Every potter wants to be certain that the products they are selling are safe and will stand up to the rigors of use in the kitchen. But because there are so many variables involved in ceramic glazes—(clay body and glaze fit, variability in chemical content, temperature variability in kilns, organic materials, etc.), it can be challenging to know whether the food safe ceramic glaze recipe you found online is truly safe when you use it on your work, with your clay body, in your kiln. But there are tests you can do at home to determine if your glaze is a food safe glaze under your firing conditions.
In today’s post, an excerpt from the Pottery Making Illustrated archive, Gabriel Kline shares two tests that can help you determine if you’re using food safe ceramic glazes in your practice. – Jennifer Poellot Harnetty, editor.
PS. For more ways to test your glazes, check out the May/June 2019 issue of Pottery Making Illustrated.
Two Food Safe Glaze Tests
If you’re a maker of functional ware to be used with food or drink, it’s important to test the durability of your product. While laboratories exist that will perform a battery of food safe glaze tests for a fee, there are several tests you can perform at your home or in your studio that indicate whether your work is appropriate for functional use. Once you’re satisfied with the visual look of your piece coming out of the kiln, subject the work to the following tests to ensure that it will hold up in a number of different scenarios. If these tests are inconclusive, or if the glaze seems at all questionable, send a piece off to a professional lab.
Food Safe Glaze Test #1: Lemon Test
Squeeze the juice of one lemon onto a horizontal, glazed surface of your piece (1). Place the rest of the lemon in the juice, and leave it out overnight. The next morning, remove the lemon and rinse the piece. If you notice any change in the color of the glaze (2), this indicates that the acid in the lemon juice was able to leach other materials out of the fired glaze. If this glaze were to be used for functional work, the acid in a cup of coffee could cause similar leaching, creating a beverage fortified with a number of unwanted and potentially hazardous materials. If your work fails this test, don’t use that glaze on functional work. It’s important to note that if your glaze passes this test, it doesn’t necessarily mean that materials aren’t leaching out of the glaze at all, just that they aren’t leaching badly. If in doubt, use a stable liner glaze on all surfaces that will come into contact with food and drink, and save any questionable glazes for the exterior of your vessels.
Food Safe Glaze Test #2: Microwave Test
You may also want to test your work in a microwave, as many cups, bowls, and plates may end up there at some point. Fill a test piece with water and microwave it for one minute. If the clay body isn’t sufficiently vitrified, the water will absorb into the surface of the work and become very hot. The expansion of this water into steam when heated can compromise the bond between the clay and glaze, causing the glaze to chip off. The thermal shock of hot water may also cause cracking if there’s a misfit in the thermal expansion of the glazes on the clay, such as a tight liner on the inside and a crazed matte on the outside. Also, it should be noted that some glazes, including lusters and other metallics, will cause sparking in a microwave and should be labeled as such when being sold or gifted.