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This article is about the substance consisting of mostly carbon. For the typeface, see Charcoal (typeface).
Charcoal is the blackish residue consisting of impure carbon obtained by removing water and other volatile constituents from animal and vegetation 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 soft, brittle, light, black, porous material is 85% to 98% carbon, the remainder consisting of volatile chemicals and ash, and resembles coal.
The first part of the word is of obscure origin, but the first use of the term "coal" in English was as a reference to charcoal. In this compound term, the prefix "chare-" meant "turn", with the literal meaning being "to turn to coal". 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, and the ships that transported it.
Production
Wood pile before covering it by turf or soil, and firing it (around 1890)
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 or moistened clay. 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; small scale production on the spot often yields only about 50%, large scale was efficient to about 90% even by the 17th century. The operation is so delicate that it was generally left to professional charcoal burners. These often worked in solitary groups in the woods and had a rather bad social reputation, especially travelling ones who often sold a sack (priced at about a day's wage) with lots of rubbish mixed in to farmers and townfolk.
Historically the massive production of charcoal (at its height employing hundreds of thousands, mainly in Alpine and neighbouring forests) has been a major cause of deforestation, especially in Central Europe, but to a lesser extent even before, as in Stuart England. The increasing scarcity of easily harvested wood was a major factor for the switch to the fossil-fuel equivalents, mainly coal and brown coal for industrial use.
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°.
Uses
One of the most important historical 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. Commercially, charcoal is often found in either lump, briquette or extruded forms. Lump charcoal is made directly from hardwood material, burns hotter than briquettes and produces far less ash than briquettes. While some briquettes are made from a combination of charcoal (heat source), brown coal (heat source), mineral carbon (heat source), borax (press release agent), sodium nitrate (ignition aid), limestone (uniform visual ashing), starch (binder), raw sawdust (ignition aid) and possibly additives like paraffin or lighter fluid to aid in lighting them, other "natural" briquettes are made solely from charcoal and a starch binder. Extruded charcoal is made by extruding either raw ground wood or carbonized wood into logs without the use of a binder. The heat and pressure of the extruding process hold the charcoal together. If the extrusion is made from raw wood material, the extruded logs are then subsequently carbonized.
The porosity of activated charcoal accounts for its ability to readily adsorb gases and liquids; charcoal is often used to filter water or adsorb odors. Its pharmacological action depends on the same property; it adsorbs 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 also to some extent removes coloring material from solutions, but animal charcoal is generally more effective.
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. It must usually be preserved by the application of a fixative. Artists generally utilize charcoal in three forms:
- Vine charcoal is created by burning sticks of wood (usually willow or linden/Tilia) into soft, medium, and hard consistencies. Bamboo charcoal is the principal tool in Japanese Sumi-e (炭絵 lit: charcoal drawing) art.
- Compressed charcoal is charcoal powder mixed with gum binder compressed into round or square sticks. The amount of binder determines the hardness of the stick. Compressed charcoal is used in charcoal pencils.
- Finely powdered charcoal is often used to "tone" or cover large sections of a drawing surface. Drawing over the toned areas will darken it further, but the artist can also lighten (or completely erase) within the toned area to create lighter tones.
One additional use of charcoal rediscovered recently is in horticulture. Although American gardeners have been using charcoal for a short while, research on Terra preta soils in the Amazon has found widespread use there by natives to turn otherwise unproductive soil into very rich soil.
Sources, references and external links
- Natural History of Europe - 2005 Tv co-production including BBC and ZDF
- On Charcoal
- The Valley - BBC TV - one year of life on a 17th century farm reenancted by archaeologists and historians
- The Lump Charcoal Database - Information about lump charcoal.
- [1] - Information about charcoal.
- Photo of traditional charcoal production A forest kiln
- The River Wey and Wey Navigations Community Site — a non-commercial site of over 120,000 words all about the Wey Valley and includes a photofile on charcoal production and information relating to gunpowder manufacture at Chilworth.
Categories: Art materials | Fuels | Carbon forms | Pyrotechnic compositions
