading into the calibration function and
deducing the concentration of the analyte.
drinking rim: 20 mm wide section of the external surface of a drinking
vessel, measured downwards from the upper edge along the wall of the
vessel.
extraction solution: acetic acid, 4% (V/V), recovered after the extraction
test and which is analyzed for lead and cadmium concentration.
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flame atomic absorption spectrometry (FAAS): Atomic absorption
spectrometry that uses a flame to create free atoms of the analyte in the gas
phase.
flatware: Ceramic or glass ware having an internal depth not exceeding
25 mm, measured from the lowest point to the horizontal plane passing
through the point of overflow.
foodware: Articles which are intended to be used for the preparation,
cooking, serving and storage of food or drinks.
glass ceramic: Inorganic material produced by the complete fusion of raw
materials at high temperatures into a homogeneous liquid which is then
cooled to a rigid condition and temperature treated in such a way as to
produce a mostly micro crystalline body.
glass: Inorganic material produced by the complete fusion of raw
materials at high temperature into a homogeneous liquid which is then cooled
to a rigid condition, essentially without crystallization. The material may be
clear, colored, or opaque, depending on the level of coloring and opacifying
agents used.
hollowware: Ceramic ware having an internal depth greater than 25 mm,
measured from the lowest point to the horizontal plane passing through the
point of overflow. Hollowware is subdivided into three categories based on
volume:
·
small: hollowware with a capacity of less than 1,1 litres.
·
large: hollowware with a capacity of 1,1 litres or greater;
·
storage: hollowware with a capacity of 3,0 litres or greater; cups and
mugs: small ceramic hollowware commonly used for consumption of
beverages, for example, coffee or tea at elevated temperature. Note:
cups and mugs are vessels of approximately 240 ml capacity with a
handle. Cups typically have curved sides whereas mugs have
cylindrical sides.
optimum working range: Range of concentrations of an analyte over
which the relationship between absorption and concentration is practically
linear.
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reference surface area: The area that is intended to come into contact with
foodstuffs in normal use.
test solution: The solvent used in the test to extract lead and cadmium
from the article. Acetic acid, 4% (V/V)
vitreous enameled ware: Metallic articles coated with a vitreous inorganic
coating bonded by fusion at temperatures above 500o C.
4 Principle
Silicate surfaces are placed in contact with 4 % (V/V) acetic acid solution
for 24 h at (22 ± 2)oC to extract lead and/or cadmium, if present, from the
surfaces of the articles or test specimens.
The amounts of extracted lead and cadmium are determined by flame
atomic absorption spectrometry (FAAS). In routine tests other equivalent
analysis methods may be used.
5 Reagents and Materials
5.1 Reagents
All reagents shall be of recognized analytical grade. Distilled water or
water of equivalent purity (grade 3 water complying with the requirements of
ISO 3696) shall be used throughout.
Acetic acid, (CH3COOH), glacial, r = 1.05 g/ml.
Acetic acid test solution, 4% (V/V) solution. Add 40 ml of acetic acid
(5.1.1) to distilled water, and dilute to 1 litre. This solution shall be freshly
prepared for use. Proportionately greater quantities may be prepared.
Lead stock solution Prepare analytical stock solutions containing 1000 ±
1 mg of lead per litre in the test solution (5.1.2). Alternatively, an
appropriate commercially available standardized lead AAS solutions may be
used.
Cadmium stock solution Prepare analytical stock solutions containing
1000 ± 1 mg of cadmium per litre in the test solution (5.1.2). Alternatively,
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an appropriate commercially available standardized cadmium AAS solution
may be used.
Lead standard solution Dilute the lead stock solutions ten-fold with test
solution (5.1.2) to produce a lead standard solution which is 100 mg/l Pb, or
0,1 g of lead per litre.
Cadmium standard solution Dilute the cadmium stock solutions 100fold with test solution (5.1.2) to produce a cadmium standard solution which
is 10 mg/l Cd, or 0,01 g of cadmium per litre.
Notes:
Standard solutions may be kept in suitable, aged, tightly closed containers
(i.e. polyethylene) for four weeks without loss of quality. New containers
may be aged by filling with standard solution and allowing to stand for 24 h.
The aging solution is discarded.
Use one-mark glass pipettes or precision piston pipettes with a fixed
stroke, typically 1000 ml and 500 ml, and appropriate volumetric glassware
(e.g. 500 ml to 2000 ml) to prepare proper calibration solutions by dilution of
the standard stock solutions (5.1.5 and 5.1.6) with test solution (5.1.2). Keep
the solutions in suitable and aged containers. Renew these solutions every
four weeks.
5.2 Materials and Supplies
5.2.1 Paraffin wax, with a high melting point.
5.2.2
Washing agent, commercially available non-acidic manual
dishwashing detergent in dilution recommended by manufacturer.
5.2.3
Silicone sealant, capable of forming a ribbon of sealant
approximately 6 mm in diameter. This sealant shall not leach acetic acid,
cadmium or lead to the test solution (5.1.2).
6 Apparatus
6.1 Atomic absorption spectrometer,
Atomic absorption spectrometer equipped with light sources [hollow
cathode or electrodeless discharge lamps] specific for lead and cadmium,
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instrumental background correction, and a single slot [approximately 100
mm] or Boling burner head. Digital concentration readout may be used. Use
air-acetylene flame and operating conditions recommended by the instrument
manufacturer. Using these conditions, characteristic concentration
[concentration that gives 0.0044 absorbance] should be approximately
[±20%] 0.2 mg/l for Pb measured at 217.0 nm. Characteristic concentration
should be approximately [±20%] 0.02 mg/l for Cd measured at 228.8 nm.
Note: Where appropriate, a wavelength of 283,3 nm may be used for the
analytical confirmation of lead.
