Coke

Coke is the residue produced by heating coal out of contact with air till all volatile matter has been removed; it is thus a by-product in the manufacture of coal-gas. Coke is often made also by burning coal in covered heaps after the fashion of charcoal-burning, or by heating it in special coke-ovens. Coke made in ovens is hard, lustrous, and dense, prismatic in structure, silvery-grey and metallic-looking; it is a good conductor of heat and electricity, and difficult to burn. When made by burning coal in heaps coke is dull black, spongy, and easily combustible.

The chief use of coke is for the smelting of iron in the blast-furnace; but for this purpose only the hard coke made in ovens can be employed.