Fresh-burnt charcoal rapidly absorbs from 9'6 to 18 per cent, of its weight of atmospheric moisture, of which the commercial article usually contains about 12 per cent. It has also a remarkable absorptive action on gases, condensing them within its pores. To the heat thus developed has been attributed the occasional spontaneous ignition of charcoal that has been stacked too soon after manufacture. Recently-made box-wood charcoal absorbs, at 12° C. and 28'5 in. pressure, 90 volumes of ammonia, 85 of hydrochloric acid, 65 of sulphurous anhydride, 55 of sulphuretted hydrogen, 35 of carbonic anhydride, 9∣ of oxygen, and 6½ of nitrogen; only 1½ vols., however, of hydrogen (Saussure). The absorptive power for gases is nearly in the ratio of the pressures to which the charcoal is exposed, the temperature being constant.

Charcoal can be made at a temperature as low as 300° C. It is produced in greatest quantity at a heat just sufficient thoroughly to char the wood. The charcoal so made contains a larger percentage of hydrogen, oxygen, and nitrogen than that formed at higher temperatures, and, being readily inflammable, is adapted for the manufacture of gunpowder. The quantity of charcoal obtainable from wood varies according to the nature of the wood employed and the method of manufacture. By distillation 25 to 27 per cent, of the weight of the wood can be obtained. Ordinarily, beech yields about 15 per cent, of its weight, box 20∣ per cent. Mr Mushet obtained in his experiments the following percentage composition of various kinds of wood, the amount of each kind taken being 1 lb avoirdupois :— [table]

Rumford, who employed a long-continued moderate heat, obtained a much larger quantity of charcoal from wood. During the charring of the wood, pyroligneous or acetic acid, creosote, pyroxylic spirit, and empyreumatic oils are produced, and much carbonic oxide, carbonic anhydride, hydrogen, and water are evolved.

Charcoal can be prepared in a variety of ways. In its purest form it is obtained by the ignition of organic substances such as starch and sugar. That made for powder-mills, as also that produced in acetic acid factories, is the product of the distillation of wood in iron cylinders. In China charcoal is prepared in pits, preferably in a clayey ooih In some parts of Sweden it is made from rectangular piles of wood, the process being regulated by the careful admission of air through holes in the covering of the wood. In Foucauld’s process, a portable shroud or abri of wood coated With a mixture of loam and grass is used to cover the heap to be charred. In most countries where wood is abundant, charcoal-burning is carried on by firing conical piles of billets of wood, about 12 feet in height, and 10 to 40 feet in diameter, from the top of a central hole or chimney. The wood is felled in winter, and must be tolerably dry; it is built up with the bark outermost, the largest billets being placed in the interior of the pile; over the whole is laid a covering of turf, or of charcoal-dust (“breeze ”) and soil. The combustion of the wood is conducted from above downwards, and from the exterior towards the centre; so that the charcoal in a half-burnt heap forms an inverted cone. At the sides of the heap are holes for the admission of air, the number and size of which are a matter of importance. The first or “sweating” process lasts three or four days, during which the cover becomes moist with condensed water. The openings round about the base of the pile are then covered, and a series of holes is made about half-way from the top of the heap; as the smoke ceases to issue from these they are closed, and other series of holes are made below, as required. The tarry products which collect towards the close of the operation are removed from the heap by means of gutters or pipes.

Sometimes the base of the heap is made in the shape of aflat funnel, from which proceeds a channel for the tar and acid. When the air-holes of the burning heap no longer emit smoke and flame, they are carefully stopped, and the pile is allowed to cool for two or three days. The charcoal is then drawn, and any pieces which may still be glowing are quenched by plunging them into water or sand. By the above-described method, 128 cubic feet (one cord) of wood yield about 30 bushels of charcoal

