|Monthly Tech-Tip |
The glaze on the right is crawling at the inside corner. Why? Multiple factors contribute. The angle between the wall and base is sharper. A thicker layer of glaze has collected there (the thicker it is the more power it has to impose a crack as it shrinks during drying). It also shrinks more during drying because it has a higher water content. But the leading cause: Its high raw clay content increases drying shrinkage. Calcining part of the raw clay destroys its affinity for water (which is what makes it plastic), this is an effective way to deal with this. Or doing a little chemistry to source some of the Al2O3 from materials other than clay (e.g. a frit having a higher Al2O3 content).
In ceramics, glazes are slurries. They consist of water and undissolved powders kept in suspension by clay particles. You have much more control over the properties than you might think.
Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
A ceramic glaze fault that occurs during firing of the ware, the molten glaze pulls itself into islands leaving bare patches of body between.
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