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.

Science week in the studio

For the past week it seems that the studio has been a place for science demonstrations.

Biology
During our clay class critique one student was asked about her mug and its interesting handle shape . Her explanation was that the piece was inspired by the lower intestine, including the appendix. The handle shape was visually interesting, but after I knew the inspiration, I thought differently about it.

In the design studio students were finishing their book project (they create a book that involves actual and implied time and motion examples). One student was expressing her frustration with the Design class. She said that design doesn't come easily to her and that to relax, she does her microbiology homework.

Chemistry
Later in the clay critique another student was showing her teapot. She had attached a thrown handle onto the side like a small Chinese teapot. During the discussion, her classmate pointed out that the shape of the rounded teapot with the two tube shaped spouts looked like it was almost the shape of a water molecule. She suggested adding another tube. Perhaps the second student had been studying the night before.

Physics
Of course the clay studio is always a good place for a practical physics demonstration. The other day we unloaded a glaze firing. Usually there are a few pieces glazed too thickly. The glaze runs down and sticks to the shelf or fuses the lid in place. This quarter we actually had very few problems with glaze running, but we had a couple in the last firing.

last glaze kiln ready to be unloaded

We also had a couple pieces warp during firing. The kiln reaches temperatures hotter than 2300 degrees Fahrenheit by heating it up with gas and forced air. This is a turbulent atmosphere and pots can warp or subtly change shape during firing, usually an open round shape can become oval shaped at the top while the base stays round. We usually combat this by firing lids on their pieces (as long as the glaze doesn't run, this isn't a problem). The lids and pots either warp together or the lid prevents the base from warping. In this batch I had a student fire her piece with the lid beside it. The top warped differently than the base and the two no longer fit right.

We also fired our first bisque firing in the gas kiln (which is usually used only for glaze firing). The electric kilns fire efficiently and can be fired without a lot effort by the instructor. We can set a firing program or we can do simple turn-ups each hour. I also have several students who know how to fire the electric kilns and they can help. However, this quarter I had two students make work too tall for the electric kilns, so we fired a gas bisque. I did a preheat (called a candle) overnight with just the pilot lights lit, then I turned the kiln up to low on one side, waited several hours and turned it up on the other side. Despite this very slow, gentle firing, all four pieces had some damage from the firing. Two pieces cracked at the seams and two had pieces blow off the sides during the firing.

large pieces after firing

Usually damage like the latter comes from the piece being too thick. Some part of the pot has a contained air pocket or moisture which expands as it is heated and forces the clay to move. The cracks in the former pots usually come from weak seams or a fast firing, or both. The gas bisque was a faster bisque than normal. Also, the pots were made from several different pieces thrown, then attached together and worked some more. I recommend that my students use overlapping seams where the top piece sits not just on, but in the wall of the lower piece. This student did not take that extra precaution. I would be curious to know whether the firing or the joining method were more at fault. Regardless, I can't fire the gas kiln much slower than I did.

Our last physics lesson came during the most recent glaze firing. I tell all my students to bisque their work before glazing. This first firing chemically changes the clay to ceramic and makes it stronger. As ceramic, it is porous enough to accept glaze but it won't break or slake (dissolve) if accidentally glazed incorrectly (i.e. dropped into the glaze bucket). Assuming that all my students were following this rule, and assuming that they would tell me if they somehow glazed greenware (unfired work), I didn't worry much about it.

On Friday we were firing the glaze kiln and I noticed that a piece had exploded during firing. I could see the broken pieces as I looked into the kiln. I was surprised that something had broken, since this very rarely happens. One of my students found a piece of the explosion that had fallen out of the back hole of the kiln and onto the floor. She picked it up and discovered that it was still clay. It had not been bisqued, but had been glazed. Of course what must have happened is that someone didn't bisque their work, but somehow still glazed the work and put it in the kiln. They were either confused or were trying to move more quickly. Neither I nor my student helper noticed as we loaded the kiln on Thursday. I fire the glaze kiln faster than a bisque kiln, since I have to get it hotter. Not knowing their was an un-bisqued piece in the kiln, I fired normally. I expect this morning will be interesting as I open the kiln and find the damaged piece and the neighbors that were also likely to sustain damage.