6.2 Accessories
Assorted glassware as required, made of borosilicate glass as specified in
ISO 3585.
Burette of capacity 25 ml, graduated in divisions of 0,05 ml, complying
with ISO 385-2, class B or better.
Covers for the articles under test, e.g. plates, watch-glasses, Petri dishes of
various sizes. Covers must be opaque if a darkroom is not available.
One-mark pipettes of capacities 10 ml and 100 ml, complying with ISO
648, class B or better. Other sizes as required.
One-mark volumetric flasks of capacities 100 ml and 1000 ml, complying
with ISO 1042, class B or better. Other sizes as required.
Precision piston pipettes with a fixed stroke, typically 1000 ml and 500 ml.
Straight edge and depth gage calibrated in millimeters.
7 Sampling
7.1 Priority
When selecting samples from a mixed lot of foodware, articles having the
highest surface area/volume ratio within each category should be given
preference. Articles that are highly colored or decorated on their food
contact surfaces should be especially considered for sampling.
7.2 Sample size
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It is desirable to develop a system of sampling control that is appropriate
to the circumstances. In no case shall less than four items be measured.
Each of the articles shall be identical in size, shape, color and decoration.
7.3 Preparation and preservation of test samples
Samples of ware shall be clean and free from grease or other matter likely
to affect the test. Briefly wash the specimens at a temperature of about 40o C
with a solution containing a non-acidic detergent. Rinse in tap water and
then in distilled water or water of equivalent purity. Drain and dry in either a
drying oven or by wiping with a new piece of filter paper. Do not use any
sample that shows residual staining. Do not handle the surfaces to be tested
after cleaning.
If an area of the surface of the sample is not intended to come into contact
with foodstuffs in normal use, other than the interior of any lid, cover this
area after the initial washing and drying with a protective coating such as
paraffin wax or silicone which will withstand the effect of the test solution
and which will not release any detectable levels of lead or cadmium into the
test solution.
8 Procedure
8.1 Determination of reference surface area for flatware
Place a specimen on a sheet of smooth paper and draw a contour around
the rim. Determine the enclosed area by a suitable means. One
recommended method is to cut out and weigh the enclosed area and to
determine the area by comparison of the weight with the weight of a
rectangular sheet of known area. Record this area, SR, in square decimeters
to two decimal places. For circular articles the reference surface area may be
calculated from the diameter of the article.
8.2 Preparation of articles which cannot be filled.
Articles are normally filled to within 6 mm of overflowing as measured
along the sloping side of flatware, or to within one mm of the rim as
measured vertically for hollowware. Articles which cannot be filled in this
manner to produce an acid depth at the deepest point of at least 5 mm are
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defined as non-fillable. Articles of this type may be tested by one of the
following methods.
Standard articles may be fitted into a silicone rubber mold which forms a
water-tight seal with the article and which encroaches no more than 6 mm
from the rim and forms a depth of at least 5 mm but no more than 25 mm.
Specimens prepared in this way are tested as fillable flatware articles.
A bead of silicone sealant may be formed around the edge of the article to
permit filling of the article to a depth of at least 5 mm but no more than 25
mm. The bead shall encroach no more than 6 mm from the rim of the article.
Specimens prepared in this way are tested as fillable flatware articles
The article may be coated on all surfaces except the reference surface with
melted paraffin wax and subsequently tested by immersion in test solution.
Specimens prepared in this way are tested as non-fillable flatware articles.
8.3 Extraction
8.3.1 Extraction temperature
Conduct the extraction at a temperature of (22 ± 2)oC. When cadmium is
to be determined, conduct the extraction in the dark.
8.3.2 Leaching
8.3.2.1 Fillable Articles: Fill each specimen with test solution (5.1.2), to
1 mm of overflowing measured vertically for hollowware or 6 mm from
overflowing as measured along the surface of flatware. For flatware
determinations measure and record the volume of acetic acid, 4%, used to fill
the article. Cover the specimen. Leach for 24 h ± 30 min.
8.3.2.2 Non-Fillable Articles: These articles, which have been masked
with paraffin wax according to 8.2.c, are placed in a suitable vessel such as
borosilicate glass of suitable size and test solution (5.1.2) is added in
sufficient quantity to completely cover the sample. Record the amount of
acetic acid added to an accuracy of 2%. Leach for 24 h ± 30 min.
8.3.3 Sampling of the extraction solution for analysis
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Prior to sampling, mix the extraction solution by stirring or other
appropriate method that avoids loss of the extraction solution or abrasion of
the surface. Remove a sufficient amount of the extraction solution with
pipette and transfer it to a suitable storage container.
Analyse the extraction solution as soon as possible since there is a risk of
adsorption of lead or cadmium onto the walls of the storage container,
particularly when Pb and Cd are present in low concentrations.
8.4 Drinking rim and other special tests
Note: This is an optional procedure for evaluating drinking rims.
Cups may be tested by marking each of four units 20 mm below the rim
on the outside. Each cup is placed inverted in a suitable laboratory glassware
container with a diameter between 1.25 and 2.0 times that of the cup. Add
sufficient 4% acetic acid to the glassware container to fill to the 20 mm mark
on the cup. Let stand for 24 h at (22 ± 2)oC (in the dark for cadmium
determinations) and protect from excessive evaporation. Before sampling the
leachate, add 4% acetic acid to the glass container as necessary to re-establish
the 20 mm level. Determine lead and cadmium by AAS and report the
results as mg/article.
8.5 Calibration
Set up the atomic absorption spectrometer according to the
manufacturer’s instructions using wavelengths of 217 nm for lead
determination and 228,8 nm for cadmium determination with an appropriate
correction for background absorption effects. Note: Where appropriate, a
wavelength of 283,3 nm may be used for the analytical confirmation of lead.
