a sort of artificial coal, or fuel, consisting of wood half burnt; chiefly used where a clear strong fire, without smoke, is required; the humidity of the wood being here mostly dissipated, and exhaled in the fire wherein it is prepared.

The microscope discovers a surprising number of Char pores in charcoal: they are disposed in order, and traverse it lengthwise; so that there is no piece of charcoal, how long forever, but may be easily blown through. If a piece be broken pretty short, it may be seen through with a microscope. In a range the 18th part of an inch long, Dr Hook reckoned 150 pores; whence he concludes, that in a charcoal of an inch diameter, there are not less than 5,724,000 pores. It is to this prodigious number of pores that the blackness of charcoal is owing: for the rays of light striking on the charcoal, are received and absorbed in its pores, instead of being reflected; whence the body must of necessity appear black, blackness in a body being no more than a want of reflection. Charcoal was anciently used to distinguish the bounds of estates and inheritances; as being incorruptible, when let very deep within ground. In effect, it preserves itself so long, that there are many pieces found entire in the ancient tombs of the northern nations. M. Dodart says, there is charcoal made of corn, probably as old as the days of Caesar: he adds, that it has kept so well, that the wheat may be still distinguished from the rye; which he looks on as proof of its incorruptibility.

The operation of charring wood is performed in the following manner: The wood intended for this purpose is cut into proper lengths, and piled up in heaps near the place where the charcoal is intended to be made: when a sufficient quantity of wood is thus prepared, they begin constructing their stacks, for which there are three methods. The first is this: They level a proper spot of ground, of about 12 or 15 feet in diameter, near the piles of wood; in the centre of this area a large billet of wood, split across at one end and pointed at the other, is fixed with its pointed extremity in the earth, and two pieces of wood inserted through the clefts of the other end, forming four right angles; against these cross pieces four other billets of wood are placed, one end on the ground, and the other leaning against the angles. This being finished, a number of large and straight billets are laid on the ground to form a floor, each being as it were the radius of the circular area: on this floor a proper quantity of brush or small wood is fired, in order to fill up the interstices, when the floor will be complete; and in order to keep the billets in the same order and position in which they were first arranged, pegs or flumes are driven into the ground in the circumference of the circle, about a foot distant from one another: upon this floor a stage is built with billets set upon one end, but something inclining towards the central billet; and on the tops of these another floor is laid in a horizontal direction, but of shorter billets, as the whole is, when finished, to form a cone.

The second method of building the stacks for making charcoal is performed in this manner: A long pole is erected in the centre of the area above described, and several small billets ranged round the pole on their ends: the interstices between these billets and the pole is filled with dry brushwood, then a floor is laid on that stage, in a reclining position, and on that a second floor, &c. in the same manner as described above; but in the lower floor there is a billet larger and longer than the rest, extending from the central central pole to some distance beyond the circumference of the circle.

The third method is this: A chimney, or aperture of a square form, is built with billets in the centre, from the bottom to the top; and round these, floors and inclined slates are erected, in the same manner as in the stacks above described, except that the base of this, instead of being circular like the others, is square; and the whole stack, when completed, forms a pyramid.

The stack of either form being thus finished, is coated over with turf, and the surface plastered with a mixture of earth and charcoal dust well tempered together.

The next operation is the setting the stack on fire. In order to this, if it be formed according to the first construction, the central billet in the upper stage is drawn out, and some pieces of very dry and combustible wood are placed in the void space, called, by workmen, the chimney, and fire set to these pieces. If the stack be built according to the second construction, the central pole is drawn out, together with the large horizontal billet above described; and the void space occupied by the latter being filled with pieces of very dry combustible wood, the fire is applied to it at the base of the stack. With regard to the third construction, the square aperture or chimney is filled with small pieces of very dry wood, and the fire applied to it at the top or apex of the pyramidal stack. When the stack is set on fire, either at the top or bottom, the greatest attention is necessary in the workman; for in the proper management of the fire the chief difficulty attending the art of making good charcoal consists. In order to this, care is taken, as soon as the flame begins to issue some height above the chimney, that the aperture he covered with a piece of turf, but not so close as to hinder the smoke from passing out; and whenever the smoke appears to issue very thick from any part of the pile, the aperture must be covered with a mixture of earth and charcoal dust. At the same time, as it is necessary that every part of the stack should be equally burnt, it will be requisite for the workman to open vents in one part and shut them in another. In this manner the fire must be kept up till the charcoal be sufficiently burnt, which will happen in about two days and a half if the wood be dry; but if green, the operation will not be finished in less than three days. When the charcoal is thought to be sufficiently burnt, which is easily known from the appearance of the smoke, and the flames no longer issuing with impetuosity through the vents; all the apertures are to be closed up very carefully with a mixture of earth and charcoal dust, which, by excluding all access of the external air, prevents the coals from being any further consumed, and the fire goes out of itself. In this condition it is suffered to remain, till the whole is sufficiently cooled; when the cover is removed, and the charcoal is taken away. If the whole process is skilfully managed, the coals will exactly retain the figure of the pieces of wood: some are said to have been so dexterous as to char an arrow without altering even the figure of the feather.

