Earthenware glazes are made mostly of frit, which is a kind of man-made feldspar. Borax, soda and other water soluble materials can be made insoluble by heating together with silica. The resulting glass is ground up and used together with clay and silica to make low temperature earthenware glazes. Frits are also used to lower the melting temperature of mid-range glazes. The raw mineral borates are found in dried up lake basins in Turkey and California.
Opacifiers can be added to make the glaze opaque. Tin oxide is the most effective opacifier but zirconium silicate is less expensive.
A respirator mask should be worn when weighing dry materials. The dry powders are added to water and left to slake before sieving. 
