Soft sodium borosilicate frit for glazes. Since it is a good source of Na2O it is very commonly found in glaze recipes all temperatures. Often used in crystal glazes. It has a high thermal expansion, therefore useful for substitution into glazes that are shivering. Since it is somewhat soluble some precipitation can occur in glazes stored for lengthly periods. 

This frit can be very useful to reduce the feldspar content in glazes (since many high feldspar glazes have low clay content and therefore poor slurry suspension properties and dried hardness). The chemistry of this frit is similar to a feldspar (but with low alumina and CaO in addition to the alkali fluxes).