Ancient milk glasses used crystals of calcium antimonate, formed in the melt from calcium present in the glass and an antimony additive. Opaque yellow glasses contained crystals of lead antimonate; bindheimite mineral may have been used as the additive. Under oxidizing condition, lead also forms incompletely dissolved lead pyroantimonate (PbSbO). From 2nd century BC tin oxide appears in use as opacifier, likely in the form of cassiterite mineral. Opaque yellow can be produced as lead stannate; the color is paler than the lead antimonate one. Later calcium and sodium phosphates became used; bone ash contains calcium phosphate in a high proportion. Calcium fluoride was also used, especially in China.
For dental ceramics, several approaches are in use. Spodumene or mica crystals can be precipitated. Fluorides of aluminium, calcium, barium, and magnesium can be used with suitable heat treatment. Tin oxide can be used, but zirconia and titania give better results; for titania, the appropriate resulting particle size is between submicron to 20 μm. Another desirable opacifier is zinc oxide.
Opacifiers must also form small particles in the system. Opacifiers are generally inert.