It was spray applied on the dried bowl (no bisque fire) an was too thick (not to mention under fired). But the main problem was a glaze recipe having too high a clay content. If a glaze has more than about 25% clay, consider a mix of the raw clay and calcined. For example, you can buy calcined kaolin to mix with raw kaolin. Or you can calcine the clay in bowls in your kiln by firing it to about 1200F.
A ceramic glaze fault that occurs during firing of the ware, islands of glaze form as it crawls, leaving bare patches of body.
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.
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