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Glaze Stability and Food Safety

If you're going to make pots for food (and let's face it, you are), then you need to make sure you're doing everything you can to ensure they are safe for use. This baker, made by Marci Paul, appeared in the book In the Potter's Kitchen.

If you’re going to make pots for food (and let’s face it, you are), then you need to make sure you’re doing everything you can to ensure they are safe for use. This baker, made by Marci Paul, appeared in the book In the Potter’s Kitchen.

While there are no specific regulations on chemicals leaching from ceramic objects into food, there are National Primary Drinking Water Regulations (NPDWRs or primary standards) that are legally enforceable standards applying to public water systems. We recommend using the maximum containment levels for inorganic chemicals listed there as benchmarks for safety when testing ceramic glazes for leaching, since anything that is likely to leach out of ceramic ware is going to do so into liquid that comes into contact with it. We also recommend adhering to the Secondary Drinking Water Standards in order to cover even more of the materials we use on a regular basis, but which may not be specifically regulated in a legally enforceable manner.

Glazes for Food-Contact Surfaces

For glazes that come into contact with food, we recommend using glazes that not only have been tested for being a stable glass that will not leach toxic materials, but glazes that don’t even include toxic materials. If you don’t use toxic materials in your glaze to begin with, half the battle is won—but there is still the issue of the glaze needing to be useful and durable and pleasing to the user, so a stable glass is still a desirable goal.

  • The glaze needs to stand up to washing, and therefore needs to resist degradation from detergents.
  • Some surfaces (in plates and bowls, for example) need to resist abrasion from metal utensils (and vice versa); not only is the sound of contact annoying, the metal can mark the surface, either by abrading it or by leaving visible bits of metal on the surface. In general, you should steer clear of matte glazes for lining forms used with metal utensils.

A Quick Test

One way to quickly rule out a recipe (one that is smooth—not matte—and contains no compounds listed in the drinking water standards and regulations listed above),  is to line a small cup with the glaze, fire it, fill it halfway full of vinegar and let it sit for 24 hours. Empty it, dry it, and look at it. If the texture or appearance of the glaze in contact with the vinegar changes in that time (becomes cloudy, chalky, etc.), discount it as a stable glaze and do not use it on food-contact surfaces, as it will likely leach compounds into acidic foods and not resist washing detergents.

If you find a glaze that passes this test, it is likely to be food safe in its current state. However, if you change anything about how it is formulated or fired, it will need to be tested in this way again—and only if it still contains no toxic compounds. If you find a glaze that passes this test, and then you add 1% cobalt oxide to it, then this test will not tell you anything about whether or not any cobalt actually leached out of the glaze (although you would be wise to assume that some did). This new glaze needs to be tested in a lab for leaching of cobalt before you can know how much, if any, cobalt is likely to leach from it. The same is true if you have the glaze tested for leaching cobalt, it turns out to be food safe, and then you swap out the cobalt for chrome; your leach test is, for all practical and legal purposes, useless for telling you anything about how chrome oxide will affect the chemistry of the glaze.

Laboratory Leach Testing

In short: Do it.

Most labs that test for leachates from ceramic glaze will test only for one specific material per sample, so you need to know what you are putting into a glaze in order to know what to ask them to test for. If you don’t know what is in your glaze materials, find out before using it on a piece of ware intended for food use, and have it tested for leaching of any potentially hazardous material before letting it out into the world.

Lab for Testing

There are several labs that will test your glazes, but we recommend Brandywine Science Center because they cater specifically to potters (they have a web page that actually has the word “pottery” on it), and have reasonable pricing compared to a lot of other labs.

Here is some great information lifted directly from the Brandywine Pottery Testing page on how to prepare your test for leaching, regardless of which lab you will be sending to:

“Make a small cup. The cup can be most any size: however, most labs have agreed to a standard size to make our results more consistent and comparable. Unless you have a special reason to use a different size, we recommend you throw cups that are straight-walled cylinders 4 inches in diameter by 3 inches tall (wet dimensions). This size cup can be thrown from less than 1 pound of clay by someone with modest throwing skills. Dry and Bisque fire the cup, glaze it and glaze fire it at your standard conditions. Make sure the glaze completely coats the inside of the cup. It is best to put a pyrometric cone pack right next to the cup so you will know exactly what temperature it saw during firing.”

 Remember, testing is good—retesting is better!

Glaze Stability and Food Safety

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