A refractory powder that can be mixed with water and painted on kiln shelves to prevent ware and accidental glaze drips from sticking. Porcelain clays, for example, melt enough during firing that they tend to stick onto the kiln shelf. Certain clays contain soluble salts which fire to a glaze-like sheen, these also tend to stick ware to shelves.
The foot ring on the left is plucking, the right one is not. Why?
These are translucent porcelains, they are vitreous. The firing is to cone 10. The one on the left is a cone 6 body, and, while it survives to cone 10 it does warp. But this problem is fairly serious, making it very difficult to get a good foot ring. The other, which has only slight plucking is also a little over vitreous (having too much feldspar). While the one on the right could likely be fired with no plucking at all using kiln wash powder on the shelves, the other will likely pluck even if the shelves are coated.
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