This section is from the book "An Elementary Outline Of Mechanical Processes", by G. W. Danforth. Also available from Amazon: An elementary outline of mechanical processes.
This is a process much used for uniting copper, brass or iron in a solid metallic joint of considerable strength, though the strength of the joint is not equal to that of the solid metal. The brazing material used, known as hard solder, spelter, or brazing metal, is variable in its. composition. It may contain copper, zinc, tin, and silver, according to the melting point required and to the required strength of joint. The usual composition for brazing brass and copper is about 60% copper and 40% zinc. This alloy is melted together, and, after cooling it is heated to redness and broken into small lumps in a heavy mortar or on the anvil.
The essentials in brazing are (1) the metals to be brazed must be filed or scraped to a clean metallic surface and must be protected from becoming re-coated with oxide during the process by means of a flux, and (2) the metals to be joined together must have higher points of fusion than that of the brazing metal.
Brazing is accomplished by applying a considerable degree of heat to the parts to be brazed. This heat melts the brazing metal and allows it to run into the joint. The two parts to be brazed must be suitably held together by wires, tongs or clamps and the brazing metal must be so placed on the joint that gravity will cause it to flow into all parts of the joint when melted.
Borax, fused to drive off the water of crystalization and powdered when cold for applying it easily, is used as a flux. It may be mixed with the broken-up brazing metal if desired. If water of crystalization is not driven off, the borax swells and bubbles, causing more or less annoyance.