Kaolin: China Clay Washed Out of Decomposed Granite

Kaolin, china clay, is granite whose felspar has rotted to a soft white clay. Washed in water, the fine clay stays suspended while sand and mica sink. Andrew Ure (1839) describes the Sèvres way: stir it hard in a cask, let the grit settle a few minutes, run the milky clay off through a very fine sieve and let it settle in covered vats. The picture: the white china clay pit at Wheal Martyn, Cornwall.