Glaze Leaching Tests

The big clay class project this quarter was an extensive leaching test on all our functional glazes. In January I read an article about liner glazes and testing functional glazes. The article, written by Deanna Ranlett, was in November/December 2012 Pottery Making Illustrated. I believe you need a subscription to read it online.

Sets of glazed bowls after setting up the test

I had my intermediate, advanced and most of my independent study students read the article, then we met as a group to discuss how we might implement testing of our glazes. Most of the current studio glazes were used at YVCC before I was hired, though we've added a few more over the years. The glazes are supposed to be food safe, but we are basing this assumption on historical use or on what the glaze recipes suggested when we first started mixing them for studio use. To my knowledge, we have never actually tested them ourselves.

A reduction copper glaze that has been in the studio for at least 7 years.

After reading the article, the students discussed what needed to happen and I was pleased to find that I didn't need to direct them much, mostly the students were able to determine how to divide the project between themselves. Each student make six bowls or plates for the project and each student was responsible for glazing their two sets of three pieces using the same coordinated glazing approach.

glazed bowls and plates with detergent and lemon

The students also worked together to set deadlines for their throwing, firing and glazing. One set needed to be re-glazed and was finally out of the kiln on Thursday, so we set up the actual glaze testing that morning. Each glaze was applied to three bowls, marked "control," "lemon," and "detergent." The control was left empty, a lemon was squeezed into "lemon" and left to sit, and a small amount of powdered dish detergent was poured into the third, then wet with a small amount of water and mixed lightly.

our glaze testing station

We left the whole set of bowls out on the counter, labeled, over the weekend. When we came back, we cleaned out each bowl, to get rid of the sticky lemon and bits of mold and the crusty line of dried detergent. Then we compared our results for each set of bowls, looking for signs of leaching.

new studio glazes. I expected the glaze on the right to leach

Glaze leaching was supposed to be shown by changes in the color or surface qualities (shininess) of the glaze. I anticipated that most of our long-time studio glazes would show no changes, but I guessed that some of the new, untested glazes would show signs of leaching. I was actually hoping that at least one glaze would leach, since 18 sets of unchanged pottery wouldn't prove a very interesting test.

Blue Matte glaze with leaching effects shown in middle bowl

The obvious flaw in our plan was the timeline, since students were using these glazes during the planning and execution of our tests. Any pots with glazes that leached should not be used for eating, serving or cooking and any functional pottery students making work, would likely expect the pots to be used for these very tasks. The article recommended that we use what are called liner glazes, to line the interior of funtional works. These liner glazes should not leach and, since they are generally white glazes, are cheaper to make than colorful glazes with a variety of expensive oxides and colorants added.

discoloration in Blue Matte glaze

The results of our test were unfortunate for a few students, but good overall. In our testing, almost none of the glazes leached. The newest glaze, just starting to be added to the studio options this quarter, was the only one with obvious discoloration with the lemon juice test. This new "Blue Matte" glaze contains lithium and copper carbonate, is a beautiful color, but obviously shouldn't be used functionally--who wants copper and lithium in their Cheerios?

Mary's Special (on bottom) with signs of texture change (center)

One other glaze, "Mary's Special" also appeared to have a change in surface sheen after a weekend's contact with dish detergent. I would consider this test result worrisome but inconclusive and I plan to re-test another set of bowls using this glaze while also urging students not to use the glaze functionally.