Charcoal is a dark grey residue containing carbon and remaining ash. It is obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen (see pyrolysis, and charring). It is usually an impure form of carbon as it contains ash.
 Charcoal production
 Charcoal in the Appropriate Technology context
An obvious question to ask is why we need charcoal. The answer is that as an energy source, it's more energy dense, and most importantly, the calorific value is higher (heat per weight). This means that a higher temperature can be achieved.
This makes has made the fuel more suitable than wood for transport purposes, at least if it is burned as is (ie for fueling a steam engine). If it was to be compared to ie a wood gas system, the total amount of energy that can be extracted from charcoal (which is also the original source of charcoal) would without question be lower than that of wood. Perhaps confusing at first glance, this can be explained quite simply: energy is needed to transform wood to charcoal (the first burn (or the "pyrolysis") will waste a lot of energy. Also, a wood gas system is much more efficient overall (steam engines are only 15% efficient, IC engines are 30% efficient).
For other tasks as blacksmithing, the firing of material (ie using a kiln, casting, ...) charcoal is the only natural material that can be used (wood can not be used). Nowadays, blacksmithing is no longer practiced much, and for casting and firing materials, other (non-natural) materials are being used instead (usually as it allows to better control the temperature, use more efficient kiln/forge designs, ...)
Other, more useful purposes of charcoal (from a AT-standpoint) are of a different kind. Water for example can be filtered by means of charcoal. In developed countries, activated charcoal is mostly used rather than regular charcoal, but charcoal can also be used (although it is less efficient).
 Further uses
- It can be used as a soil amendment; it is then called "biochar".
- Charcoal is useful for other purposes, e.g. metal smelting
- Charcoal as filter: it has very high absorptive capacities and therefore can be used for water purification and filtration; it can also be used to filter pyrolysis gas itself, although it may not be suitable as biochar later (depending on feedstock and potentially hazardous polycyclic aromatic hydrocarbons produced during combustion)
- Lastly, charcoal powder can also neutralize smell when put into a composting toilet.