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Charcoal Burning
The reason for the creation of charcoal is that it burns at a higher temperature than ordinary seasoned wood, mainly because there is no moisture left within the charcoal.
Charcoal is created by partially burning the wood, then restricting air before the fire has a chance to destroy the wood. When ventilation has been restricted, it is the heat alone that drives out the remaining moisture within the wood, leaving only the carbon behind.
The resulting charcoal was traditionally used in the production of iron, by acting as a reducing agent with Iron ore. Charcoal was also required along with Sulphur and Saltpetre in the making of gunpowder.
Today, charcoal is more commonly used for barbeques and in small quantities as artist materials.
When to collect wood and when to burn
Charcoal burning can be carried out at anytime of the year, although it is preferable to make charcoal during the summer when there are not so many strong winds or rainy days. The ideal time for cutting wood is during the winter when trees become dormant and coppicing can be carried out to gather wood. The wood should be allowed to season in log form for at least a year or preferably 2-3 years, as dry wood burns easier and cleaner than green wood. Logs can be cut to 2ft (60cm) long and are generally no more than 6” (15cm) in diameter. Wider logs can be used, but would be better split with an axe.
Trees
The main trees used for charcoal production in Great Britain include:
Ash (Fraxinus excelsior)
Beech (Fagus sylvatica)
Hornbeam (Carpinus betulus)
Oak (Quercus robur and petraea)
Hazel (Corylus avellana)
Methods
There are two main methods of burning charcoal:
1. Metal kiln
Kiln
The metal kiln is a steel ring that stands about 5ft (1.5m) high and has a diameter of about 6 ½ ft (2m). There are several vents around the base of the kiln.
Site
The kiln should be sited on a clear, brash free area of soil, often within the woodland.
Logs
Logs are cut to approximately 50cm in length. A single layer of logs is laid perpendicular to the inner kiln wall all the way around, with gaps where the vents exist. The vents must not be blocked to ensure there is enough air flow to get the wood burning. The logs are then stacked on top of this layer, all the way to the top. A central flue is left to allow space for some old charcoal and paper to start the fire. The kiln is filled above the level of the rim to allow for the natural loss of water and therefore wood size when burning. The lid is placed roughly on top of the wood.
Starting the fire
To start the fire a lit torch is pushed through one of the vents into the centre of the kiln.
Burning
When burning, large amounts of white steamy gases will be released from the kiln, the greener the wood, the more steam there will be.
Once the wood has partly burned the lid should fit tightly on the top of the kiln. To ensure an air tight fit, shovel soil around the rim of the lid. Depending on design, there should be a recess where soil can go. All but four of the basal vents should be covered at this point with soil to restrict the amount of air that enters the kiln. The remaining vents are fitted with chimney pipes to direct steam up and away from the floor. The kiln now relies on the heat to burn the remaining wood until all moisture has gone and only carbon is left. This will take 24 – 36 hours.
All the vents are then closed by covering with soil and the kiln is allowed to cool. Once cooled the kiln is removed and the charcoal is sieved to remove ashes. The resulting charcoal can be graded further or packed straight into bags.
2. The Wood Stack and clamp
The Traditional and more self sufficient method of burning wood for charcoal. This method requires careful management and regular maintenance but does not require heavy or expensive kilns.
A circular pit is dug to approximately 12ft (3.6m) across and 18” (45cm deep) The soil is piled around the pit to be used later.
Logs are cut to about 2ft (60cm) in length and are stacked in a triangular fashion to 4ft (1.2m) high to create the central flue.
Logs are then stacked vertically around the central flue to create a circular stack of logs to fill the pit.
A roof is created using 5ft or 6ft (1.5-1.8m) logs laid pointing towards but not covering the central chimney.
The stack is then covered with bracken and other brash material and then covered with a layer of soil or turf to create what is known as a clamp. Remembering to keep the chimney open.
Hot charcoal and dry brash is dropped down the central shaft to start the fire. Once the fire has taken hold and spread well and truly throughout the stack, the shaft is covered with a metal sheet and covered with soil to prevent air from entering. Over the following days it is important to maintain the clamp by recovering any cracks within the soil wall with fresh soil. Maintain the clamp to prevent the fire from restarting and potentially destroying the charcoal.
Once fully cooled the earth can be removed and the charcoal can be removed and sieved to remove soil, ash and debris.