Charcoal is an amorphous form of carbon produced when wood, peat, bones, cellulose, or other carboniferous substances are heated with little or no air present, or are destructively distilled, for example in a kiln or retort. A highly porous residue of microcrystalline graphite remains. Charcoal has a high energy density and is used as a fuel and was used in blast furnaces in the manufacture of steel until the advent of coke. Charcoal is also used as a thermal insulator, by artists for drawing, in the manufacture of gunpowder, as a decolorizing agent, and in sugar refining and solvent recovery. A highly porous form, activated charcoal, is made by heating charcoal in steam; it is used for adsorption in refining processes and in gas masks.