Welding is the union of pieces of metal by fusing the opposing surfaces that have been made molten by heat. Processes similar to welding include brazing and soldering. Allied processes include the removal and cutting of metals. Common heat sources are the electric arc and oxy - gas flames. Gas welding/cutting entails heat being applied to the metal by the flame from a torch in which a gas, such as acetylene, is burnt with a supply of oxygen or air.