Hydrous Kaolin it is whitish soft plasticity clay derived from the decomposition of granite or similar rock. Most kaolin is wet refined to produce a range of products to improve both physical and chemical properties to meet specific customer applications.
Calcined kaolin is a powdered white non-plastic material. It is raw kaolin that has been fired (in a rotary calcining kiln) high enough to remove the 12-14% (approx.) crystal water. The material is a good example of how we can alter the mineralogy of a material to affect its working properties while maintaining the chemistry to maintain fired properties.