Borax Decahydrate is used as a flux when forge welding or making Damascus.
The role of a flux in the joining processes is typically dual: dissolving of the oxides on the metal surface, which facilitates wetting by molten metal, and acting as an oxygen barrier by coating the hot surface, preventing its oxidation.
Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. Powdered borax is white, consisting of soft colorless crystals that dissolve easily in water. A number of closely related minerals or chemical compounds that differ in their crystal water content are referred to as borax, but the word is usually used to refer to the decahydrate. Commercially sold borax is partially dehydrated.
Borax was first discovered in dry lake beds in Tibet and was imported via the Silk Road to the Arabian Peninsula in the 8th Century AD.