Zircopax is no longer produced. In lieu of Zircopax, there are several alternatives that will serve as an opacifier in your glaze. These substitutes include:
Zircopax is a brand name version of zirconium silicate. It is often used for semi-opaqueness, finer materials like Superpax being used for full opacity. Zirconium silicates are used primarily as opacifiers in glazes at all temperatures. Zircopax Plus is a cheaper alternative to tin oxide.