The economical use of fuel requires that none of it should be wasted in an unburned state, as is frequently the case in the escape of unburned gases up the chimney. Combustion is the chemical union of an oxidizable substance with oxygen, and when this is accomplished rapidly, as in ordinary burning, an intense degree of heat is generated. Flame is caused by the burning of gas. The flame from solid or liquid fuel is caused by the burning of gas distilled from the fuel. The presence of fuel in many metal producing and metal shaping operations is objectionable (though unavoidable) for the following reasons, viz.: in the presence of high heat (1) the oxygen necessary for combustion consumes more or less of the metal or other material subjected to the operation, and (2) the substance of the fuel, and particularly the impurities in the fuel, combine with a metal in the process of smelting and in subsequent heating or melting operations and impair its quality.

The essentials for burning fuel completely, with no visible smoke and no waste of combustible substance, are (1) every particle of combustible substance must be supplied with the oxygen necessary for complete oxidation, and (2) the fuel and the oxygen must be hot enough to combine.

Every combustible substance must be heated up to a definite temperature, called the temperature of ignition, before it will burn. Each burnable substance has a temperature of ignition differing more or less from that of other substances.

In the confined space of a furnace, coal in small lumps or porous coke present much surface to combustion and therefore burn rapidly, but if coal is too fine, it masses together and leaves too little passage for air.