Feldspar Feldspar is a common raw material used in glass making, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber. In glass making, alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. About 66% of feldspar is consumed in glass making, including glass containers and glass fiber. Pottery (including electrical insulators, sanitary ware, tableware, and tile) and other uses, such as fillers, accounted for the remainder.