Kaolin clay is commonly differentiated as "hard" clay or "soft," according to terminology borrowed from the rubber industry. Hard clay is relatively poorly crystallized, very fine-grained kaolin, about 0.2 to 0.4 æm median particle size by sedimentation. It provides reinforcement in rubber, resulting in hard, uncured compounds. Soft clay is a better-crystallized, coarser kaolin, about 1.3 µm median particle size by sedimentation. It has a low reinforcing effect in rubber, resulting in softer uncured compounds.
The basic differentiation of coatings-grade kaolins is hydrous clay and calcined clay. Hydrous clays are structurally unmodified, retaining their hydrophilic surface hydroxyls, the so-called water of crystallization. Most of the hydrous clay used in coatings is water-washed. Water-washed grades are made by slurrying the clay in water and then centrifuging or hydrocycloning it to remove impurities and produce specific particle size fractions. The refined slurry is either dewatered (to reduce soluble impurities) and dried, or concentrated to 70% solids and sold in slurry form. Water-washed clays are often treated to improve their brightness. This includes chemical bleaching and/or high-intensity magnetic separation to remove iron and titanium impurities.