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Safe for Dinnerware Use
This is what most potters are interested in. Can you use a certain glaze on a piece which will contain food and beverages?
Toxicity is one aspect of this. If there is no lead or cadmium in your glaze (including no Frits which contain lead), and your kiln is not contaminated with lead, then you pass one toxicity test (for lead and cadmium). (If you have fired leaded glazes before, your kiln brick may have absorbed lead and could be depositing it on current firings. You can get an inexpensive kit at the hardware store to test your kiln for lead release. And of course you should never fire dinnerware in a kiln with other leaded glazes.)
There are some glazes that have lead or cadmium and still say they are dinnerware safe. They have been fired and tested, and found to pass the test for lead and cadmium release. (A small amount of leaching is allowed by law.) There are also some glazes where the cadmium is encapsulated in other glaze ingredients which traps it when fired. The only caution here is that your firing conditions will be different, so it is possible that your pieces could leach when the test pieces did not. For this reason, it is best to have a sample tested anytime you use glazes which contain lead or cadmium. Later on I will tell you how to do that.
Any time you begin to layer glazes, you are pretty much on your own. Any testing that the manufacturer did will not be applicable. If you don't use any glazes with lead or cadmium as ingredients, you are pretty safe (with the caveats above.) Otherwise, test!
Many potters believe that you should never use these ingredients in dinnerware period. Who knows what may happen to the glaze after years of use, after going through the dishwasher 30 times, after the glazes craze, after they are microwaved and frozen and bombarded with acidic food. It is always possible that a piece will leach lead or cadmium at some point in the future. So to be safe, just avoid them.
Then of course there are the ingredients which are not regulated, but may be toxic especially in high amounts. Unfortunately there is no simple answer to this problem. If you are producing dinnerware then it is advised that you have sample pieces tested for the various ingredients which might be leaching.