1911 Encyclopædia Britannica/Charcoal

CHARCOAL, the blackish residue consisting of impure carbon obtained by removing the volatile constituents of animal and vegetable substances; wood gives origin to wood-charcoal; sugar to sugar-charcoal; bone to bone-charcoal (which, however, mainly consists of calcium phosphate); while coal gives “coke” and “gas-carbon.” The first part of the word charcoal is of obscure origin. The independent use of “char,” meaning to scorch, to reduce to carbon, is comparatively recent, and must have been taken from “charcoal,” which is quite early. The New English Dictionary gives as the earliest instance of “char” a quotation dated 1679. Similarly the word “chark” or “chak,” meaning the same as “char,” is also late, and is probably due to a wrong division of the word “charcoal,” or, as it was often spelled in the 16th and 17th centuries, “charkole” and “charke-coal.” No suggestions for an origin of “char” are satisfactory. It may be a use of the word “chare,” which appears in “char-woman,” the American “chore”; in all these words it means “turn,” a turn of work, a job, and “charcoal” would have to mean “turned coal,” i.e. wood changed or turned to coal, a somewhat forced derivation, for which there is no authority. Another suggestion is that it is connected with “chirk” or “chark,” an old word meaning “to make a grating noise.”

Wood-charcoal.—In districts where there is an abundance of wood, as in the forests of France, Austria and Sweden, the operation of charcoal-burning is of the crudest description. The method, which dates back to a very remote period, generally consists in piling billets of wood on their ends so as to form a conical pile, openings being left at the bottom to admit air, with a central shaft to serve as a flue. The whole is covered with turf of moistened soil. The firing is begun at the bottom of the flue, and gradually spreads outwards and upwards. The success of the operation—both as to the intrinsic value of the product and its amount—depends upon the rate of the combustion. Under average conditions, 100 parts of wood yield about 60 parts by volume, or 25 parts by weight, of charcoal. The modern process of carbonizing wood—either in small pieces or as sawdust—in cast iron retorts is extensively practised where wood is scarce, and also by reason of the recovery of valuable by-products (wood spirit, pyroligneous acid, wood-tar), which the process permits. The question of the temperature of the carbonization is important; according to J. Percy, wood becomes brown at 220° C., a deep brown-black after some time at 280°, and an easily powdered mass at 310°. Charcoal made at 300° is brown, soft and friable, and readily inflames at 380°; made at higher temperatures it is hard and brittle, and does not fire until heated to about 700°. One of the most important applications of wood-charcoal is as a constituent of gunpowder (q.v.). It is also used in metallurgical operations as a reducing agent, but its application has been diminished by the introduction of coke, anthracite smalls, &c. A limited quantity is made up into the form of drawing crayons; but the greatest amount is used as a fuel.

The porosity of wood-charcoal explains why it floats on the surface of water, although it is actually denser, its specific gravity being about 1.5. The porosity also explains the property of absorbing gases and vapours; at ordinary temperatures ammonia and cyanogen are most readily taken up; and Sir James Dewar has utilized this property for the preparation of high vacua at low temperatures. This character is commercially applied in the use of wood-charcoal as a disinfectant. The fetid gases produced by the putrefaction and waste of organic matter enter into the pores of the charcoal, and there meet with the oxygen previously absorbed from the atmosphere; oxidation ensues, and the noxious effluvia are decomposed. Generally, however, the action is a purely mechanical one, the gases being only absorbed. Its pharmacological action depends on the same property; it absorbs the gases of the stomach and intestines (hence its use in cases of flatulence), and also liquids and solids. Wood-charcoal has also the power of removing colouring matters from solutions, but this property is possessed in a much higher degree by animal-charcoal.

Animal-charcoal or bone black is the carbonaceous residue obtained by the dry distillation of bones; it contains only about 10% of carbon, the remainder being calcium and magnesium phosphates (80%) and other inorganic material originally present in the bones. It is generally manufactured from the residues obtained in the glue (q.v.) and gelatin (q.v.) industries. Its decolorizing power was applied in 1812 by Derosne to the clarification of the syrups obtained in sugar-refining; but its use in this direction has now greatly diminished, owing to the introduction of more active and easily managed reagents. It is still used to some extent in laboratory practice. The decolorizing power is not permanent, becoming lost after using for some time; it may be revived, however, by washing and reheating.

Lampblack or soot is the familiar product of the incomplete combustion of oils, pitch, resins, tallow, &c. It is generally prepared by burning pitch residues (see Coal-tar) and condensing the product. Thus obtained it is always oily, and, before using as a pigment, it must be purified by ignition in closed crucibles (see Carbon).