At present charcoal is manufactured to a small extent only; and its cost is so high as greatly to restrict its application.
The ordinary process of manufacture, although extremely simple, requires great care and attention. The wood is cut into billets from two to four feet in length, and dried by exposure to the air; when dried it is closely stacked in conical mounds from six to twelve feet high, and from ten to forty feet in diameter.
The ground is first cleared and levelled; a small framework is erected in the centre of the space, about three feet square, and consisting of four forked-sticks standing two and a half feet out of the ground, and connected at the top by four stout rods. The billets are compactly stacked round the frame until the entire area is covered, all the billets sloping towards the centre; the stack is then completed to the desired height by billets arranged horizontally, and the whole covered by a layer of earth, finished off with sods when it is practicable to obtain them.
The heap is kindled by an opening made at the top, and others near the base; after burning for three or four days these are closed, and other holes are made in the sides about half-way between the base and the apex. The holes must be closed whenever it is seen that combustion is too rapid, and care must be taken to fill up any depression that may arise from this cause.
When smoke ceases to be given off all the holes are closely stopped, and the heap is allowed to cool for three or four days, when the cover is removed and any charcoal that may still be in a burning condition is extinguished by water.
In many places the site of the mound is formed into a funnel-shapad depression with a hole in the centre, which communicates with a ditch dug on the outside to enable the tarry matters to be drained off.
Charcoal intended to be used in the manufacture of the finer kinds of gunpowder is subjected to combustion in large iron retorts furnished with refrigerating condensers, by which means nearly the whole of the volatile products can be readily obtained.