Assessing the fusibility of feldspathic fluxes for ceramic tiles by hot stage microscope

The fusibility and firing behaviour of several types of feldspathic fluxes used in ceramic tile manufacturing were appraised by hot stage microscopy. Each flux (nepheline syenite, sodic feldspar, potassic feldspar, quartz-bearing mixed sodic-potassic feldspar, and quartz-feldspathic sand) is characterized by different sintering, softening and melting temperatures. Fusibility depends basically on the amount and composition of feldspars and feldspatoids as well as on the quartz/feldspars ratio. Bloating phenomena fundamentally occur to silica-rich and to clay- or carbonate-bearing raw materials. A simple model was set up by multiple regression analysis in order to predict the melting temperature of feldspathic fluxes on the basis of their composition.