There are considerable differences in the coals of different vegetables, in regard to their habitu to fire: the very light coals of linen, cotton, some fungi, &c., readily catch fire from a spark, and soon burn out; the more dense ones of woods and roots are set on fire more difficultly, and burn more slowly: the coals of the black berry-bearing alder, of the hazel, the willow, and the lime tree, are said to answer best for the making of gunpowder and other pyrotechnical compositions, perhaps from their being easily inflammable: for the reduction of metallic calces those of the heavier woods, as the oak and the beech, are preferable, these seeming to contain a larger proportion of the phlogistic principle, and that, perhaps, in a more fixed state; considered as common fuel, those of the heavy woods give the greatest heat, and require the most plentiful supply of air to keep them burning; those of the light woods preserve a glowing heat, without much draught of air, till the coals themselves are consumed; the bark commonly crackles and flies about in burning, which the coal of the wood itself very seldom does.

Mathematical instrument makers, engravers, &c., find charcoal of great use to polish their brass and copper plates after they have been rubbed clean with powdered pumice stone. Plates of horn are polishable in the same way, and a glost may be afterwards given with tripoli.

The coals of different substances are also used as pigments; hence the bone-black, ivory-black, &c., of the shops. Most of the paints of this kind, besides their incorruptibility, have the advantage of a full colour, and work freely in all the forms in which powdery pigments are applied; provided they have been carefully prepared, by thoroughly burning the subject in a close vessel, and afterwards grinding the coal into a powder of due fineness. Pieces of charcoal are used also in their entire state for tracing the outlines of drawings, &c.; in which intention they have an excellence, that their mark is easily wiped out. For these purposes, either the finer pieces of common charcoal are picked out and cut to a proper shape; or the pencils are formed of wood, and afterwards burnt into charcoal in a proper vessel well covered. The artists commonly make choice of the smaller branches of the tree freed from the bark and pith; and the willow and vine are preferred to all others. This choice is confirmed by the experiments of Dr Lewis, who has found that the wood of the trunks of trees produces charcoal of a harder nature than their small twigs or branches; and the hard woods, such as box and guaiacum, produced coals very sensibly harder than the softer woods. Willow he prefers to all others. The shells and stones of fruits yielded coals so hard that they would scarcely mark on paper at all; while the coals of the kernels of fruits were quite soft and mellow. The several coals produced by the doctor's experiments were levigated into fine powder, mixed both with gum water and oil, and applied as paints both thin and thick, and diluted with different degrees of white. All of them, when laid on thick, appeared of a strong full black, nor could it be judged that one was of a finer colour than another; diluted with white, or when spread thin, they had all somewhat of a bluish cast.

Horns, and the bones both of fishes and land animals, gave coals rather glossier and deeper coloured than vegetables; and which, in general, were very hard, so as difficultly, or not at all, to stain paper. Here also the hardness of the coal seemed to depend on that of the subject from whence it was prepared; for silk, woollen, leather, blood, and the flaky parts of animals, yielded soft coals. Some of these differed from others very sensibly in colour; that of ivory is superior to all the rest, and indisputably the finest of all the charcoal blacks. The animal coals had much less of the bluish cast in them than the vegetable, many of them inclining rather to a brown. Charred pit coal, on the other hand, seemed to have this blueness in a greater degree. For the chemical properties of charcoal see Chemistry Index.