Coke is a hard, gray, carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile components are driven off by heating in a special furnace at temperatures as high as 2,000°F so that the fixed carbon and residual ash are fused together. Before the exploitation of natural gas, much coal gas was thus produced for use as a fuel (heating value of 24.8 million BTU per ton) and also to make water gas. Today, most coke is used as a reducing in metallurgy, mostly in blast furnaces. Such coke must be strong (to support the weight of the charge), porous, and relatively pure.