As the rock and stone that make up the earth's crust are ground down by erosion into microscopic particles, clay is formed. Fe 2 O 3 6.88 6.70 0.57 MgO 3.49 3.08 0.42 CaO 5.08 4.26 . O 1.15 3.45 13.28 TiO 2 1.05 0.91 . The relative concentrations of these mineral components in clays is actually quite variable. Red clays generally have compositions close to that of the overall crust of the earth with a good deal of iron oxide and soluble bases in them.
Kaolin is well hydrated and has a much higher percentage of alumina than does red clay.
Oftentimes a naturally occurring clay will be modified with additives. This is done because each type of clay body will have different characteristics of plasticity, openness, and shrinkage, and these properties are important to the working potter. Furthermore, different clay bodies will fuse and mature at different temperatures. Components that lower the fusion point and allow the pottery or glaze to be fired at lower temperatures are called fluxes . The table below lists some of the fluxes commonly used in clay bodies and glaze furmulations.