|Monthly Tech-Tip |
It would craze glazes! This is fired at cone 6 and the crazing was like this out of the kiln. This is about as bad as I have ever seen. One might think that there is adequate quartz in this high of a percentage of ball clay to at least minimize crazing, but no so. This demonstrates the need for adequate pure silica powder in stoneware bodies to give them high enough thermal expansion to squeeze glaze on cooling to prevent crazing like this. This is also not proving to be quite as refractory as I thought, it looks like it will have about 3% porosity at cone 10.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
In ceramics, cristobalite is a form (polymorph) of silica. During firing quartz particles in porcelain can convert to cristobalite. This has implications on the thermal expansion of the fired matrix.
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down through 573C. Fired cracks are often related to this.