Charcoal is a term most frequently applied to charred wood, or coal produced by charring wood. Formerly, charcoal was the name for charred sea-coal or mineral coal; and the word is popularly used for the carbonaceous residue of vegetable, animal, or mineral substances when they have undergone smothered combustion.
ANIMAL CHARCOAL, BONE-BLACK, or IVORY-BLACK, is prepared from bones by heating them in close retorts till they undergo the process of destructive distillation, when combustible gases and water, together with the vapours of various salts of ammonia, and oil, are given off, and bone-black is left in the retort. It is generally reduced to coarse grains from about the size of small peas down to large pin-heads, and is extensively used in the arts for decolourising liquids, such as the syrup of sugar, and solutions of argol (impure cream of tartar) and of the alkaloids, as also in Filters (q.v.) for separating chemical impurities from water. The general mode of using the bone-black is to allow the coloured liquid to percolate through a layer of the charcoal, when all colour is arrested, and the syrup or water runs clear and colourless from under the stratum of charcoal. This power of absorbing colouring matters is also observable in vegetable (peat or wood) charcoal, but not to such an extent as in bone-black. The application of heat to the liquids before filtration greatly facilitates the decolourisation, and where the volume of liquid to be operated upon is not great, the most expeditious method is to boil the liquid and bone- black together, and then strain through filtering-paper or cloth. The composition of bone-black in 100 parts is 10 of pure charcoal, associated with 90 of earthy salts—i.e. in the proportion of one of pure charcoal in 10 of the commercial bone-black. The power of absorbing colours appears to be due to the porosity of the substance, and is not resident simply in the pure charcoal; indeed, the earthy matters (principally phosphatic of lime and carbonate of lime) can be dissolved out of the bone-black by dilute hydrochloric acid, and the pure charcoal thus obtained only possesses about one-third the decolourising power of the total amount of bone-black it was obtained from. Thus, if 100 parts of ordinary bone-black have the power of arresting the colour from ten volumes of a given coloured liquid, then the 10 parts of pure charcoal which can be obtained from the 100 parts of bone-black will be found to decolourise only three volumes of the same coloured liquid; so that it is apparent the earthy matters in the bone-black influence and increase the absorption of the colouring matter, and thus render a given weight of the charcoal of greater commercial value. When syrup of sugar and other liquids have been run through bone-black for some time, the pores of the latter appear to get clogged with the colour, and the clarifying influence ceases, and then the bone-black requires to undergo the process of revivification, which consists in reheating it carefully in ovens, or iron pipes inclosed in a furnace, when the absorbed colour is charred, and the bone-black can be of service once again as an arrester of colour. After several reburnings, the bone-black becomes of very inferior absorptive quality, and is then disposed of for the manufacture of bone-ash and dissolved bones (see BONE MANURES). Bone-black has likewise a great power of absorbing odours, especially those of a disagreeable nature, and can thus be employed to deodorise apartments, clothing, outhouses, &c., or wherever animal matter may be passing into a state of active putrefaction.
WOOD CHARCOAL is the most important, though not the purest kind of Carbon (q.v.). 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 exception of a small white ash; but if the supply of air be limited, only the more volatile matters burn away, and most of the carbon remains. This is the principle of the process of charcoal-burning in countries where wood is abundant. 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, and holes left at the bottom for the air to get in. An open space is also left in the middle of the heap to serve as a flue. The heap is set on fire by putting burning wood into the top of the central opening. The combustion proceeds gradually from the top to the bottom, and from the centre 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 smoke is thick and white when the process is going on properly; if it becomes thin, and especially if a blue flame appears, the wood is burning away too fast, and the combustion must be checked by closing the holes at the bottom, or by heaping fresh ashes on the top and sides. As soon as the combustion is completed, 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. It is found that 100 parts of wood yield on the average from 61 to 65 parts by measure, or 24 parts by weight, of charcoal. 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 colour, and hence termed charbon roux or red charcoal, is frequently prepared 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 gunpowder.
The general properties of wood-charcoal are, that it is black and brittle, and retains the form of the wood from which it is derived; it is insoluble in water, infusible and non-volatile in the most intense heat; its power of condensing gases is noticed under Carbon (q.v.); 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 charcoal, 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 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 at which it is formed; thus, charcoal charred at ( C.) contains 65 per cent. of carbon, while that charred at ( C.) contains 80, and that charred at ( C.) contains 96; but the loss of charcoal occasioned by these high temperatures is very great, the percentage yield of charcoal corresponding to these temperatures being 50, 20, and 15.
The uses of wood-charcoal are numerous and extensive. It is very largely employed as a fuel, taking the same place in many countries that coal occupies here. From its being proof against all ordinary chemical agencies, superficial charring is often employed to protect wood from decay, as in the case of fence-posts, of telegraph poles, or of piles which are driven into mud or into the beds of rivers to serve as foundations. With the same design it is not unusual to char the interior of tubs and casks destined to hold liquids. In a finely-divided state it is commonly regarded, as has been already stated, as an antiseptic; and there is no doubt that the offensive effluvia from animal matter in an advanced stage of putrefaction disappear when the putrefying substance is covered with a layer of charcoal; but in reality the decay goes on, without the emission of any odour, till at length the whole of the carbon is dissipated as carbonic acid gas, and the hydrogen as water, while the nitrogen remains as nitric acid. It has been shown that the action consists in a rapid process of oxidation, dependent upon the power which finely-divided charcoal possesses of condensing oxygen. In a finely-divided state, charcoal not only condenses gases to a marvellous extent, but has the power of absorbing colouring matters, bitter principles, &c.; and hence it is of extensive use in the laboratory. From the rapidity of its absorbing action, the use of a respirator filled with charcoal has been suggested to protect the mouth and nostrils in an infected atmosphere; trays of powdered wood-charcoal in dissecting-rooms, in the wards of hospitals, and in situations where putrescent animal matter is present, exert a most beneficial influence in sweetening the atmosphere, by absorbing and decomposing the offensive gases. Charcoal is accordingly valuable in filters, not only for decolourising purposes, but likewise for assisting in purifying water for domestic use. It is also successfully used to prevent the escape of noxious vapours at the ventilating openings of sewers, as it allows the free passage of air, but condenses the offensive effluvia in its pores, where they are destroyed by a process of oxidation. Besides its employment in the manufacture of gunpowder it has many applications in the arts. In medicine it is used to destroy fetor, applied in the form of powder or poultice to gangrenous sores, ulcers, &c.; it is also largely employed in tooth-powders. In indigestion accompanied by flatulence it may be given in doses of two or three teaspoonfuls suspended in water, or as charcoal-biscuits (see BISCUITS). For the charcoal blacks, see BLACK.