CLAY: (p) Types.
Preparing clay for use
Clay is the fundamental material for all types of work in ceramics, from industrial brick and tiles to decorative jars and plates. The two main types of clay are primary and sedimentary. Clay is a damp plastic material that can be manipulated into any shape, left to dry and then fired to become bisque, which is not pliable, but breakable and porous. To stop it being porous it has to be covered with a glaze and fired again.
Primary Clays are also known as residual clays. They are clays that have remained in the same ground in which they were formed. There are few and the main one is China clay. Its characteristics are whiteness and strength and it is the main base for porcelain china.
Secondary clays are also known as sedimentary clays. They are clays have that been displaced and eroded by earth movements throughout thousands of years.
All clays have to be prepared before using or purchased already prepared. Clay is the material out of which all ceramic work is made. It is heavy, damp and has to be prepared to have a workable consistency that can be molded and shaped. When fired, clays change after reaching 600º C into bisque, which is hard, breakable and porous. There are many different types of clays, with different colors, textures, plasticity and shrinking capacity that become fusible at different temperatures. All clays are fusible, this means that they can melt and disintegrate if fired above a certain heat.
It becomes bisque at 600º C, but the more it is fired the stronger it becomes. When using new clay always check to what temperature it can be fired. Normally clays for pottery can be fired up to 1050º C and there are refractory clays that can stand temperatures of up to 1300º C.