(Na2B40 ,oH20) A hydrated sodium borate, crystalline and slightly alkaline, used as a flux for brazing and welding. Borax, or mixtures of borax and boric acid, as well as calcine borax and borax glass are among the most successful fluxes. They are used to restrict the formation of metallic oxides and exert a solvent action on these oxides as they are formed.
Borax is fluid at 760°C (1400 F), but begins to thicken below this point. The addition of boric acid tends to produce a more viscous flux, but it is not as active as borax in dissolving oxides of metal. When heated, boric acid does not bubble like common borax, but spreads over the surface of the joint, thus protecting it from oxidation. The bubbling of heated borax results from driving off the water of crystallization, but this can be avoided by using fused borax. Alcohol
is added to fused borax; water must not be added, as it will rapidly cake.
Dry borax flux can be sprinkled along the joint, but if a torch is used for heating, it is better to warm the joint so that the flux will adhere and not blow away. A thin paste is a satisfactory form of borax, because an even coating of the paste can be brushed over the surface.
Bare spots must be carefully avoided. See FLUX.