Ferro Frit 3134

This is a popular frit and has been used for many years. Equivalents are made by many frit companies. The manufacturer says that it is "intended for use as a lime and borate source in partially fritted glazes
 
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