Kaolin (01C, 02C, 03C, 04C, 05C, 06C, 07C, 08C, 09C, 10C, 11C, 12C, 13C, 14C, 15C, 16C)
Kaolin
A clay that is essentially kaolinite, a hydrated aluminum silicate.  It has a high fusion point and is the most refractory of all clays.