Glaze Testing Pt. 1

Glaze Test Corrections for Oxidation

After the destructive flooding in Vermont which took place back in July (2023) I was displaced with my work as I fully relied on the use of the gas kiln to produce my work. We thankfully had an extra electric kiln and despite its small size, it allowed us to still produce work. Because of the situation I was forced to try my hand in oxidation firing. I had been very dismissive about oxidation firing for a while as I felt there wasn’t much life to the pots when fired this way. Granted, I also didn’t try to make it work before so of course that would be my expectation.

Within the first couple firings I found some extra test pots laying around, glazed them in my satin white glaze, and put them in the electric kiln. The glaze was honestly pretty bad. It had pinholing and a plethora of crazing. My attention shifted from the seeming failure of the glaze to the success of the clay body. It was a beautiful reddish brown with tiny hues of purple. The clay is what was bringing the life to the pot. Since I really had no other option I shifted gears and started the journey of altering the glaze in hopes to make it function on my clay.

Below is the original test in Oxidation, Fired to 2300 degrees Fahrenheit:

The first step to fixing crazing was to increase silica. Crazing comes from a difference in expansion between the glaze and the clay body. Crazing occurs when the glaze shrinks more than the clay body. To break the tension the glaze cracks which creates the crazing effect. My mentor always explained it like a super muscly dude wearing a really tiny shirt, it doesn’t take much movement for the shirt to rip. I always thought that was a funny analogy.

Additions of silica help reduce the expansion of the glaze. My tests added increments of silica starting at 5% and went up to 14% in the recipe. When the tests came out i thankfully fixed the crazing but ran into another problem. Silica itself is a very refractory material. It needs feldspar and other fluxing materials to actually bring it down to melt into a glass. So by saturating the glaze with more and more refractory material (silica) I ended up raising the maturity temperature enough that the finished glaze couldn’t heal blisters and pinholes. It fixed one problem but created or exaggerated another.

Fixing the melt while maintaining the surface ended up being a much harder problem to solve than I expected. I don’t have a crazy amount of chemistry knowledge but even with all the research I’ve been doing I have gotten stumped one time after the other, again and again.

I began researching avenues to go down in order to fix my issue. One fix I found was on Digitalfire.com and the remedy was to do a drop hold in the kiln once it reached its temperature. The firing program wanted you to drop the temperature by 100 degrees and then hold it there for another 30+ minutes. I tested this and had very good results! The blisters and pinholes healed over much nicer!

Strangely enough about a week or so later I ended up testing the same exact glaze mixture and for some reason it came out blistered and scummed like never before! I was extremely confused because I didn’t change anything. It was the same glaze mixture on top of the same clay (batch of clay and pots were made at the same time), same kiln, same firing schedule. The only variable was time. I found out one possibility was the formation on soluble salts coming to the surface from leaching out of the dry materials. It made sense having time being the only real culprit.

A remedy for this was to add small additions of barium carbonate. Barium Ions and sulfur ions have opposing charges. Additions of barium create a bond between the two and prevent sulfur from moving to the surface of the glaze. At least, on a very dumbed down explanation that’s what happens. I tested this with success for the most part. Even after a week the glaze was firing the same as the first time.

Shown below:

Left photo - Base glaze and added Barium Carb 2%, thin dip

Right photo - Base glaze and added Barium Carb 2%, thick dip

I noticed after a little while I was getting a bit carried away with “fixing” the glaze. I was definitely paying attention to how my alterations affected the glaze result but I was too wrapped up in understanding the numbers to realize that the actual color, look, and feel of the glaze was becoming more and more distant to what I actually wanted. This is a recurring phenomenon with me. It’s a bit of a vicious cycle.

What the target glaze would be is a satin glaze, slightly opaque, which could theoretically work well in both reduction and oxidation atmospheres. The glaze is smooth and buttery to the touch. The glaze will break nicely on edges to highlight the dark clay underneath. The light color of the glaze also helps to emphasize the dark clay where there is no glaze. This would be the best description of a goal I can make. It is specific but has plenty of wiggle room.