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The manufacture of charcoal

The collection and preparation of the right timber is important to ensure a consistent charcoal product that meets both high quality standards as a fuel, as well as helping improve the woodland environment. Today, timber is sourced from woodland regeneration projects, coppicing and thinning. The image below illustrates storm damaged derelict woodland undergoing restoration. Most of the fallen timber in this picture will have been used for charcoal manufacture.

wood stackBefore timber can be loaded into a kiln it has to be prepared. After the initial cutting, the wood is fine cut into lengths of around eight feet and is then stacked to dry for a period of 6-9 months. Towards the end of the period it is chopped and spit into sections of about three feet in length and five inches in diameter.

wood stack ready for burningCharcoal is made when wood is heated under conditions where there is insufficient air for complete combustion. In this process the water contained in the wood is first driven off before the wood begins to break down. A series of chemical reactions then follow which result in the release of volatile products. Charcoal – a solid black residue is left.

Manufacture of charcoal in the UK is now largely carried out in portable kilns. These kilns are circular steel drums of about eight feet diameter and a least four feet in height. There is a removable lid and a number of chimneys, which vent from the bottom of the kiln. The timber is carefully placed in the kiln leaving a central void which forms the starting point for the fire. When the kiln is full a fire is lit in the central void and allowed to establish before the lid is loosely placed on the top of the kiln. Over the next couple of hours the amount of air to the kiln is carefully controlled before the lid is sealed using earth from nearby. The kiln then vents from ground level and burns for around 24 hours providing perfect conditions for the production of charcoal.

arly stages of the combustion processAll vents are then sealed and the combustion process dies down. The kiln is then allowed to cool before it is safe to remove the charcoal. The cooling process can take several days. The image illustrates the early stages of the combustion process before the lid is properly sealed.

Quality Charcoal:
Charcoal demand currently favours large fairly evenly sized pieces. This can only be produced from timber that has been cut to consistent sizes and generally is no smaller than 4 inches diameter. In addition the charcoal out of the kiln has to be run over a sieve before it can be bagged. This removes the smallest pieces (known as fines) which are then bagged off and utilised in mulches. Both plastic and paper bags are used. Whilst paper bags have the advantage that they can be burnt on a fire, plastic bags offer better long term storage of the charcoal by preventing moisture ingress.

Timber Choice:
Although any species of wood can be converted to charcoal the best charcoal is usually manufactured from fine grained hardwood species. In the UK the most commonly used wood for this purpose are ash, beech, oak, chestnut and hazel.