Ferro Frit 3249

Frit 3249 is a valuable source 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. 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. This frit is also an excellent source of boron to produce a much stronger and utensil-resistant glaze surface. 

Frits are manufactured (melted and fused in a furnace, quenched and ground to a powder) precision stable mixtures of inorganic substances. Basically, they are complex combinations of oxides normally including silica (glass). With the addition of 10% clay, most are clear low-fire glazes by themselves. They offer stability and relative safety in their powdered form by rendering solubles and hazardous components insoluble by combining them with silica. Formulated and manufactured to exacting specifications, they are the exception to the usual impure variety of most ceramic raw materials.
Safety Data Sheet