Calcined kaolin is a powdered white non-plastic material.
Kaolin is pure clay mineral, having a fired chemistry of 1 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. Al2O3 is essential to the chemistry of the vast majority of glazes and kaolin is ideal source material.
Calcined kaolin has many other uses in ceramics. They key property it has is that it is refractory. It is useful in refractory castables and furniture, thermal insulation bodies, low expansion bodies and permeable ceramic compositions.