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Having Your Glaze Tested for Toxic Metal Release

Section: Materials, Subsection: Safety


Having Your Glaze Tested for Metal Release

Article Text

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) use drinking water standards as a safe-side limit (however there are not 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. Raw metallic colorants like cobalt, manganese, chrome are a concern if on food surfaces. The manufacturer will have information of the contents of any stains you use, however stains are pre-fired at high temperatures and are supposed to be much more inert than raw colorants.

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 linked on this page. 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 (3M bought Hibrivet Systems, they used to have kits for lead, barium, chromate, cadmium, nickel, cobalt, iron, and mercury, however it is not clear what has happened to these).

Please read the linked articles Is Your Ware Safe and Are Your Glazes Food Safe. No single thing is more important than knowing more about the materials you use and how to make glazes that have a balanced chemistry that is less likely to leach. The best piece of advice is to do what industry does: 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.

Copper can destabilize a glaze and make it soluble

Copper can destabilize a glaze and make it soluble

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.

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By Tony Hansen

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