Glassy Phase in Alumina–Silica Ceramic Shell Molds

Variation of the amount and the composition of the glassy phase in high‐purity Al₂O₃‐SiO₂ shell molds and the crystallization of the glassy phase have been studied. The results indicate that the two main modes of transformation from a glassy phase to a crystalline phase during firing are (1) formation of mullite through a chemical reaction and (2) devitrification of the glassy phase that comes from the original materials. The transformation is mainly influenced by the S/(A + S) value (A and S are the weight percentages of Al₂O₃ and SiO₂, respectively), the Na₂O content, and the firing temperature. Low S/(A+S), low Na₂O, and high firing temperature are favorable to the transformation.