CHARCOAL. The relation which char coal bears to carbon has been already ex plained. [CARBON]. As now conducted, charcoal is prepared by two different methods. One is that of piling the wood in a heap, which is covered with turfs and sand, to allow of the entrance of such a portion only of atmospheric air as is sufficient to carry on the imperfect combustion required. The heap is fired at several holes left near the bottom, and a draught of air is obtained by at first leaving an orifice at the top of the heap ; this is afterwards covered, and, when it is found that the flame has prevailed the heap entirely, the bottom holes are also closed. In the other method, the wood is put into iron cylinders, which are set in brick-work, and surrounded by fire : the wood in the cylinders has no communication with the external air, and they have only a small opening to allow of the escape of the residual products, which con sist chiefly of water, pyroligneous acid, creosote, pyroxylic spirit, and empyreumatic oil.
Violette communicated an important paper to the French Academy of Sciences in 1848, on the manufacture of charcoal for gunpowder. He found that when wood is exposed to heat in close vessels, it does not become charred till near 500° F., when it produces an imperfect charcoal; at near it yields a brown charcoal; while the best black charcoal requires a temperature of nearly 700° F. He also invented a remark able mode of making charcoal by the heat of steam. The wood is put into a horizontal cylinder, round which is coiled a steam-pipe, and through this pipe is sent a constant flow of steam of a determinate temperature, according as a greater or lesser degree of charring is required. M. Violette has found that fuel is saved, and better charcoal produced by this method; he has adopted it at the powder mills of Esquerdes, which are under his charge.