Fritting involves the melting together of glaze materials, some of which are body-soluble, to produce a combination which is relatively insoluble.
Lead Bisilicate is the frit generally used in Low Solubility glazes, and Standard Borax Frit for unleaded glazes. The Alkaline Frit will give beautiful crackle blue from copper oxide addition, and the Soft Borax will give turquoise.
Most frits, with the exception of lead-based ones, can be used in stoneware glazes since the typical % addition would be less than 70%, quite often around 10-20%.
See our Technical Information Sheet "Safe Handling of Powdered Ceramic Materials"