Coke is the infusible, solid residue remaining after the distillation of certain bituminous coals, or as a by-product of petroleum distillation. Coke may also be obtained from petroleum residue, pitch, and other materials representing the residue of destructive distillation. Coke will ignite more quickly than anthracite, but less readily than bituminous. It burns rapidly with little draft. As a result, all openings or leaks into the ash pit must be closed tightly when the coke is being burned.

Since less coke is burned per hour per square foot of grate than coal, a larger grate is required and a deep fire-pot is necessary to accommodate the thick bed of coal. Since coke contains very little hydrogen, the quick flaming combustion, which characterizes coal, is not produced, but the fire is nearer even and regular. The best size of coke recommended for general use, for small fire-pots where the full depth is not over 20 inches is that which passes over a one-inch screen and through a one and a half inch screen. For large fire-pots where the fuel can be fired over 20 inches deep, coke which passes over a one inch screen and through a three inch screen can be used, but a coke of uniform size is always more satisfactory.