Aspirate the zero member of the set of calibration solutions and adjust
zero. Aspirate the set of calibration solutions, prepared by dilution of the
standard solution with test solution (5.1.2) and prepare calibration curves
over a linear range. Suggested ranges:
0,5 - 10,0 mg/l Pb
0,05 - 0,5 mg/l Cd
8.6 Determination of lead and cadmium
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Set up the spectrometer as described previously. Aspirate distilled water
and then acetic acid, 4%, and verify the absorbance is zero. Aspirate the
extraction solution, interspersed with test solution (5.1.2) and record the
absorbance values of the extraction solutions.
If the lead concentration of the extraction solution is found to be higher
than 10 mg/1, dilute a suitable aliquot portion with test solution (5.1.2) to
reduce the concentration to less than 10 mg/1.
Similar considerations apply to the determination of cadmium.
9 Expression of results
9.1 Bracketing technique
The lead or cadmium concentration, ro, expressed in milligrams per litre
of the extraction solution, is given by the formula
éæ A - A1 ö
ù
r o = êç o
÷ ( r 2 - r1 ) + r1 ú d
ëè A2 - A1 ø
û
where
Ao is the absorbance of the lead or cadmium in the extraction solution;
A1 is the absorbance of the lead or cadmium in the lower bracketing
solution;
A2 is the absorbance of the lead or cadmium in the upper bracketing
solution;
r1 is the lead or cadmium concentration, in milligrams per litre, of the
lower bracketing solution;
r2 is the lead or cadmium concentration, in milligrams per litre, of the
upper bracketing solution
NOTE: If the extraction solution was diluted, an appropriate correction
factor, d, is used in the formula.
9.2 Calibration curve technique
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Read the lead or cadmium concentration directly from the calibration
curve or from the direct read-out.
9.3 Calculation of release of lead and cadmium from flatware
The lead or cadmium released per unit area from flatware, Ro, expressed
in milligrams per square decimeter, is given by the formula
R
o
=
r
o
×V
S
R
where:
ro is the lead or cadmium concentration, expressed in milligrams per
litre, of the sample extract solution.
V is the filling volume of the specimen, expressed in litres
SR is the reference surface area of the article, expressed in square
decimeters.
For hollowware articles report the result to the nearest 0,1 mg of lead per
litre and to the nearest 0,01 mg of cadmium per litre
For flatware report the result to the nearest 0,1 mg of lead per square
decimeter and to the nearest 0,01 mg of cadmium per square decimeter. Also
report the concentration of lead and cadmium in the leach solution to the
nearest 0,1 mg of lead per litre and to the nearest 0,01 mg of cadmium per
litre.
10 Reproducibility And Variability
Lead and cadmium release measurements from ceramic foodware are
subject to analytical reproducibility errors and sampling variability. The
material presented in this section are of scientific and technological interest
but are not of normative or statutory value in the context of this ISO standard.
10.1 Reproducibility
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Three types of determination errors occur in the analytical measurement
of lead and cadmium concentrations. Each is listed in table 1 with an
approximate value for the standard deviation of each1.
TABLE 1 -- SOURCES OF VARIATION IN ANALYTICAL
DETERMINATION OF PB AND CD
Source of Variation
Analysis, within laboratory
Analysis, between laboratories
Laboratory x Sample Interaction
Reproducibility
Standard
Deviation, Pb
Determination
, [mg/l]
0,04
0,06
0,06
0,094
Standard
Deviation, Cd
Determination,
[mg/l]
0,004
0,007
0,01
0,012
The statistical interaction term, row 4 in table 1, reflects the failure of the
differences in sample analyses to be the same from laboratory to laboratory.
A detailed discussion may be found in elementary statistical texts which
address Analysis of Variance (ANOVA) methods. The reproducibility is the
square root of the sum of the squares of the standard deviations from the
three sources of variation.
10.2 Variability
Analytical reproducibility is quite good compared to the intrinsic
variability of the extraction behavior of glass and ceramic surfaces. This
variability, termed sampling variability, is by far the greatest source of
experimental error. Moore2 has shown that the coefficient of variability for
lead and cadmium release for large samples is typically 60%. Thus, the true
average lead release value for a large population must be approximately 0,58
mg/l in order to avoid one of four test specimens from exceeding a 2 mg/l
limit 1 in 10 000 times. The table 2 illustrates the effect of population mean
1
ASTM Standard Test Method for Lead and Cadmium Extraction from Glazed
Ceramic Surfaces, C738-94, American Society for Testing and Materials,
Philadelphia, PA 1994.
2
Moore, F., Transactions, Journal of British Ceramic Society, Vol. 76 (3), 1977, pp.
52-57.
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and standard deviation values on the probability that 1 in 4 or 1 in 6
specimens will exceed a 2 mg/l limit value.
TABLE 2: PROBABILITIES OF EXCEEDING 2 MG/L LIMIT
Population Mean
0,4
0,8
1,2
0,4
0,8
1,2
Population
Std. Dev.
0,24
0,48
0,72
0,12
0,24
0,36
Probability of
1 in 4 > 2 mg/l
<0,00001
0,13826
0,75836
<0,00001
0,00002
0,32568
Probability of
1 in 6 > 2 mg/l
<0,00001
0,20005
0,88122
<0,00001
0,00004
0,44627
11 Test Report
The test report shall include the following information:
·
a reference to this part of ISO 6486;
·
identification of the sample, including type, origin, and destination;
·
the surface area or the reference surface area and the filling volume
or contact volume for non-fillable articles and test specimens;
·
·
the number of samples tested;
the test results, expressed as individual values for each specimen and
the mean value for test sample groups. Test values for hollowware
articles should be reported to the nearest 0,1 mg of lead per litre and
to the nearest 0,01 mg of cadmium per litre. Test values for flatware
should be reported to the nearest 0,1 mg/dm2 of lead and to the nearest
0,01 mg/dm2 of cadmium. NOTE: As supplementary information, the
concentration of solutions from tests on flatware articles should also
be included and reported to the nearest 0,1 mg/l of lead and to the
nearest 0,01 mg/l of cadmium.
