WOOD-CHARCOAL is the most important, although not the purest, kind of charcoal. Wood consists of carbon, hydrogen, and oxygen, the last two being in the proportion to form water. When heated in the open air, it burns completely away, with the excep tion of a small white ash; but if the supply of air he limited, only the more volatile matters burn away, and most of the carbon remains. This is Ury principle of the proc ess of charcoal burning in countries where wood is abundant, as, for example, in the Harz mountains. "A number of billets of wood are built up vertically in two or three rows into a large conical heap, which is covered over with turf or moistened charcoal ash, holes being left at the bottom for the air to get in. A hollow space is also left in the middle of the heap, to serve as a flue for the gaseous matters which are evolved. The heap is set on fire by throwing burning pieces of wood into the central opening, near the top of which, however, a kind of grate, made of billets ;,-f wood, is placed, to pre vent the burning fuel from falling at once to the bottom. The combustion then pro ceeds gradually from the top to the bottom, and from the canter to the outside of the heap; and as the central portions burn away, fresh wood is continually thrown in at the top, so as to keep the heap quite full. The appearance of the smoke shows how the combustion is proceeding: when it is going on properly, the smoke is thick and white; if it becomes thin, and especially if a blue flame appears, it is a sign that the wood is burning away too fast, and the combustion must then be checked, by partially stopping up the holes at the bottom, or by heaping fresh ashes on the top and sides, and pressing them down well, so as to diminish the draught. As soon as the combustion is com pleted, the heap is completely covered with turf or ashes, and left to cool for two or three days. It is then taken to pieces, and the portions still hot are cooled by throwing water or sand upon them: 100 parts of wood yield on tne average from 61 to 65 parts by measure, or 24 parts by weight of charcoal."—Watts's Dictionary of Chemistry, vol. i.
p. 759. The charcoal thus prepared is the best suited for fuel. In England, a large quantity of charcoal is obtained in the dry distillation of wood in cast-iron cylinders, for the preparation of crude acetic acid. the charcoal thus prepared is preferable for making gunpowder, but is inferior for other purposes. A peculiar kind of charcoal of a reddish-brown color, and hence termed charbon roux, is prepared in France for the manufacture of the gunpowder used for sporting purposes, by subjecting wood in iron cylinders to the action of superheated steam under a pressure of two atmospheres. Powder made with this charcoal absorbs moisture more rapidly than ordinary gun powder.
The general properties of wood-charcoal are, that it is black and brittle, and retains the form of the wood from which it was derived; it is insoluble ix water, infusible and non-volatile in the most intense heat; its power of condensing gases is noticethin the article on that subject; and from its power of destroying bad smells, it has been regarded as possessing considerable antiseptic properties. It is frequently stated that charcoal is a bad conductor of heat, but a good conductor of electricity; these properties depend upon the nature of the chareqal, the lighter wood, such as willow. yielding a porous charcoal, with little power of conducting heat or electricity; while boxwood yields a very compact charcoal, which is a good conductor of heat and electricity, and v is admirably adapted for the exhibition of the electric light. Charcoal never consists entirely of pure carbon, the degree of purity varying directly with the temperature; thus, charcoal charred at 480° contains 65 per cent of carbon, while that charred at 750° contains 89, and that charred at 2,730° contains 96; but the loss of charcoal occasioned by these high temperatures is very great, the three percentages of charcoal correspond ing to these temperatures being 50, 20, and 15.