Forge welding is a welding process of heating two or more pieces of wrought
iron or steel until their surfaces are malleable and then hammering them
together. Its use is ancient, doubtlessly being the first method devised for the
joining of metals. Until the invention of electrical and gas welding methods
during the Industrial Revolution, it was the only available method.
Often a flux is used to keep the welding surfaces from oxidizing and
producing a poor quality weld. The flux also mixes with the oxides that do form
and lowers the melting temperature and the viscosity of the oxides. This enables
the oxides to flow out of the joint when the two pieces are beaten together. A
simple flux can be made from borax, sometimes with the addition of iron filings.
The welding temperature is above the forging temperature, and not so very
far away from the melting point of the metal. Steel welds at a lower temperature
than iron. The metal may take on a glossy, or wet, appearance at the welding
temperature. Care must be taken to avoid "burning" the metal, which is
overheating to the point that it gives off sparks from rapid oxidation.
Multiple layers of different kinds of iron and steel can be forge welded
together to produce pattern welded or Damascus steel. A similar process is used
to produce the blades of katana.