Charcoal is a light, black residue, consisting of carbon and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen (see char and biochar). It is usually an impure form of carbon as it contains ash; however, sugar charcoal is among the purest forms of carbon readily available, particularly if it is not made by heating but by a dehydration reaction with sulfuric acid to minimise the introduction of new impurities, as impurities can be removed from the sugar in advance. The resulting soft, brittle, lightweight, black, porous material resembles coal.