Having Your Glaze Tested for Metal Release
Labs do not test ceramic glazes for "food safeness"; they only determine how much of a given metal leaches out into water under standardized test conditions. It is your responsibility to ask them to check for the right the metal(s), evaluate the accuracy they claim, and determine what 'safe levels' are. In the absence of legal standards for "food safeness" for glazes (with the exception of lead and cadmium) we use drinking water standards (however there are no drinking water standards for all the materials used in ceramics). If you don't feel qualified to evaluate the results yourself, contact or search writings of an industrial toxicologist.
Certain things are obviously worth testing for and others not. If you are not using any materials that contain lead, there is no point in testing for this. The only possible sources are frits, and few suppliers would even have a leaded frit in their building. If your glaze contains barium or lithium, these would be worth testing for if you use a material that sources them. Raw metallic colorants like cobalt, manganese, chrome are a concern on food surfaces. The manufacturer will have information of the contents of any stains you use. While stains are blended compounds (to add stability) and are pre-fired at high temperatures and are supposedly more inert than raw colorants, there is still a potential for leaching.
On the list of labs at https://digitalfire.com/services/database.php?list=labs, the Brandywine Science Center specializes in this type of testing. Their page details how to prepare and send your sample.
For doing your own testing, please see the GLLE test, if you are doing nothing to test right now, these simple tests are an excellent start.
3M Lead Check kits can be bought at amazon.com. Search "home water test kit" for products that can sense a range of heavy metals.
Please read the linked articles. No single thing is more important than knowing more about the materials you use and how to make glazes that have a balanced chemistry (making them less likely to leach). Using a frit about whose chemistry you know nothing is asking for trouble. The best piece of advice is to do use a liner glaze (see link below). Your clay supplier should be able to recommend safe liner glazes (and accompanying firing schedules) for each of the bodies there sell.
A closeup of a glossy Cone 6 glaze having 4% added copper carbonate. The bottom section has leached in lemon juice after 24 hours. This photo has been adjusted to spread the color gamut to highlight the difference. The leached section is now matte.
This lab is certified by the US Department of Environmental Protection (DEP) for drinking water and waste water analysis. They also provide pottery glaze leaching analyses (the water is kept in contact with the glaze for many days then analysed for trace levels of specific metals). Each suspected metal must be tested for and each entails a separate charge ($30-60 in this case). That means that testing one glaze for several metals could cost $200. How to make sense of these numbers? Google the term: "heavy metals drinking water standards", and click "Images". You will find many charts with lots of data (some of them show multiple sources). By searching pages for this term you will find books having detailed sections on each of the metals. This can seem overwhelming, but typically you are only interested on one metal in a specific glaze (often cobalt or manganese).
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