What is charcoal or biochar?
Charcoal is formed when wood is burned with insufficient air for complete combustion. As the wood burns any water it contains is drive off first (creating a dense white smoke). This is then followed by all of the volatile compounds. If the burn is arrested at that stage it leaves behind the charcoal which is almost pure, solid, black carbon.
What are the uses for charcoal?
Traditionally charcoal was used as a fuel to forge and smelt iron as it burns much hotter than wood. It was also a key ingredient in gunpowder.
In many places charcoal has become popular for barbecues as it burns with almost no smoke and imparts a beautiful flavour to the food. A lot of charcoal is imported and comes from unsustainable, environmentally damaging sources so its important not to but this type of charcoal.
There is a growing awareness of the value of charcoal as a soil conditioner - it is then known as biochar. Carbon plays a key role in maintaining soil fertility, maintaining soil structure and binding nutrients to prevent them leaching. Rich, black, fertile soils or 'terra preta' have been found in the Amazon rain forest in areas with otherwise poor soils. Terra preta are a legacy of a cut and char system which added charcoal to the soil and maintained incredible levels of fertility. Modern research has confirmed that the addition of biochar can significantly improve fertility once the biochar has been biologically activated through a compost heap or through use of compost teas and biological fertilisers.
Making charcoal on a small scale
Charcoal can be successfully made in a home made kiln made from a steel oil barrel.
The kiln is made by removing one end of the barrel completely and by knocking half a dozen holes of up to 50 mm diameter in the other end with a hammer and cold chisel. The remaining lip on the cut end can be knocked upwards with a hammer so that the cut disc will sit on it as a lid.
To operate the drum is set, open end uppermost, on top of four bricks to allow air to reach the base holes. Kindling is then used to set a small fire in the bottom of the drum.
When this fire is established, cut seasoned wood can be tumbled in to fill the drum right up and the lid placed loosely on top. The wood should be of fairly even diameter but if there are larger ones put them at the bottom where they will char for longer.
Once the fire is clearly well established and won't go out restrict the air flow to the bottom of the barrel by heaping earth around the base of the drum leaving just 100 mm gap (away from the wind). The lid can be propped up on one side with a branch across the top of the drum to allow smoke to escape.
This stage of the burn will be characterised by dense white smoke. As the smoke changes to be thin and slightly blue this signals that the charcoal is starting to burn. It can be worth giving the drum a knock at this stage to allow the wood inside to settle which may release another burst of the white smoke.
Once you think the charcoal has formed, the fire must be extinguished by excluding the air. Use more earth to pack right around the base of the drum and settle the lid on the top and use more earth and turfs to seal it so that no smoke is escaping. The process from lighting to sealing will take between 3 and 5 hours.
The drum must then be left to cool for a further 24 hours before opening it. It is worth checking it a few times through the first part of the cooling stage to ensure that air has not succeeded in penetrating again and allowing the fire to restart and burning through the wood.
