In ceramics and glass production, feldspar is used as a flux. A flux is a material that lowers the melting temperature of another material, in this case, glass. Substitutes and Alternative Sources. Feldspar can be replaced by other minerals and mineral mixtures of similar physical properties. Minerals that could be used to replace feldspar include pyrophyllite, clays, talc, and feldsparsilica (quartz) mixtures.