Abrupt expansion in heating and corresponding contraction in cooling that occurs in silica crystals in all clay and glazes at around 1063° F. Greatest risk is in cooling high-fired wares and in re firing previously high-fired wares.

SiO₂  The primary glass-former in clay and glazes vitrification, fluidity, transparency/opacity controlled by adding fluxes and/or refractories. Highly toxic in inhalation.