Lead in Ceramic Glazes: What Did We Learn?
Note: The CDC published a revised view of lead in 2012, so material on this page is outdated. See the glossary entry for more information.
Lead tends to have a long slowly debilitating effect on the people it poisons. Likewise the practice of using lead in glazes also died a very long and slow death during the 1980s and 90s in many ceramic manufacturing countries. Although defenders who fought for it were right that it can be used safely, the bottom line is that many manufacturers do not use it safely. The US FDA tests incoming ceramic ware from many countries, a plenty of ware still has lead release that is off the scale. Starting in 2001 an organization of 21 ceramic producing lead using countries began an organized effort to train technicians how to substitute for lead in glazes. It is thus clear that lead is still widely used around the world as a glaze flux.
It is amazing that there are still many misinformed users around the world who are complacent and just unaware. Many people today still believe that as long as fritted sources are used it is safe. This is obviously not true, PbO in a fired glaze is still PbO no matter what material sourced it.
Potters and small manufacturers are now subject to inspection by regulatory agencies and increasing scrutiny from more sophisticated and less trusting customers. They must be much more concerned about the leachability of their ware.
Public and industry attitudes toward lead have shifted since the early 90s, and finally most potters and companies are realizing that the narrow parameters within which lead can be used safely (or perceived to be used safely) are just too difficult to work within. Public paranoia is common even though, for example, there are no known cases of lead related illness in the US for domestic manufactured ware.
Inhalation exposure to lead is considered to be significant if the amount of lead in the air is more than half the lead-in-air standard i.e.. >0.075 mg/m3 over an 8 hour working day. Concentration in the human body is significant if it exceeds 40 micrograms/100ml of blood.
The US FDA (Food and Drug Administration) has been a driving force in the move to eliminate lead worldwide. European standards (i.e. (European Community Directive 1984) are less stringent and will be modified to allow export to the US. The Compliance Policy Guide adopted by the FDA in 1992 reduces allowable limits dramatically. It is based on the standard 4% acetic acid text as follows:
Category, Samples, Lead Release (micrograms/ml), Pre 92, Current
The US Consumer Product Safety Commission and the Environmental Protection Agency has also issued voluntary standards.
In the early 90's, industry was in a compliance mode, adopting quality assurance standards and formulation expertise (i.e. BS 5270, ISO 9000) to meet the challenge and increase credibility. However, environmental concerns and political pressure are forcing the industry to eliminate lead completely or face a total ban. The sectors which most depend on lead are bone china, vitrified hotelware, feldspathic porcelains, and earthenware.
International industry leaders (Corning, Lennox, Mikasa, Noritake, Pfaltzgraff, Royal Doulton, Villeryoy & Boch, Wedgewood) have formed the Coalition for Safe Ceramic Ware to represent the interests of the industry. Only companies with good reputations for producing quality ware are eligible for membership. The CSC called for the reduction of lead limits and was instrumental in the FDA's revisions in Apr 1992. The CSC has continued to develop and apply quality and design control standards which individual members must document and validate with sampling programs. In addition, the CSC has published material to help consumers assure that tableware products are safe to use.
CSC has also taken on the challenge of educating the public as to what it sees as inappropriate. Examples are California's Proposition 65 which requires that consumers be warned if lead levels are more than one-one thousandth of the level at which there is observable effect on human health. For example, a restaurant must warn its patrons with signage. Lead levels of this nature are difficult to measure with test equipment.
Lead compounds have a TLV (threshold limit value) of 0.05 milligrams per cubic meter of air breathed. By comparison iron oxide is considered a safe-to-use material at 5.0, kaolin is 2.0, barium carbonate is 0.5, quartz is 0.1-0.05.
Non-fritted lead compounds can be partially soluble and thus be absorbed into the body; but the major risks are associated with inhaling dust or aerosol. Lead carbonate is an extremely fine powder which can easily become airborne. It decomposes at 400C and a product of the decomposition is the oxide PbO. It is much more likely to vaporize in a reduction atmosphere (which can easily happen in electric kilns).
Potential effects of lead exposure are tiredness, abdominal cramps, constipation, anemia, nerve damage, anorexia, vomiting, convulsions, brain damage. Lead is a cumulative poison.
Although lead can be used safely in circumstances where stringent testing and controls are in place, in general, there are too many variables to use lead safely in a small operation.
Testing Kits: Lead release is normally tested by measuring how much of the glaze will dissolve in a dilute acid mixture. Testing kits are available for $25 and come with a neutralizer (sodium hydrate) and an indicator (sodium sulfuret). They have warnings about flushing skin or eyes thoroughly with water if splashed on and drinking 3 or 4 glasses of water or milk if ingested (do not induce vomiting). One set of instructions, for example, says to wash the object, immerse in white distilled vinegar (5% acetic acid - any vinegar will do) for 18 hours (or longer), stir the vinegar, fill the supplied test tube to the etched line (about 2 cc), add 7 drops of neutralizer, invert to mix. After 2 minutes, add 7 drops of indicator and invert to mix. After 2 minutes a clear or milky solution is OK, any shade of brown indicates lead according the comparison chart provided.
Lab testing can be done at:
A Relationship Between Acid Test and Glaze Solubility?
In 1997 two lawsuits were settled in which children were allegedly harmed by lead glazes used by their mothers. One suit was brought by Sherrell McClendon and her husband Richard Duggan on behalf of their three children, one of which allegedly was harmed in utero. They sued Duncan Ceramics, Mayco Colors, and Allstate Insurance. The case settled in Summer 97 for around $500,000.
The second suit was for harm that Ashley Rose Witt allegedly sustained in utero, and for harm to her mother, Patty Moore. The child's mother and father sued Duncan Enterprises, American Art Clay Co., Mayco colors, C and R Products, and Robert R. Umhoefer, Inc, in Florida. A settlement in December, 1997 provided $750,000 for the child and $115,000 for the mother.
The injuries claimed in these suits occurred before 1991 when lead glazes were commonly tested for acid solubility. Lead glazes could even be labeled "lead free" if they did not release more than 0.06% on the acid leach test. Experts in the lawsuits testified that the acid test was shown to be faulty in 1992 when a nursing home patient's blood was tested after she swallowed some 'lead free' glaze. One expert, Dr. Woodhall Stopford, toxicologist for the Arts and Creative Materials Institute, referred to the incident in deposition as follows:
These statements and testimony suggest that manufacturers who still use acid tests to determine glaze toxicity risk their liability. Paint manufacturers also should not use a similar acid test, ASTM D-5517. Ceramicists need to be aware that labels on old bottles of glaze may thus be inaccurate.
California Tableware Education and Enforcement Program
Visit http://www.dhs.ca.gov/childlead for the California Dept. of Health Services, Childhood Lead Poisoning Prevention Branch. Phone: (510) 622-5059 Fax: (510) 622-4888 This site has been established as a source of information about the issues of lead poisoning and ceramics, offering a variety of resources, information and links to other related sites.
An Interesting Book
As evidence that hazards of lead have been known for a long time, the book "Research on Leadless Glazes" by William James Furnival was written in 1898 and was dedicated "To all who ardently desire and are working for the betterment of mankind". The book contains much interesting historical information about the use of lead in industries at the time.
Out Bound Links
In Bound Links