Rarely does a week go by at the studio where I don't learn something. Usually it is a new technique, or a new tool, or maybe trying a new clay. Once in a while it is something much more challenging to understand and this was one of those weeks.
We unloaded the glaze kiln a few days ago and included was a dog bowl that I had created as a prototype for a new technique and style. It was using one of the iron heavy clays that I like with white and colored underglazes applied using brushes, sponges, etc. The end result looked pretty good and I was actually quite pleased. Other people also commented on the piece and the comments were all very positive.
I put the bowl on my side-desk in my office so I would remember to look at it much more closely the next day. The following day I noticed something sitting about 8" away from the bowl and after a moment or two I realized it was part of the glaze from the bowl! The bowl was untouched, but the clear glaze and some of the underglaze had just sprung off of the rim! With the help of one of my fellow potters, we determined that this was called "shivering". It is an indication that the glaze and the clay do not fit each other well. Shivering means that the glaze is compressing at a rate that is greater than the clay and in doing so, it shivers off of the surface of the clay and pops off.
OK, that stunk. Buy why was it doing this?
Upon further research we found information on Carbon Core(ing). In our studio, we use a gas fired reduction kiln. This means that during the firing process, we are intentionally using the O2 in the kiln as part of the fuel and we are allowing the firing to consume all of the oxygen in the atmosphere. A reduction firing can cause carbon core at both low and high temps. At higher temps, smoking can cause the center core to blacken which results in a brittle body. Some clays cause worse carbon core than others based on their chemistry. Clay that is higher in iron will typically have a greater risk of carbon core than non-iron clays. If you look at the photo that accompanies this post you will see the bottom of the piece is still a redish brown...but the cross section is black. THAT is carbon core.
So what does this mean to me? First, the piece that brought all of this to my attention had a combination of elements that I had not previously used all together in one piece. I have used this specific clay many times without issue. I have used the clear glaze on this clay in the past without issue. I have used underglaze in the past without issue. But I have never used underglaze on this high-iron clay with the clear glaze.
I need to be aware that carbon core is something I have no control over as it is purely a combination of the clay and the firing. I don't do the firing. It can also depend on where the specific piece is sitting inside the kiln as some areas will be hotter than others due to thermal mass. However, I can start paying closer attention to the color of the clay after the firing. If it is a dark red/brown I will know that carbon coring is likely present. If it is a lighter toast red/brown, carbon coring is likely not present. I can choose to do my underglaze pieces using a clay that has no iron. That will likely prevent the shivering issue.
Whew...who knew that the science behind this ancient art was so complex????
Next week, we will address crazing and crawling! Until then...CHEERS!