ZIRC

Zircopax is milled zircon, and the usual way a glaze is made white and opaque. The particles stay suspended in the glass rather than dissolving, and that is what scatters the light.

Zircopax works by staying undissolved: zircon bends light far more strongly than glaze glass does, so particles held in suspension scatter it instead of letting it through, and the glaze goes white. A glaze commonly carries between a twentieth and an eighth of its weight in it, a dense white at the upper end; past that the opacity has little further to go and the extra mostly stiffens the melt. Colours under it come out paler and softer rather than duller, which is the scattering lifting the lightness rather than anything happening to the colourant. It will not carry a copper red, though — the reduction a red needs does not happen with zircon in the glaze.