Glaze-body expansion mismatch is the root cause of both crazing (glaze expansion too high) and peeling/shivering (glaze expansion too low).
Understanding This Defect
The key insight is that flux materials are the primary tool for adjusting expansion in both body and glaze. Na2O and K2O increase expansion strongly; SiO2 decreases expansion; CaO has moderate effect. To raise body expansion: increase nepheline syenite (high Na2O+K2O). To lower glaze expansion: replace nepheline with K-feldspar (higher SiO2, lower total alkali) or increase quartz in the glaze. To raise glaze expansion: increase nepheline in the glaze (rarely recommended — better to raise body expansion instead).
Solution & Prevention
The ideal combination for most tile production: nepheline (K100 or K200) in the body for efficient vitrification and raised body expansion, plus K-feldspar (SY101) in the glaze for controlled expansion and good surface quality. This body-nepheline/glaze-feldspar approach is used by leading tile manufacturers worldwide..
