Ferro Frit 3249

"No potter or manufacturer should be without this type of frit (despite how expensive it is). Every frit manufacturer will be making a magnesia borosilicate like this (because it is needed to lower the thermal expansion of glazes and glasses).

Frit 3249, from Ferro (now Vibrantz), is an example of this type. It is valuable because it introduces a form of MgO (the lowest expansion flux) that will melt at much lower temperatures than MgO-sourcing raw materials like dolomite or talc (trading some of the high-expansion fluxes in a glaze for MgO is the most effective way to reduce glaze thermal expansion). Also, high MgO is the mechanism of some of the best matte glazes and a frit like this is often the best way to source it." - Source Digital Fire