The use of alumina in the composition of glazes is essential to provide high mechanical strength, wear resistance, thermal shock and chemical corrosion resistance. The glaze could be matt or glossy, transparent or opaque.
Hard calcined alumina gives a shiny effect, while reactive alumina gives a matt and opaque effect.
Hard calcined alumina controls thermal expansion of the engobes.
Special medium calcined alumina – ARZ® series - gives opacification and replaces totally or partially zircon sands in the formulation of engobes and glazes to give whiteness and opacification.
Note that glaze is also used in digital printing applications.
Aluminas with intermediate to relatively high specific surface areas (BET 3- 75m²/g) with alpha alumina contents varying between 20% and 95%.These are produced by controlled rotary calcination of aluminium trihydroxide.
These aluminas have low specific surface area (BET < 1m²/g) and are almost entirely alpha alumina. Standard soda content is 3000-4000 ppm and intermediate soda content is 2000-3000 ppm. Hard calcined aluminas are produced in rotary kilns.