Borax or sodium tetraborate is the sodium salt of boric acid.
The name comes from the same mineral (Persian burah), which has been used since antiquity in the manufacture of glass. Also the name boron is derived of the principal element borax.

Borax is a salt that is poorly soluble in cold water, but well in warm water. After evaporation of a borax solution a glassy layer on metal surfaces prevents strong oxidants and promotes the outflow of metal. Borax is, therefore, known as a flux in the soldering.
Pesticides against woodworm based on borax and boric acid are very toxic. Because of the toxicity borax is used as a disinfectant.

The mineral borax (Na₂B₄O₅(OH) ₄.8(H₂O))is the main source of borax. Also colemanite (Ca₂B₆O₁₁.5(H₂O)), kernite (<Na₂B₄O₆(OH)₂.3(H₂O), and tincalconite (<Na₂B₄O₅(OH) .3(H₂O)) were mined for it.