·
any unusual features noted during the determination;
·
any optional tests, or tests not included in this International Standard
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ISO6486: Permissible limits
12 Permissible limits
The permissible limits for lead and cadmium release are given in the following table.
Type of Ware
n
Permissible Limit Criterion
Unit of
measure
Lead
Limit
Cadmium
Limit
Flatware
Small Hollowware
Large Hollowware
Storage Hollowware
Cups and Mugs
Cooking Ware
4
4
4
4
4
4
Mean £ Limit
All specimens £ Limit
All specimens £ Limit
All specimens £ Limit
All specimens £ Limit
All specimens £ Limit
mg/dm2
mg/l
mg/l
mg/l
mg/l
mg/l
0,8
2,0
1,0
0,5
0,5
0,5
0,07
0,50
0,25
0,25
0,25
0,05
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Appendix C:
Materials Handling
Every type of lead material can be handled and controlled with safety if
proper modern equipment is provided for the protection of the health of the
industrial worker. This is borne out by the many plants that have as their
primary function the processing and handling of lead and its compounds and
do so with entire success and safety.
Industrial health precautions are by no means confined to the use of lead
and its compounds. Silica, beryllium, cadmium, antimony, selenium,
tellurium and other elements and compounds present extremely hazardous
exposure problems. While this text primarily addresses the handling of lead
compounds, it should be remembered that similar precautions are necessary
in plants where other compounds are used that generate dust hazards.
Proper handling of lead compounds in the ceramic industry requires:
1. Proper Plant Hygiene
2. Proper Instruction And Supervision Of Workers
3. Regular Checks By Plant Physician Or Medical Director.
Proper Plant Hygiene
·
Adequate pre-employment examination of all plant workers. This
should include previous working histories, chest X-rays and blood
examinations.
·
All operations which disperse dust should be controlled by closed
systems of local exhaust ventilation.
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Lead compounds for the ceramic user are conveniently packaged so that
unloading, plant storage and movement to the location where the packages
are to be emptied present few exposure problems.
Dust hoods should be located where dry materials are charged or
discharged from processing equipment and should surround the operation as
completely as possible. Air velocity of 200 linear feet per minute at the hood
face is the generally accepted standard requirement. Exhaust from hood face
is the generally accepted standard requirement. Exhaust from hood
ventilating systems must be discharged outside of the plant, preferably into
cloth-screen dust arrestors having not less than one-half square foot of
effective area of cloth per cubic foot of dust-laden air per minute passing
through it.
Bins, elevators, chutes, covered conveyors, covered mixers etc., can
usually be made dust-free by drawing off dust-laden air at critical points. By
maintaining a slight negative pressure in the closed equipment, some air may
enter the system, but dust will not escape. Relatively small air volumes are
required to achieve dustless operation for closed equipment. Exhausted dustladen air should be properly filtered before discharge outside the plant.
Dust control and elimination are important to all industries today and
there are many reliable and qualified manufacturers of equipment to do the
job properly.
·
Only where local or general control is impossible, workers should be
provided with respirators specifically approved by United States
Bureau of Mines for this type of protection.
·
Adequate washroom facilities should be provided. Hot water and
soak should be provided, along with individual hand towels. The
hand towels may be paper. Shower facilities are recommended.
·
Proper locker room facilities should be provided. Workers handling
toxic materials should have separate lockers for street clothes and
work clothes to prevent contamination.
·
A suitable and separate place should be provided for the workers to
eat.
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Workers should not eat or smoke on the job while handling toxic
materials.
Proper Instruction and Supervision
The purpose, correct use, and maintenance of respirators should be
explained to the workers and enforced.
There are several good types of approved respirators on the market. Be
sure that workers are provided with respirators that fit the individual.
Respirators assigned to one individual should be regarded as personal as a
toothbrush and should not be interchanged. Respirators worn by a single
workman usually tend to fit better and more comfortably after repeated use.
Spare respirator filters, head bands and other parts should be readily
available. Respirators must be maintained in order to be effective.
Air supplied respirators are preferred where it is practical to use them as
they are generally more comfortable and positive in their action.
Most authorities agree that the use of respirators is no substitute for
adequate plant engineering. In practical plant operation, however, situations
do arise during which engineering controls are being developed, or where
repairs are necessary. The use of respirators in these situations is extremely
important for proper plant hygiene.
Regular Checks By Plant Physician Or Medical Director
Physicians in the developed countries rarely have occasion to treat cases
of heavy metal intoxication or poisoning since these events are few and
isolated. Therefore, the plant doctor should be informed of any toxic
materials being handled and instructed on symptoms and treatment for the
specific hazards at a given plant. The plant doctor will then be in a position to
study the problem and outline a regular safety program.
Health Risks of Lead Compounds
Human exposure to lead occurs through a combination of inhalation and
oral exposure, with inhalation generally contributing a greater proportion of
the dose for occupationally exposed groups, and the oral route generally
contributing a greater proportion of the dose for the general population. The
effects of lead are the same regardless of the route of exposure (inhalation or
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oral) and are correlated with internal exposure, as blood lead levels. The main
sources of information for this section are the US EPA's Integrated Risk
Information System (IRIS), which contains information on the carcinogenic
effects of lead, the Agency for Toxic Substances and Disease Registry's
(ATSDR's) Toxicological Profile for Lead, and EPA's Supplement to the
Criteria Document on Lead. Other secondary sources include the Hazardous
Substances Data Bank (HSDB), a database of summaries of peer-reviewed
literature, and the Registry of Toxic Effects of Chemical Substances
(RTECS), a database of toxic effects that are not peer reviewed.
