These glazes are
commonly called alumina matts as Al2O3 is the primary amphoteric oxide
present in all glazes.
with Al2O3 can be very stiff and stable sometimes with pinhole defects.
The suspended alumina forms crystals producing soft to dry surfaces
depending on the amount in excess.
Too much alumina
can inhibit the melting of a glaze to the extent that a poor quality
matt glaze results which is prone to discolouration.