Charcoal is the dark grey residue consisting of impure carbon obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood, sugar, bone char, or other substances in the absence of oxygen (see pyrolysis, char and biochar). The resulting soft, brittle, lightweight, black, porous material resembles coal and is 50% to 95% carbon
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 is assumed to be a back-formation from the earlier charcoal. It may be a use of the word charren or churn, 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 (also as a wood collier). The word “collier” was also used for those who mined or dealt in coal, and for the ships that transported it.
Charcoal, or biochar, is also an essential element in the composition of terra preta.
Historically, production of wood charcoal in districts where there is an abundance of wood dates back to a very ancient 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 seventeenth century. The operation is so delicate that it was generally left to colliers (professional charcoal burners).
The massive production of charcoal (at its height employing hundreds of thousands, mainly in Alpine and neighbouring forests) was a major cause of deforestation, especially in Central Europe. In England, many woods were managed as coppices, which were cut and regrew cyclically, so that a steady supply of charcoal would be available (in principle) forever; complaints (as early as the Stuart period) about shortages may relate to the results of temporary over-exploitation or the impossibility of increasing production to match growing demand. 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 for 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 °C, and an easily powdered mass at 310 °C. Charcoal made at 300° is brown, soft and friable, and readily inflames at 380 °C; made at higher temperatures it is hard and brittle, and does not fire until heated to about 700 °C.
In Finland and Scandinavia, the charcoal was considered the by-product of wood tar production. The best tar came from pine, thus pinewoods were cut down for tar pyrolysis. The residual charcoal was widely used as substitute for metallurgical coke in blast furnaces for smelting. Tar production led to rapid deforestation: it has been estimated all Finnish forests are younger than 300 years. The end of tar production in the end of the 19th century meant also rapid re-forestation.
The charcoal briquette was first invented and patented by Ellsworth B. A. Zwoyer of Pennsylvania in 1897 and was produced by the Zwoyer Fuel Company. The process was further popularized by Henry Ford, who used wood and sawdust byproducts from automobile fabrication as a feedstock. Ford Charcoal went on to become the Kingsford Company.
Commercial charcoal is found in either lump, briquette, or extruded forms:
- Lump charcoal is made directly from hardwood material and usually produces far less ash than briquettes.
- Briquettes are made by compressing charcoal, typically made from sawdust and other wood by-products, with a binder and other additives. The binder is usually starch. Some briquettes may also include brown coal (heat source), mineral carbon (heat source), borax, sodium nitrate (ignition aid), limestone (ash-whitening agent), raw sawdust (ignition aid), and other additives like paraffin or petroleum solvents to aid in ignition.
- 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 characteristics of charcoal products (lump, briquette, or extruded forms) vary widely from product to product. Thus it is a common misconception to stereotype any kind of charcoal, saying which burns hotter, etc.
Charcoal is sometimes used to power commercial road vehicles—usually buses—in countries where oil is scarce or completely unavailable. In the years immediately after the Second World War, charcoal buses were in regular use in Japan and are still used today in North Korea.
In the developed world, charcoal briquettes are widely used for outdoor grilling and barbecues in backyards and on camping trips.
In many non-industrialized countries, charcoal is used for everyday cooking by a large portion of the population. This is a serious health problem when used indoors. Amy Smith reports that acute respiratory infection from indoor cooking fires is the number one cause of death in children under 5 in the third world. This represents nearly 2 million deaths per year. In addition, carbon monoxide (CO) is a combustion product. It also contributes to outside air pollution.
Historically, charcoal was used in great quantities for smelting iron in bloomeries and later blast furnaces and finery forges. This use was replaced by coke during the Industrial Revolution. For this purpose, charcoal in England was measured in dozens (or loads) consisting of 12 sacks or shems or seams, each of 8 bushels.
Purification and filtration
Charcoal may be activated to increase its effectiveness as a filter. Activated charcoal readily adsorbs a wide range of organic compounds dissolved or suspended in gases and liquids. Charcoal is often used to filter water to remove bacteria and undesired tastes. In the East, in countries like China, Japan, Taiwan and Korea, water purified by Charcoal (or bamboo charcoal) is used to make tea. In certain industrial processes, such as the purification of sucrose from cane sugar, impurities cause an undesirable color, which can be removed with activated charcoal. It is also used to absorb odors and toxins in gases, such as air. Charcoal filters are also used in some types of gas masks. The medical use of activated charcoal is mainly the adsorption of poisons, especially in the case of suicide attempts in which the patient has ingested a large amount of a drug. Activated charcoal is available without a prescription, so it is used for a variety of health-related applications. For example, it is often used to reduce discomfort (and embarrassment) due to excessive gas in the digestive tract.
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. Wood charcoal also to some extent removes coloring material from solutions, but animal charcoal is generally more effective.