Assessing Personal Exposure
The amount of lead in the blood can be measured to determine if exposure
to lead has occurred
Exposure to lead can also be evaluated by measuring erythrocyte
protoporphyrin (EP), a component of red blood cells known to increase when
the amount of lead in the blood is high. This method has been commonly
used to screen children for potential lead poisoning.
Methods to measure lead in teeth or bones by X-ray fluorescence
techniques are available.
Health Hazard Information
Acute Effects:
Death from lead poisoning may occur in children who have blood lead
levels greater than 125 µg/dL and brain and kidney damage have been
reported at blood lead levels of approximately 100 µg/dL in adults and 80
µg/dL in children.
Gastrointestinal symptoms, such as colic, have also been noted in acute
exposures at blood lead levels of approximately 60 µg/dL in adults and
children.
Short-term (acute) animal tests, such as the LC50 test in rats, have shown
lead to have moderate to high acute toxicity.
Chronic Effects (Noncancer):
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Chronic (long-term) exposure to lead in humans can affect the blood.
Anemia has been reported in adults at blood lead levels of 50 to 80 µg/dL,
and in children at blood lead levels of 40 to 70 µg/dL.
Lead also affects the nervous system. Neurological symptoms have been
reported in workers with blood lead levels of 40 to 60 µg/dL, and slowed
nerve conduction in peripheral nerves in adults occurs at blood lead levels of
30 to 40 µg/dL.
Children are particularly sensitive to the neurotoxic effects of lead. There
is evidence that blood lead levels of 10 to 30 µg/dL, or lower, may affect the
hearing threshold and growth in children.
Other effects from chronic lead exposure in humans include effects on
blood pressure and kidney function, and interference with vitamin D
metabolism.
Reproductive/Developmental Effects:
Studies on male lead workers have reported severe depression of sperm
count and decreased function of the prostate and/or seminal vesicles at blood
lead levels of 40 to 50 µg/dL. These effects may be seen from acute as well
as chronic exposures.
Occupational exposure to high levels of lead has long been associated
with a high likelihood of spontaneous abortion in pregnant women. However,
the lowest blood lead levels at which this occurs has not been established.
These effects may be seen from acute as well as chronic exposures.
Prenatal exposure to lead produces toxic effects on the human fetus,
including increased risk of preterm delivery, low birthweight, and impaired
mental development. These effects have been noted at maternal blood lead
levels of 10 to 15 µg/dL, and possibly lower. Decreased IQ scores have been
noted in children at blood lead levels of approximately 10 to 50 µg/dL.
Human studies are inconclusive regarding the association between lead
exposure and birth defects, while animal studies have shown a relationship
between high lead exposure and birth defects. (1,6)
Cancer Risk:
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Human studies are inconclusive regarding lead and an increased cancer
risk. Four major human studies of workers exposed to lead have been carried
out; two studies did not find an association between lead exposure and
cancer, one study found an increased incidence of respiratory tract and
kidney cancers, and the fourth study found excesses for lung and stomach
cancers. However, all of these studies are limited in usefulness because the
route(s) of exposure and levels of lead to which the workers were exposed
were not reported. In addition, exposure to other chemicals probably
occurred. (1,2,5)
Animal studies have reported kidney cancer in rats and mice exposed to
lead via the oral route. (1,2,5,6)
EPA considers lead to be a probable human carcinogen (cancer-causing
agent) and has classified it as a Group B2 carcinogen.
Physical Properties
Lead is a naturally occurring, bluish-gray metal that is found in small
quantities in the earth's crust. Lead is present in a variety of compounds such
as lead acetate, lead chloride, lead chromate, lead nitrate, and lead oxide.
Pure lead is insoluble in water; however, the lead compounds vary in
solubility from insoluble to water soluble.
The chemical symbol for lead is Pb and the atomic weight is 207.2 g/mol.
The vapor pressure for lead is 1.0 mm Hg at 980 C.
Major Health Effects Noted from Lead Exposure
Blood lead levels
Health numbersa
(µg/dL)
150
Death
100.0
Death (children)
(125 µg/dL)
Brain and kidney damage (adults)
(100 µg/dL)
75.0
Brain and kidney damage (children)
(80 µg/dL)
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40.0
Increased blood pressure (40 µg/dL)
30.0
Slowed nerve conduction velocity (30
µg/dL)
20.0
Decreased IQ and growth in young
children
(20 µg/dL)
10.0
Preterm birth, reduced birthweight
(10 to 15 µg/dL)
Occupational Exposure to Lead:
Lead has been poisoning workers for thousands of years. In the
construction industry, traditionally most overexposures to lead are found in
the trades, such as plumbing, welding and painting. Significant lead
exposures can also arise from removing paint from surfaces previously
coated with lead-containing paint, such as in bridge repair, residential
renovation, and demolition.
Hobbies resulting in lead exposure may include firearm practice,
soldering in jewelry making or stained glass work, and ceramics.
Exposure to lead may result when lead or any product containing lead is
heated, especially above 500 degrees C (932 degrees F). Lead dust exposure
may result from such operations as pouring powders containing lead, sanding
or sandblasting surfaces coated with lead based paints.
Very small amounts of lead that may be unintentionally ingested via
eating, drinking, or smoking on the job or through hobbies can be harmful.
Good personal hygiene is important where lead is present.
Worker awareness and training are important so that employees can
recognize the symptoms of exposure and get prompt medical attention. Jobs
involving potential lead exposure should be targeted for detailed evaluation
of their potential for lead exposure.
OSHA regulates lead for employees who work in the private sector. The
OSHA lead regulations (29 CFR 1910.1025 and 29 CFR 1926.62) require:
If a worker works with lead, the employer must test the air for lead levels.
