Brazing, the uniting of two pieces of metal, as of brass or copper, or one piece of each, by hard solder. Hard solder is distinguished from soft by being made of metals that require a higher temperature to melt them; but all solders should melt more easily than the metals they unite; and to give the maximum of strength, they should have about the same hardness and malleability as these metals. For brass, copper, iron, German silver, etc, the solder used is an alloy of zinc and copper in equal parts, or for a harder mixture, two parts of zinc to three of copper. The two surfaces to be united are made perfectly clean and bright, then brought together and secured with wire or otherwise in their place, and covered around their edges with the granulated solder, mixed with pounded borax and wet with water. The parts are then heated; the borax melts, and runs over the bright surfaces, protecting them from oxidation; and as the heat increases it fluxes the solder, and this suddenly flushes, or runs through the joints, uniting with the two surfaces, and making with them one piece, as the parts cool and the solder sets. The pieces are then dressed with the file.

It is sometimes convenient to cover the joints and the solder with a clay lute before heating; this is done in soldering iron, to prevent a scale of iron forming on the surface. The borax may be first melted and run into glass of borax, or allowed to froth up upon the joints.