This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.
The principal property of kaolin, or clay, is that it becomes slightly soft (plastic) when wet and may be molded into various shapes. When clay is heated it shrinks and in cooling becomes very hard. The color of clay, which is due to the presence of iron and other impurities, varies from gray to red.
 
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