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If the lead level in the air is 30 micrograms per cubic meter of air or
greater, the employer must offer employees routine blood testing.
An employee must be removed from all exposure to lead if the average
blood level is 50 micrograms/deciliter or more on three tests. He or she
cannot return to an environment where lead is present until the blood level
falls to at least 40 mcg/dl.
The standard also establishes requirements for medical monitoring,
respiratory protection, protective clothing, engineering controls and
ventilation, work practice controls, hygiene facilities, and employee
education.
OSHA requires employers to reduce airborne lead exposure below the
OSHA Permissible Exposure Limit (PEL). The best way to do this is to
simply replace lead and products that contain lead with less toxic materials. If
this is not possible, the employer must provide change rooms and lockers,
showers, and a thorough cleaning of work surfaces. There can be no smoking
or eating in work areas. The employer must also provide training to
employees on the hazards of lead and on the OSHA lead standard. Currently,
there is no OSHA standard that provides a permissible limit for lead
contamination of surfaces in occupational settings.
Lead dust can settle on your clothes and if these are not changed before
going home, family members can be exposed. Lead can stay on your skin,
hair, and on your shoes, lunch bucket, bookbag, etc. Young children are more
sensitive to lead than are adults and can have health problems from exposure
to less lead.
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Appendix D:
Sections of
Materials Safety Data Sheets
for Selected Lead Compounds
[Note: The following abstracted sections from Materials Safety Data
Sheets [MSDS] are intended to summarize some of the safety issues for
compounds commonly used in lead glazes. Refer to the complete
MSDS prior to handling or using any of these materials. Complete
MSDS forms can be obtained from the raw material supplier or from
the Internet. See URL: www.hammondlead.com].
Sections of materials safety data sheets for the following lead
compounds are given in this appendix:
·
·
·
·
·
·
·
Lead Bisilicate
Litharge [PbO]
Lead Monosilicate
Lead Monoxide
Red Lead [Pb3O4]
Tribasic Lead Silicate
White Lead [lead carbonate hydroxide]
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Lead Bisilicate
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Litharge
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160
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Lead Monosilicate
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Lead Monoxide
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Red Lead
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169
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Tribasic Lead Silicate
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"WHITE LEAD" -- 2PbCO3 Pb(OH)2
SECTION 1 - PRODUCT IDENTIFICATION
White Lead
Trade Names and Synonyms
Chemical Names and Synonyms Basic Lead Carbonate
Basic Lead Chemicals
Chemical Family
SECTION 2 - INGREDIENTS
Ingredien C.A.S.
t
Number
Lead
7439-92-1
% W/W
Min.
%W/W
Max.
99.97
99.99
Exposure
LD50
Limit
oral, rat
0.05
790
mg/m3
mg/Kg
SECTION 3 - PHYSICAL DATA
Boiling Point (deg C) N/A
Vapor Pressure (mm
N/A
Hg)
Vapor Density
N/A
(Air=1)
Solubility In Water N/A
White
Odorless
Appearance
Powder
None
Odor
SolidForm
Powder
WHMIS
D2-A
Classification
NP - Not Pertinent
U - Unknown
Specific Gravity
6.8
% Volatile (By Volume) NP
Evaporation Rate
(Ether=1)
pH
Melting Point (deg C)
NP
NP
550 - 790
TDG Information
Shipping Name:
UN Number:
Class/Division:
Packing Group:
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NP
NP
NP
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SECTION 4 - FIRE AND EXPLOSION HAZARDS
Flammable Limits in Air (Vol
Flash Point (deg C) and Method %)
NP
Upper: NP
Lower: NP
Means of
Extinction:
Class D - Water Fog, Flood, CO2, and Dry Chemical
SECTION 5 - HEALTH HAZARD AND FIRST AID DATA
Ingestion
Eye Contact
Skin Contact
Skin Absorption
Inhalation
Effect of Acute
Exposure
Effects: May cause headache, nausea,
abdominal pains, fatigue, muscle/joint pain,
kidney disjunction, wrist-drop.
First Aid: Give water or milk. If conscious,
induce vomiting.
Effects: Dust or fumes may cause irritation.
First Aid: Flush eye with cool water for 15
minutes and seek immediate medical aid.
Effect: May cause local irritation.
First Aid: Remove contaminated clothing and
wash affected area with soap and water.
NP
See "Ingestion", CNS damage (results in
fatigue, tremors, hallucinations, convulsions,
delirium), weight loss, sleep disturbance.
See " Ingestion Effects " and " Inhalation
Effects ".
Effects of Chronic
Exposure
Possible anemia, central nervous system and
kidney damage.
Carcinogenicity:
IARC (Yes)
Mutagenicity: Yes
Teratogenicit Yes
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Reproductiv Yes
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y:
e Effects:
SECTION 6 - REACTIVITY DATA
Stability
Incompatible
Materials
Stable - Yes
Conditions to avoid: Hydrogen
peroxide
Water: No
Acid: No
Corrosive: No Alkali: No
Other: No
Oxidizers: Yes
Reducers: No
Hazardous Decomposition Products: Toxic lead oxide fumes will
form at elevated temperature.
Hazardous Polymerization:
May Occur: No
Will Not Occur: X
Conditions to Avoid:
NP
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Appendix E:
Pyrometric Cone Properties
Temperature Equivalents of Orton Large Pyrometric Cones
(1)
Cone
Number
6OCº
887
915
945
973
991
108Fº
1629
1679
1733
1783
1816
150Cº
894
923
955
984
999
270Fº
1641
1693
1751
1803
1830
1031
1050
1086
1101
1117
1888
1922
1987
2014
2043
1046
1060
1101
1120
1137
1915
1940
2014
2043
2079
05
04
03
02
01
1136
1142
1152
1168
1177
2077
2088
2106
2134
2151
1154
1162
1168
1186
1196
2109
2124
2134
2167
2185
1
2
3
4
5
1201
1215
1236
1260
1285
2194
2219
2257
2300
2345
1222
1240
1263
1280
1305
2232
2264
2305
2336
2381
6
7
8
9
10
1294
1306
2361
2383
1315
1326
2399
2419
11
12
176
010
09
08
07
06
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Pyrometric Cone Table Notes:
1. The temperature equivalents in this table apply only to Orton Standard
Pyrometric Cones when heated at the rates indicated.