Besides being employed as a fuel, and âs a reducing agent in metallurgy, wood-charcoal is applied to a variety of purposes. It is much used in the manufacture of Alters, and as a medicine it may be administered in some cases of dyspepsia. On account of the resistance of charcoal to the action of water, stakes for wet soils and fchs insides of casks are charred previous to use. Charcoal is valuable as an absorbent of noxious effluvia, which it decomposes by bringing them into contact with condensed oxygen within its pores. Its absorbent power is greater than that of spongy platinum, but its efficacy as a promotor of chemical union is not so great. Dr Stenhouse has been able to unite the properties of charcoal and spongy platinum in what he calls “platinized charcoal,” which is made by boiling charcoal in a solution of platinic chloride, and then heating it to redness in a closed vessel. This preparation may be employed in ventilators and respirators, and, on account of its oxidizing properties, has been proposed for use as a mild caustic.

Coal-Gas Charcoal, or Gas-Carbon, is a dense and pure variety of charcoal, of a greyish black colour, which is deposited in the inside of gas-retorts. It is used for making the negative poles of Bunsen’s batteries.

Lamp Block is a finely-divided form of charcoal, obtained by condensing the smoke of burning resinous and oily substances in cylindrical chambers hung with sacking or sheep-skins. The crude lamp black is purified by heating to redness in closed vessels.

Peat Charcoal. — This variety of charcoal is produced by the carbonization of peat in kilns or circular shafts of brick and stone-work. The ignition is made to proceed from above downwards. Peat may also be economically charred by means of the waste-gas of smelting-furnaces, or of heated gases produced by the combustion of wood. In Bohemia a muffle-like chamber, heated at the sides and end by peat-fires, has been successfully employed,—8 to 9 cubic metres of charcoal being produced from 20 cubic metres of peat, by the consumption of 10 cubic metres of the same, and at a cost of 5s. 2jcl per cubic metre of charcoal. According to Stöckhardt, 100 ft> of wood-charcoal will by their combustion evaporate as much water as 113 ft> of peat-charcoal. The use of charred peat in some metallurgical operations must depend upon the cost of its preparation. Its friability renders it unfit for the blastfurnace; but it may be advantageously used on blacksmiths’ hearths. Hitherto, it has not been employed on an extensive scale, the large amount of ash it produces (45 per cent.) being one objection to its consumption.

Animal Charcoal, or Bone-Black, is prepared by igniting fresh and coarsely comminuted bones, which have been previously boiled to remove fat, in closed vessels of iron or earthenware. The bone-black so produced, which weighs about half as much as the bones employed, is hermetically sealed, as soon as made, in iron canisters. Animal charcoal contains of carbon about 14 per cent, in a state of fine division, of calcic phosphate 80 per cent., of calcic carbonate 5 per cent., with nitrogen and minor impurities. It is largely employed as a decolorizing, deodorizing, and filtering agent. It removes many organic substances from their solutions; thus it has been found that whilst the colour of ale can be made paler by its means, the bitterness is at the same time wholly removed. This action of animal charcoal is due to the separation of the particles of carbon by the earthy matter present in it. The precipitant action of bone-black on matters in solution is much greater than that of wood charcoal. Its decolorizing properties are found to be greatly enhanced by washing with hydrochloric acid and subsequent calcination with potash. A good decolorizing charcoal is made by igniting nitrogenous animal matter, such as horn and clippings of hides, in contact with pearl-ash, and washing the product with water. Animal charcoal can be re-purified after use by treating it with acids; or by putrefying and dissolving out organic impurities, washing, and finally igniting it. Animal charcoal is used as a pigment, more especially in the form of ivory black, and also as a manure for vegetable soils; and it has been recommended by Drs Eulenberg and Wohl as an antidote in cases of phosphorus poisoning.

See Wurtz, Dictionnaire ds Chimie, vol. ii., 1868, pp. 843-847; Hunter, “On the effects of Pressure on the Absorption of Gases by Charcoal,” Jαurn. Chsm. Soc., 1871, p∙ 76; Percy, Metallurgy, Lond. 1875; Wanklyn, “On the Process of Combustion which takes place in the Interior of certain Porous Filters,” Chsm. News, vol. xxxiii. p. 243, and vol. xxxiv.