Understanding Glaze Stability
Glaze stability to acids and bases is an area of emerging importance to potters. Of course, we have been concerned with using lead and cadmium in our glazes for many years to the point where most of us no longer use either metal because of the safety concerns and the rigorous testing program required by law if we do use them. Barium is another metal that many, but by no means all, potters have stopped using on functional pottery because of concerns over its toxicity if it leaches from our glazes. But what about such metals as copper, manganese, cobalt, boron, lithium and the other metals that usually end up in the form of metal oxides in our glazes? Should we worry about them. I hasten to point out that there is no evidence of anyone ever having been harmed by these metals leaching from a pottery glaze. Most of the hazard is to we potters who handle them in powdered form.
At the same time I must point out that some potters are using glazes that are so unstable that the color leaches out with ordinary use--either in the basic environment of a dishwasher or when exposed to acidic foods such as orange juice, vinegar or tomato sauce. We also know that copper, for example, can add a bitter taste to foods at levels as low as 10 mg/l. There are also those of us who feel that we are not in the business of supplying metal supplements to the diets of our customer, regardless of the fact that it is more than likely causing no harm. We believe it is a matter of good craftsmanship to make our glazes as stable as possible and certainly durable enough to have a long life in ordinary use and to avoid adding undesirable flavor to food served in them. It is in this spirit that this body of work on glaze stability is undertaken.
What then are the goals we might set for ourselves. This is a complex question and one for which their is no definitive answer at the present time except for the regulated metals of lead and cadmium. There are those who feel if an acid leaches less metal from a glaze than is allowable in drinking water that certainly would be an unassailable position. In my opinion, this would certainly be a safe-side goal, but probably overly stringent because even lead is allowed to leach from a glaze at 7-200 times the amount allowed in drinking water depending on the specific type of vessel being tested. Others feel that if a glaze leaches well below the FDA Recommended Daily Intake level, we are certainly no in danger of harming anyone. To get a better understanding of this issue, tests are currently being done at the Alfred University Analytical Laboratory to relate the standard leaching test to what really happens in real life use of our pottery with food. When this work is complete, I will modify this section to reflect any new findings.
For the current purposes, then, all I can do is give some reference numbers for us to think about. There are no regulations we must follow except for lead and cadmium at the present time in the United States. Each potter must set his/her own goals and, of course, are even free to completely ignore this subject if they aren't using either lead or cadmium. For my part, I am going to continue to try to learn how to make my glazes as stable as possible while still being attractive.
Reference Points
Metal Maximum Contamination Levels in Drinking Water, mg/liter Level Allowed in Standard Acidic Leaching Test in the U.S., mg/liter FDA Recommended Daily Intake, mg/day Comments Barium 2.0
Boron 0.6 (health advisory only, not legally binding)
Cadmium 0.005
0.25-0.5 depending on the type of pottery Standards are most stringent for vessels holding liquids; less stringent for flatware. Calcium
1000
Chromium 0.1
0.12
Cobalt Copper 1.3
2.0
Lead 0.015
0.5-3.0 depending on the type of pottery; 0.1-0.226 in California Standards are most stringent for vessels holding liquids; less stringent for flatware. Manganese
2.0
Nickel 0.1
Zinc 15.0
Overall Comments: Empty boxes mean that there are no requirements that must be met or, in the case of Recommended Daily Intake, that none is necessary in our diet.
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