Production Methods
Charcoal has been made by various methods. The traditional method in Britain used a clamp. This is essentially a pile of wooden logs (e.g. seasoned oak) leaning against a chimney (logs are placed in a circle). The chimney consists of 4 wooden stakes held up by some rope. The logs are completely covered with soil and straw allowing no air to enter. It must be lit by introducing some burning fuel into the chimney; the logs burn very slowly (cold fire) and transform into charcoal in a period of 5 days' burning. If the soil covering gets torn (cracked) by the fire, additional soil is placed on the cracks. Once the burn is complete, the chimney is plugged to prevent air from entering.
The last section of the film Le Quattro Volte (2010) gives a good and long, if poetic, documentation of the traditional method of making charcoal. The Arthur Ransome children's series Swallows and Amazons, (particular the second book Swallowdale), features carefully drawn vignettes of the lives and the techniques of charcoal burners at the start of the 20th century, in the Lake District of the UK.
Modern methods use a sealed metal container, as this does not require watching lest fire break through the covering. However, on-site attendance is required. This is often carried out by the last forestry workers to live in working woodland in the western world. There has been a resurgence of this, particularly in the UK. A good example of this is Bulworthy Project where charcoal production supports an experiment in low-impact living and nature conservation.
The properties of the charcoal produced depend on the material charred. The charring temperature is also important. Charcoal contains varying amounts of hydrogen and oxygen as well as ash and other impurities that, together with the structure, determine the properties. The approximate composition of charcoal for gunpowders is sometimes empirically described as C7H4O. To obtain a coal with high purity, source material should be free of non-volatile compounds (sugar and a high charring temperature can be used). After charring, partial oxidation with oxygen or chlorine can reduce hydrogen levels. For activation of charcoal see activated carbon.
Read more about this topic: Charcoal
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