Up to recent years soft coal was the most extensively used fuel for forges, but petroleum residue is now much used, and natural gas is used in localities which supply it.

Soft coal is used because its gaseous constituents distil off continuously during burning and assist the solid carbon to maintain a steady fire. The coal used must be of a quality containing very little if any sulphur, and almost free from mineral matter which will not burn or which will form a pasty mass of slag or clinker in the fire. It should be a free coking coal, broken into small lumps which will stick together slightly during burning. The air blast is supplied in a forge to increase the rapidity of combustion in one spot, and thus supply a concentrated quantity of heat for needs of local heating.

Fig. 143. - Swage Block.

Oil used in forges is the same as supplied for steam boiler uses, although the grades low in sulphur are preferable. To attempt the used of undistilled, or crude, petroleum would be dangerous because of the gasolene and other highly volatile constituents.