China Clay Calcined
Kaolin is pure clay mineral, having a fired chemistry of 1 molar part Al2O3 and 2 parts SiO2. But the raw clay crystals are hydrated, having 12% crystal-bound water. This is the secret to their plasticity. Calcining the material removes that water and destroys the plasticity.
Calcined kaolin has an important use in ceramic glazes. Al2O3 is essential to the chemistry of the vast majority of glazes and its most readily-meltable natural supplier is feldspars, however they most often oversupply KNaO in providing the needed Al2O3. Raw kaolin is an ideal second or even primary source of Al2O3. Not only does it readily decompose in the melt but it also supplies SiO2 that all glazes need. And it acts to suspend the slurry and harden the dried layer. But there is a problem with raw kaolin: Once recipe percentages pass 20% shrinkage becomes too high (causing crawling). In these cases substituting part of the raw kaolin for calcined kaolin solves the problem, maintaining the chemistry of the glaze but reducing the shrinkage and cracking. Of course, mixing the raw and calcined materials must take into account the LOI of the raw material (12% less calcined is needed).