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) 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.
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.
Out Bound Links
Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.
Max Richens outlines the various mechanisms by which acids and bases can dissolve glass and glazes. He provides some information on stabilizing glazes against attack.
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside glaze at the rim.
In Bound Links
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
This glaze was developed using the 1214W glossy as a starting point. This article overviews the types of matte glazes and rationalizes the method used to make this one.
Understanding the advantages of disadvantages of stains vs. oxide colors is the key to choosing the best approach
Glazes are not as inert and stable as many people think. All are slightly soluble and will thus leach to some extent, even if minute, into liquids they come into contact with. However some glazes are dramatically more soluble than others. The subject of leaching and glaze safety can be complex, but ...
By Tony Hansen