During this last week we have carried out a charcoal burn using our large (6 ft diameter) kiln. Our reasons for producing charcoal in our woods and threefold: it helps keep a traditional craft alive; produces saleable charcoal which is bagged then sold through the Ightham Mote shop; and illustrates the technique to interested people.
As mentioned we burn using a kiln instead of the traditional method of covering tightly packed wood with turf. This is mainly a labour saving issue, and it reduces the impact of the burn to a small scorch on the woodland floor. Whilst Alder wood is reported to be the best for charcoaling (and was used in the production of gunpowder until the change to cordite) we used Birch in this instance because it is still produces very good standard charcoal compared to other woods, and we have a plentiful supply.
After splitting a good supply of similarly seasoned birch we set up the kiln just South of the Willet Memorial in a clear area of the oak thinnings with the help of our Tuesday volunteer team. The logs are stacked in as tight as possible to avoid air pockets, with tunnels built in the stack from the core of the burn to the air intakes and exhausts which are the feet around the bottom of the kiln.
There are four constituents to wood: water; volatile organic compounds; tar and carbon. As wood is heated they burn in that order although. Charcoal production is basically isolating the carbon element of wood by burning off the other parts. This is achieved through controlling the flow of oxygen into the kiln during the burn.
The first stage of the burn involves lighting the kiln through the intake/exhaust channels built into the stack. At the core of the kiln firelighters are placed with kindling and previously producted charcoal to start the burn. The slightly elevated base of the kiln is not sealed, and the lid propped up so that the maximum amount of air can flow in through the bottom and out through the top, beginning the combusion process throughout the kiln.
After about an hour and a half we observe the base of the kiln, telltale embers all the way around the bottom indicates that fire has spread throughout the kiln. Three ‘feet’ are configured as air intakes and the three others as exhausts with chimneys. The rest of the base is stoppered with ballast, and the lid lowered and made air tight with sharp sand. Now begins the main portion of the burn with the only air flow occuring through the air intakes and exhausts. Close attention is paid to the colour of the smoke eminating from the chimneys, very white billowing smoke is mainly steam; whereas more yellow and brown in the smoke inducates volatile organic compounds and tar being burnt. The burn is shut down when the smoke has a blue tinge, as this means that carbon is being burnt and that charcoal remains. At this point if the burn was not shut down (the oxygen supply cut off by sealing the intakes and exhausts) then eventually only ash would remain. In the large kiln the whole burn process takes around twelve to thirteen hours.
After a few days to cool the lid is opened and the good charcoal graded and bagged. Inevitably some wood will not have completely turned to charcoal (called brown ends) and will be used as fuel in a subsequent burn. Conversely some charcoal will have burned too far and form very small pieces and dust (called fines). Fines can be used to start the fire of another burn, minimising waste. Inefficient burns occur when the fuel inside the kiln does not burn evenly. There are many contributory factors, including wind strength and direction, air pockets in the kiln, different water contents of the wood, mis-timing of changeovers and an uneven start to the fire. An experienced collier aims to minimise these inefficiencies.
If making charcoal is of interest to you then keep an eye out for notices of charcoal demonstrations being carried out later in this summer.