Crystallization of glass of the system K2O(Na2O)Al2O3SiO2

In a whiteware composition, the glassy phase derives from the molten feldspar in which a portion of quartz, clay and other crystalline constituents are dissolved. This glass is the major and continuous phase in the whiteware body and is the potential source of the crystalline phase, viz. mullite. Synthetic glass has been prepared by melting mixtures of feldspar and quartz comparable to whiteware-glass in composition. Glasses were also synthesized by melting mixtures of feldspar, quartz and mineralizers. All of these glasses were heat-treated and their mullite contents were estimated by X-ray analysis supported by TEM & SEM study. Among thirtyone (31) different mineralizers tried, only thirteen (13) are found to be effective mullite is builders. The cation of the mineralizer helps replacement of the Al³⁺ ion from the glass which subsequently diffuses through the residual SiO₂ and mullite is formed. The replacement reaction and hence the degree of mullitization is dependent on factors, e.g. charge, radius, field strength of the cation as well as on the cation-oxygen bond strength. This last appears as the most predominant factor for mullitization.