This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.
Porcelain is a glazed material used for insulators, etc. It is made by mixing kaolin, fine sand, and powdered feldspar, shaping the mass, and then heating it to a high temperature. The surface is glazed by being coated with a mixture of salt and heated. The heat causes the glaze to melt and penetrate the surface.
 
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