Borax Decahydrate is the refined form of natural sodium borate. Composed of boric oxide (B2O3), sodium oxide, and water, it is a mild, alkaline salt, white and crystalline, with excellent buffering and fluxing properties. Borax Decahydrate is an important multifunctional source of B2O3 (e.g. frits), particularly for processes in which the simultaneous presence of sodium is beneficial.
Borax is available in large crystal, powder, and granular form, the latter being the most practical for ceramic use. Although the 10 molecules of water in the theoretical formula vary somewhat, they are the source of the designation "10 Mol Borax". The water content can vary with storage (it tends to loose water with time). Where precision is required, it is necessary to measure the water content just before use.
It begins to melt in its own water of crystallization at 60.8C. It is soluble in water, acids, glycol and other solvents. Almost all American borax comes from deposits of the crystalline precipitate mineral tincal in the Mohave Desert of California (Asian borax is called Tincal).
The solubility of borax normally disqualifies it for use in clay bodies and glazes (since during drying it will come to the surface with the water and be concentrated there). Actually, this phenomena can be used to advantage to make self glazing products like Egyptian Paste and it can produce a hardened surface in non-fired products. In addition, the migration issue during drying can be tolerated in some circumstances, borax can be employed in a traditional ceramic slip or glaze blend as a low-cost super-flux to produce a glaze or slip for low temperatures.
Rio Tinto Borax has many technical, granular and powder grades of this material. "20 Mule Team" borax is this material. It is also produced at the Bandirma Borax Plant in Turkey.