Most welding (fusing two pieces of steel together) nowadays is done with welding torches, such as TIG or MIG torches. Torches such as these melt both steels whilst a third metal (normally in the form of a rod) is melted into the cavity. Forge welding was mainly done in the 1800’s, before torches of that heat were invented. Still today many bladesmiths use the ancient technique to weld bars of steel together to make patternweld, or damascus, steel.
In forge welding damascus, the individual bars of steel are laid one on top of the other and given a simple weld with a torch to keep them all together. This stack, called a billet, is then placed into the forge to get up to welding temp. Every few minutes, the bladesmith takes the billet out of the forge and sprinkles a flux (normally Borax) over the hot surface. The flux melts, and flows between the individual bars, keeping the oxygen out of the way. Once the metal is up to welding heat (where the steel looks like it is just barely melted; looks shiny and almost watery) the smith forges the billets together with heavy whacks of the hammer. The force and incredible heat cause all the bars to fuse together, and are completely welded into one firm billet. The smith then forges out the bar, twists it, cuts it, and does whatever is necessary to get a beautiful Damascus pattern in the steel.