Two Tests to Determine if Your Glaze is a Food Safe Ceramic Glaze

Find out if your food safe glaze is really food safe!

food safe ceramic glaze

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 May/June 2019 issue of Pottery Making Illustrated, 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.

33 Tried & True Glaze Recipes

Pick up 33 of our favorite pottery glazes when you download this freebie33 Tried & True Glaze Recipes.



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.

To test a glaze’s acid resistance, squeeze a lemon wedge onto a horizontal, glazed surface.

Changes in the glaze color indicate that acids from foods can leach materials from the glaze, and that it is not food safe.

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.

Comments
  • Sharyn M.

    This article couldn’t have been better timed, I’m setting up my home studio and the kiln will be installed this weekend. I will be doing test pieces, then test cups and bowls to see how glaze colors work. Now I’ll be able to also test for food safety.

  • Hawi W.

    …and if water gets absorbed into a pot due to insufficient vitrification, it gets trapped in a capillary maze in the ceramic body. Microwave heating can heat this trapped water to temperatures well above the boiling point, as it happens in a pressure cooker, up to a point when the trapped water explosively boils and changes into a great volume of pressurised steam. This of cause happens on a small, molecular level, but it is sufficient to pop off glaze particles, which would end up in your food.

  • Randall M.

    Remember that if a body (earthenware for example) with high iron content will also get hot in a microwave.

  • Claire O.

    Thank you for this excerpt from larger article. However can ask for clarification? In the article you state “If the clay body isn’t sufficiently vitrified, the water will absorb into the surface of the work and become very hot.”
    How do you know the water is hot? Are you saying the piece will become hot to the touch?
    Thank you

Leave a Reply

You must be logged in to post a comment.

Enter Your Log In Credentials
This setting should only be used on your home or work computer.

Larger version of the image

Send this to a friend