Below you can see the effect of adding opacifier to a transparent glaze containing different colored pigments
Top = transparent glaze Bottom = opaque glaze
Typical glaze opacifiers are zircon (zirconium silicate), tin oxide, and titania (titanium dioxide).
|Zircon opacifier powder|
ZirconZircon is the preferred opacifier for glaze due to the low cost, inertness and stability in glaze. To achieve full opacification, the opacifier content and the particle size are important factors. For addition of zircon to the glaze mill during grinding a particle size of 95% less than 3 microns and a content of 5-10% ensures sufficient opacifier is dispersed to create optimum opacity.
However an exceptional level of opacity can be achieved by pre-melting the zircon into a glass (called frit) and then using the frit as part of the glaze recipe. These frits are transparent before firing but crystallize zircon during the glaze firing process to give a high level of opacity.