ZrSiO4. Zirconium silicate. Zircon. A purified, micronized glaze opacifier. 10%-15% Ultrox Extra added to most clear glazes produces a good white. Add to coloured glazes to make opaque pastels. It can be used in recipes calling for Zircopax, Superpax, etc. (alternate brands differ somewhat in effective opacifying power depending on the fineness of their grind).