I have done alot of work recently comparing the sourcing of fluxing oxides from frits vs. raw materials. The differences are often incredible. The oxides MgO and CaO normally come from materials that melt high. But common frits that source them melt low. The chemistry in the two cone 6 glazes (comparing on this melt flow tester) are the same. But Y4 sources the KNaO from Ferro Frit 3110 instead of feldspar and alot of the MgO from Ferro Frit 3249 instead of talc. Y4 would flow even more but also has 10% calcined alumina! Sourcing Al2O3 from such a refractory material should make it melt less than sourcing it from kaolin, but the frits overcome even that to create this amazing fluidity in a matte glaze. The lesson: All glazes have a chemistry, but that cannot be taken in isolation from the materials that source it.
Ferro Frit 3110
High sodium, high thermal expansion low boron frit. A super-feldspar in clay bodies.
Ferro Frit 3249
A magnesia borosilicate frit having very low thermal expansion and melting point. Invaluable in pottery as to increase the MgO in glazes to prevent crazing.
Frits are used in ceramic glazes for a wide range of reasons. They are man-made glass powders of controlled chemistry with many advantages over raw materials.
Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.