ZnO HT flux that promotes brilliant glossy surfaces. In some glazes can encourage opacity. With titanium in low-alumina glaze can encourage macro crystalline growth (crystalline glazes). Volatizes in high-fire reduction. Toxic in inhalation.

Common opacifier, often used in place of the more expensive tin oxide. Sold as Opax, Super pax, Ultrox, etc., use 7% in a slip and 10 to 12% in a clear glaze to get pure opaque white. Zirconium particles are extremely refractory and remain as inert inclusions in the glaze-melt. They will not affect mattness or gloss, but will increase viscosity.

ZrSiO₄ zircon opacifier inert dispersoid in glaze melt low-cost substitute for tin oxide use double the recipe weight of tin. Includes Zircopax, Opax, Super pax, Ultrox. All are toxic in inhalation.

Zircon opacifier, no longer being manufactured. See zirconium silicate. Toxic in inhalation.

Period during cooling ramp when a particular material tends to crystallize out of glaze-melt.