... raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber. In ... , the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture ... electrical insulators, sanitaryware, tableware, and tile) and other uses, such as fillers, accounted for the remainder. SiO2: 70-75% K2O: 3 ...