Kaolin, also called china clay, is a pure white clay mineral that fires white and has a very high melting point. It is the main ingredient of porcelain and a common source of alumina in glazes.
What Is Kaolin?
Kaolin is a primary clay, meaning it is found close to where it formed from decomposed granite, so it has fewer impurities than clays carried by water. That purity gives it its whiteness, and its name comes from Gaoling, the hill in China where it was famously mined for porcelain.
On its own kaolin is not very plastic and is hard to throw, so porcelain bodies blend it with feldspar, silica and often some ball clay or bentonite. Different kaolins vary in particle size, plasticity and whiteness, which is why porcelain recipes often specify a particular one.
In glazes, kaolin adds alumina and silica, keeps the glaze suspended in the bucket and gives the dried coat strength. Calcined kaolin, which has been heated to remove its chemical water, is used where raw kaolin would make a glaze shrink and crawl. Kaolin is also used in slips and engobes for a white surface.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.