Borax (sodium tetraborate) is the sodium salt of pyroboric acid. It has the formulae Na2B4O710H2O and a relative hardness of 3. It forms large transparent six-sided prisms which have an alkaline reaction, effloresce in air, and when heated swell-up and melt to a transparent glass. Borax is used in the manufacture of enamel-ware, glass, as an antiseptic and is a food preservative. It is also useful in brazing and silver soldering as it dissolves metalic oxides, thus cleaning the surfaces of the metals to be united.
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