Charcoal is the blackish residue consisting of impure carbon obtained by removing water and other volatile constituents of animal and vegetable substances. It is usually produced by heating wood in the absence of oxygen (see char), but sugar charcoal, bone charcoal (which contains a great amount of calcium phosphate), and others can be produced as well. The light, black, porous material is 85% to 98% carbon, and resembles coal.
The first part of the word is of obscure origin. The independent use of "char", meaning to scorch, to reduce to carbon, is comparatively recent and must be a back-formation from the earlier charcoal. It may be a use of the word charren, meaning to turn, i.e., wood changed or turned to coal; or it may be from the French charbon. A person who manufactured charcoal was formerly known as a collier, though the term was used later for those who dealt in coal.
Production of wood charcoal in districts where there is an abundance of wood dates back to a very remote period, and generally consists of 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 pile 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 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 practiced where wood is scarce, and also by reason of the recovery of valuable byproducts (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. It is also used in metallurgical operations as a reducing agent, but its application has been diminished by the introduction of coke, anthracite smalls, etc. A limited quantity is made up into the form of drawing crayons; but the greatest amount is used as a fuel, which burns hotter and cleaner than wood. Charcoal is often used by blacksmiths, for cooking, and for other industrial applications.
The porosity of wood charcoal explains why it floats on the surface of water, although it is actually denser. This also accounts for its ability to readily absorb gases and liquids; charcoal is often used to filter water or absorb odors. Its pharmacological action depends on the same property; it absorbs the gases of the stomach and intestines, and also liquids and solids (hence its use in the treatment of certain poisonings). Charcoal filters are used in some types of gas mask to remove poisonous gases from inhaled air. Wood charcoal has also the power of removing coloring 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% 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 and gelatin 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.
Charcoal is used in art for drawing, making rough sketches in painting, and is one of the possible media for making a parsemage. Bamboo charcoal is the principal tool in Japanese Sumi-e (炭絵 lit: charcoal drawing) art.