Why does clay become hard and waterproof when it’s heated? Why does temperature affect different clays in different ways?

Question asked by Volunteers at the Idaho Art Lab | Answered by Dr. Paul Link, ISU Department of Geosciences The purest clay is made of the mineral kaolinite, KAlSi3O8. Ceramic clays also contain quartz and feldspar minerals. During the heating (firing) process to about 1400 degrees F, the feldspars melt, along with some of the…