Agree Babs: but he said "unstable glaze- not enough alumina and/or silica." Alumina and silica are refractory materials. Depending on the ratio of flux/ refractory materials, the melting point will rise and fall....... I took a short cut in my explanation. hope it is now clearer what I meant.
Yep. If it was fired cooler it might be fine, but at that temp it needs more silica or alumina to stabilize it. If we had the recipe we could compare it to limit formulas, but without a recipe you could run tests adding equal parts alumina and silica in 2% each increments. At some point, if that really is the problem, the craters will stop. Equal parts clay and silica will generally keep the si:al ratio the same as the original, or at least very close.