When a glaze
is fired and it does not melt sufficiently to form a good glass
there is likely to be excessive amphoteric and/or acid oxides (i.e.
Al2O3 and/or SiO2) causing stiffness and opacity.
to think of this is that there is insufficient flux oxides in the
glaze to take up the available alumina and silica.
It is a question
of getting the right balance of fluxes on the one hand and alumina
and silica on the other.