Zircopax is primarily used in ceramic to opacify glazes. These vary according to particle size, the finer the size the better the opacification. Additionally, the finer sized materials contain a little extra silica for maximum whiteness. 

The amount of zircopax in a glaze determines the opacity. Small amounts (1-3%) may give no noticeable difference but it can improve glaze hardness. Zircopax is refractory. The more that is added the more the degree of glaze melting (and melt viscosity) is going to be affected. Up to 15% or more might be needed to fully opacify a glaze. If higher amounts are needed the glaze formulation may need to be adjusted to reduce the amount of SiO2 or increase flux (to melt the glaze better). 

Zircopax is a very effective body whitener, especially for casting porcelain. Excessive additions give the porcelain an artificial "white plastic" appearance.