A clean fire of incandescent coal all around a piece to be forged will insure even heating. There must be a substantial layer of burning coal between the forging and the tuyere, else the oxygen of the air entering through the tuyere will burn the forging. Also the supply of air must be regulated by the damper to avoid letting more in than can be consumed by the burning fuel, as any excess will be taken up by the hot iron, forming a scale of iron oxide over the surface.

The simple means of regulating the air and oil, or air and gas supply to oil and gas furnaces by merely turning the controlling valve of each, makes these forges superior to coal forges.

The degree of heat to which a forging should be raised varies somewhat for different kinds of work. Welding requires a high heat to bring the parts to be joined near the fusing temperature, but for shaping it is sufficient to bring the piece to a red heat. A large forging should be heated as bright as can be without burning and it is often heated yellow because fewer heats are necessary and the more plastic condition at high heat insures the hammer impact reaching further into the mass of metal. The finishing of work at a low red heat shapes the surface, and the blows then should not be heavy. No forging should be done below a red heat, except that light blows may be given to smooth the surface.

The heating of forgings is a part of the subject of the heat treatment of steel, which is now a matter of careful study among those experienced and interested in steel working. Wrought iron is less affected by overheating because it has no carbon to be burned out, and is not subject to the internal crystalline conditions to anything like the degree affecting steel, if at all.