CHARCOAL.
Charcoal.
Wood charcoal is the woody fibre that remains after the liquid and more volatile parts have been driven off by the fire in the process of charring. The temperature resulting from the combustion of charcoal is much higher than that from burning wood, in consequence of the absence of the large quantity of water which wood contains, amounting to between 50 and 60 per cent.; Object of charring.the object, therefore, of charring wood is the removal of moisture, and also, what is of great importance, the expulsion of those matters contained in it which become volatile before they are burned, thus rendering a large amount of heat latent. Best wood for charcoal.The woods generally used in this country in making charcoal for gunpowder are the alder, willow, and dogwood. There are about 60 acres of wood grown for charcoal at Waltham Abbey. The alder is cut every eight years, and the willow in six years. It is used after one year. Other woods are sometimes used by English and foreign manufacturers, but none produce a powder of such quality as obtained from the above. It is usually considered that better charcoal is distilled when the wood is allowed to season for a time; but recent experience has shown that wood only lately cut and peeled, after being desiccated in a hot chamber, will make equally good charcoal with that which has been seasoning for three or four years.
First process.
All the wood which is cut in the Government grounds or purchased from merchants, is stripped of the bark, on account of its being impregnated with salts and gummy substances, cut into lengths of 3 feet for the convenience of loading the iron slips, which are a little above this length, and stacked in the wood-yard.
Cylinder charcoal.
Cylindrical cases of the required size, fitted with lids, are filled with wood. These cases are made to fit easily, and slide horizontally into iron retorts built in the wall, which admit of the accurate regulation of heat (communicated to them by furnaces underneath) throughout the operation of charring. A great saving of time and heat is effected by their use, as when the wood has been properly charred the case or slip containing it may be easily withdrawn, and another containing a fresh charge at once introduced into the retort, without allowing the latter to cool down, as would otherwise be necessary. When it has been sufficiently charred (which is known by experience, in watching the burning of the gas that is produced and is conducted into the fire), the slip is withdrawn by tackling, and at once lowered down into iron coolers or cases, which are immediately covered up with close-fitting lids, and then allowed to remain until all fire is extinguished. The goodness of charcoal is an essential point in the manufacture of gunpowder. Quantity produced.About twenty-five to thirty per cent. is obtained; and one cord will produce about four cwt. of charcoal. Qualities.If properly charred, it should have a jet black appearance, and when powdered a lustre resembling velvet; it should be light and sonorous when gently dropped, and its fracture should exhibit the same appearance throughout; it should be so soft as not to scratch polished copper, and ought not to exhibit any alkali when treated with pure distilled water. Charcoal is very porous, and absorbs very greedily gases and moisture from the atmosphere; no large store therefore is ever kept, and particular care is taken to prepare it only in proportion as it is required for use. Kept dry.At all times it must be kept exceedingly dry; Absorbs.when whole it will absorb about eight per cent. its weight of moisture, and when in powder 15 per cent., so that the fresher the charcoal is the better for the powder.