2. Temperature Equivalents are given in degrees Centigrade (ºC) and the
corresponding degrees Fahrenheit (ºF) . Rates of heating shown at the head of
each column of temperature equivalents are expressed in Centigrade degrees
(Cº) and Fahrenheit degrees (Fº) per hour. These heating rates were
maintained uniformly during the last several hundred degrees of temperature
rise in the test. All determinations were made in an air atmosphere.
3. The temperature equivalents are not necessarily those at which cones
will deform under firing conditions different from those under which the
calibrating determinations were made.
4. For reproducible results, care should be taken to insure that cones are
set in a plaque with the bending face at the correct angle of 82º from the
horizontal, with the cone tips at the correct height above the top of the
plaque. (Large Cones 2”, Small and P.C.E. Cones 15/16”. )
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Cone Position Diagram
The illustration indicates a method of designating or recording the
position or degree of bending of a cone on a plaque. Beginning at the left, the
cone is in the original position with an 8 degree inclination. In the next figure
the cone has deformed to the 1 o’clock position. In the next four figures, the
cones have successively deformed to the 2, 3, 4 and 5 o’clock positions. The
last figure shows the end of cone deformation -- 6 o’clock -- as far as a
reading can be obtained. Temperatures on cones as given in the Temperature
Equivalents Table were determined when a certain cone had reached the 6
o’clock position.
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Appendix F:
Glossary Of Terms
Amphoteric -- Relating to an acid/base behavior intermediate
between acids and bases. Aluminum oxide often behaves in an
amphoteric way, sometimes behaving like an acid oxide such
as SiO2 and other times behaving in a more basic way, such as
MgO.
Bisilicate – A compound with two silica [SiO2] units in the formula,
such as lead bisilicate, PbO . 2SiO2.
Bisque -- The kiln firing, or “fire”, in which the ceramic ware is
matured before the glaze is applied.
Bridging – In the silicate molecular structure bridging refers to
oxygens that are bonded to two silicon atoms, i.e. they bridge
between the silicons.
Carboxy Methylcellulose – A cellulose polymer use as a volatile
binder in ceramic glazes.
Cone – A small, tall pyramidal cone of a controlled ceramic material
that has very specific thermal softening characteristics that can
be used to monitor the time x temperature work product during
a firing cycled.
Devitrification – Crystallization of a glass or glassy phase.
Devitrified glazes are glazes in which crystals have formed.
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Earthenware – Ceramic ware based on coarse or unrefined raw
materials, often indigenous naturally-occurring mixtures of
clay, flint and feldspar, that matures at low temperature [e.g.
cone 06] and is porous and colored by impurities.
Engobe – A clay based coating applied to green ceramic ware to either
mask the color or texture of the underlying body in preparation
for glazing, or to produce special effects.
Frit – A glass that is formulated and melted from a specific
composition of glaze oxides and then pulverized to a fine
power for use in glazing. Frits are prepared to stabilize oxides
that would otherwise cause problems during glazing.
Glost – The kiln firing, or “fire”, in which the glaze is matured.
Green – Not fired. A clay body before firing is said to be “green”.
Inglaze – A reference to decorations that react with the glaze and form
a durable glaze/decoration combination.
Interdiffusion – The simultaneous counter movement ions or other
molecular size particles in a material. Use in this book to refer
to the movement of lead ions from the glass to the aqueous
solution and the counter flow of hydrogen ions during acid
leaching of glazes.
Knoop – A system for the measurement of hardness.
Leaching – The selective extraction of lead and other modifier ions
from the silica glaze network during acid
Matts – Not glossy, i.e. possessing the property of diffuse light
scattering.
Methylcellulose – See carboxy methylcellulose.
Nonbriding -- In the silicate molecular structure bridging refers to
oxygens that are bonded to only one silicon atom.
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Onglaze – See overglaze.
Opacifier – A substance which reduces the transparency of a glaze.
Opacifiers are generally crystalline materials such as SnO2 or
ZrO2 that scatter light and convert a clear glaze to an opaque
glaze.
Overglaze – A reference to decorations that are applied over the glaze,
i.e. after the glost fire. These decorations are then fired at a
lower temperature to fix them to the surface.
Polymorphic – Of many forms. Polymorphic materials such as SiO2
have different structural forms depending on temperature.
Porcelain – Ceramic ware based on highly refined raw materials,
mostly kaolin [china clay] and feldspar, and fired at high
temperature [e.g. cone 2 – 12] to produce a strong, vitreous,
fully dense, and translucent fired body that is white to off-white
in color.
Potentiometric – relating to electrical potential, or voltage.
Rutile – a raw material containing principally TiO2, but often with
other impurities.
Spinels – A specific ceramic crystalline structure, as that found in
magnesium aluminate.
Stoneware – Ceramic ware based on coarse raw materials, often with
significant amounts of grog [coarse non-plastic], and fired at
high temperature [e.g. cone 6 – 10] to produce an opaque fired
body low in porosity [impervious to water] and ranging from
pale to moderately dark in color.
Ulexite/colemanite – sodium calcium borate compounds used in raw
glazes, i.e. not fritted glazes.
Underglaze – A reference to decorations that are applied under the
glaze to achieve the protection afforded by the glaze.
