Forge welding is a welding process of heating two or more pieces metal and then hammering them together. Its use is ancient, being one of the least technologically complex methods of joining metals. The metals to be welded can be well below their melting points, greatly reducing the infrastructure needed. Forge welding is versatile, being able to join a host of similar and dissimilar metals. However, with the invention of electrical and gas welding methods during the Industrial Revolution, forge welding has been largely replaced.
Forge welding between similar materials is caused by solid-state diffusion. This results in a weld that only consists of the materials without any fillers or bridging materials.
Forge welding between dissimilar materials is caused by the formation of a lower melting temperature eutectic between the materials. For this reason many dissimilar metals welded together with superior properties of the weld.
The welding temperature is above the forging temperature, and not so very far away from the melting point of the metal. It is typically 50 - 90% of the melting temperature. 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.

Flux
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.
Early examples of flux used different combinations and various amounts of iron fillings, borax, sal ammoniac, balsam of copaiba, cyanide of potash, and soda phosphate. The 1920 edition of Scientific American book of facts and formulae indicates a frequently offered trade secret as using copperas, saltpeter, common salt, black oxide of manganese, prussiate of potash, and "nice welding sand".