These situations can be fixed using glaze chemistry to source Al 2 O 3 more from kaolin or ball clay and less from feldspar (the Na 2 O/K 2 O can come from a much lower alumina material, like a frit ( Ferro Frit 3110). 2) High feldspar glazes have high amounts of Na 2 O and K 2 O. Yes, these are good melters (fluxes) but they have by far ...