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Appendix G:
Bibliography and References
[1.]
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lead-free glaze by the sol-gel method”, Ceram. Int., volume 27,
pp 681 – 687 (2001)
[2.]
Agency for Toxic Substances and Disease Registry (ATSDR). Case
Studies in Environmental Medicine, Lead Toxicity. U.S. Public
Health Service, U.S. Department of Health and Human Services,
Atlanta, GA. 1992.
[3.]
Agency for Toxic Substances and Disease Registry (ATSDR).
Toxicological Profile for Lead (Draft). U.S. Public Health Service,
U.S. Department of Health and Human Services, Atlanta, GA. 1993.
[4.]
Ainsworth, L. “A Method for Investigating the Structure of Glazes
Based on a Surface Measurement”, Trans. Brit. Cera. Soc., 55, 66173 (1956).
[5.]
Anderson, O. “The Volatilization of Lead Oxide from Lead Silicate
Melts”, J. Am. Ceram. Soc., 2 (10) 784-89 (1919).
[6.]
Andrews R. and R. S. Murray, “Method of Improving Acid
Resistance of a Glass Color”, U. S. Patent 2, 724, 662, November 22,
1955.
[7.]
Azzoni, CB., Delnero, GL., Krajewski, A., Ravaglioli, A., “A
Diffusive Model Of Pb+2 Release By Lead-Ceramic Glazes”,
Journal Of Materials Science, Volume 16, pp. 1081 – 1087
(1981).
183
Lead Glazes for Ceramic Foodware
An ILMC Handbook
[8.]
Bartel, P. “Lead Solubility of Fritted Glazes”, Sprechsaal, 51, 25-43
(1918).
[9.]
Bartel, P. “Literature on the Lead Questions as to Laws and
Regulations”, Ber. Deut. keram. Ges., 3, 86 (1922).
[10.]
Bennett H. and F. Vaughan, “Solubility of Lead Glazes, Part IV,
Investigation of Certain Chemical Methods of Lead Determination”,
Trans. Brit. Ceram. Soc., 52, 578-87 (1953).
[11.]
Bennett, H. “The Solubility of Lead Glazes, Part V, Chemical
Factors Affecting Solubility Determinations”, Trans. Brit. Ceram.
Soc., 53, 203-17 (1954).
[12.]
Berger, R. “Colors for Porcelain and Glazes”, Silihat J, 3 (5) 405
(1964).
[13.]
Binns C. F. and F. Lyttle, “A Vermillion Color for Uranium”, J. Am.
Ceram. Soc., 3 (11) 913-14 (1920).
[14.]
Bishop, D. F. W. “Lead and Silicosis, Factory Precautions”, Trans,
Brit. Ceram. Soc., 37, 17-26 (1937-38).
[15.]
Bloor, E. C. “Glaze Composition, Glass Structural Theory, and its
Application to Glazes”, Trans, Brit. Ceram. Soc., 55, 631-60 (1956).
[16.]
Brandt, A. “Lead Poisoning in the Ceramic Industry, Part H”, Ber.
Deut. keram. Ges. , 19, 46 (1938).
[17.]
Burke, Francis M., “Leachability of lead from commercial glazes”,
Ceram. Eng. Sci. Proc., 6[11-12]1394 (1985).
[18.]
Carr, Dodd S.; Cole, Jerome F.; McLaren, Malcolm G, “Ceramic
foodware safety: III, Mechanisms of release of lead and cadmium”,
Ceramica (Sao Paulo), 28[N 148]151-5 (1982).
[19.]
Cordt, F. W. “Production of Red Glazes”, Ceram. Ind., 21 (4) 172-75
(1933).
[20.]
Cos, P. E. “Review of Glaze Making Aids”, Ceram. Age, 51 (3) 12728 (1948).
[21.]
Currier, A. E. “Standard Method for Determining Leachability of
Lead from Lead Frits”, J. Am. Ceram. Soc., 30 (11) 335-38 (1947).
Sponsored by the United States Potters Association.
[22.]
E.J. Calabrese and E.M. Kenyon. Air Toxics and Risk Assessment.
Lewis Publishers, Chelsea, MI. 1991.
184
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An ILMC Handbook
[23.]
Eggert, F. “The Influence of Admixtures on the Red Color of
Uranium Glazes”, Email-Keramo-Techn., 2, 74 (1951).
[24.]
Eisenlohr, H. “Acid-Resistant Colors: Testing”, Sprechsaal, 59 (39)
645 (1926).
[25.]
Eska, H. “Weather-Proof Red Uranium Glazes”, Sprechsaal, 66 (4)
59-61; (5) 77-78; (6) 93-96; (7) 109-10; (8) 127-29 (1933).
[26.]
Fajans K. and N. J. Kreidl, “Stability of Lead Glasses and
Polarization of Ions”, J. Am. Ceram. Soc., 31 (4) 105-114 (1948).
[27.]
Franklin C. E. L. and J. A. Tindall, “Attack of On-Glaze Colors by
Acid, Alkali and Washing Agent”, Trans. Brit. Ceram. Soc. , 58, 589
(1959).
[28.]
Franklin, C. E. L., J. A. Tindall and A. Dinsdale, “The Influence of
Glaze Composition on the Durability of On-Glaze Colors”, Trans.
Brit. Ceram. Soc. , 59, 401-23 (1960).
[29.]
Frey, Emmo; Scholze, Horst, “Lead and cadmium release from fused
colors, glazes, and enamels in contact with acetic acid and food
under the influence of light”, Ber. Dtsch. Keram. Ges., 56 (10): 2937 (1979).
[30.]
Funk W. and H. Mields, “Resistance of Enamel Colors to Dilute
Acids and a Method of Determining the Lead Solubility”, Ber. dent.
keram. Ges. , 12, 535 (1931).
[31.]
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