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 durability 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.
Some glazes may give an indication of their suitability for functional work simply by their recipes. Glazes with high amounts of colorants (such as those with more than 4% copper carbonate), and glazes with low silica and low alumina should be used only on surfaces that will not come into contact with food or drink. Some toxic materials may leach out of the glaze in the presence of acids such as vinegar or coffee. If you’re in doubt about a certain glaze, first use the tests below. If these tests are inconclusive, or if the glaze seems at all questionable, send a piece off to a professional lab.
Place the piece in your freezer overnight. The next morning, preheat your oven to 350°F (177°C) and heat the piece by moving it directly from the freezer to the oven. Leave it in the hot oven for a half hour. Clay and glaze used for functional work should be able to handle this stress. If the piece cracks or crazes when making the transition from extreme cold to extreme heat, you may want to consider using a different clay or a different set of glazes for your work. If the piece passes the test, you can be assured that the future owner can use it without much worry.
Take a key from your key ring and draw it back and forth over a small area of the glazed surface of your piece (1). If the key leaves a score mark that cannot be removed with a gentle buffing by your finger, flatware will also score the surface similarly and leave a darkened surface over time. While this scoring isn’t necessarily a problem in terms of function, and can be removed with commercial products such as Bar Keepers Friend, you may want to adjust your recipes to include more silica and/or flux, providing a glossier, more durable surface.
Squeeze the juice of one lemon onto a horizontal, glazed surface of your piece (2). Place the rest of the lemon in the juice (3), 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 (4), 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.
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.
Excerpted from Gabriel Kline’s book Amazing Glaze, published by Voyageur Press (an imprint of Quarto Press), Minneapolis, Minnesota, 2018, and available on HighwaterClays.com, www.amazon.com, and www.barnesandnoble.com.This article was excerpted from the May/June 2019 issue of Pottery Making Illustrated.