... Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender ... product hardness, durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and ... glassmaking, including glass containers and glass fiber. Pottery (including electrical insulators, sanitaryware, tableware, and tile) and other uses, ...