Variation of the amount and the composition of the glassy phase in high‐purity Al2O3‐SiO2shell 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 theS/(A + S) value (AandSare the weight percentages of Al2O3and SiO2, respectively), the Na2O content, and the firing temperature. LowS/(A+S), low Na2O, and high firing temperature are favorable to the transformat