Yet another National Trust working holiday, this one 8 miles south of Chesterfield, on the estate of Hardwick Hall. We spent most of our time cutting down hazel coppice and stacking anything fatter than about 3cm into piles about a metre long. After 6 or 8 weeks these partially dry stacks will be loaded into the steel charcoal kiln and undergo a controlled burn to drive off the water and volatile components of the wood, leaving just solid sticks of carbon – charcoal. At least thats the plan, but its a complex mixture of science and art and loads of variables, so an ideal burn is , well, rare.
We had a great week for weather – a short Indian Summer, low to mid twenties Celcius.
The kiln is a steel ring about a metre high and 1.3m diameter, there are four steel vents the ring sits on. The vents are extended to the centre of the kiln with logs covered by more logs, so that air can be drawn into the very centre of the kiln. Then we start adding dry brash – the smaller branches from the felled coppice stems – and a volunteer climbs in to walk round trampling the brash down. This layer will burn first and get the logs burning. Because the brash will burn out completely we stack the logs higher than the kiln, so they will fall down as the brash burns, and we’ll still have a full kiln of burning wood when the lid drops into place.
The kiln is lit by pushing a burning rag through one of the vents to the centre.
After a little while steam – cool and wet – pours out of the kiln. Gradually the steam is replaced by smoke and flames as the wood dries, gets hot and drives off the volatile resins, gums, sugars etc as gases which start burning.
When the brash is gone the lid falls – with some urgent and vigorous help – into its retaining groove. Two steel chimneys can now be fitted into the vents, on opposite sides of the kiln. Now air is drawn into the two low vents, and the fire inside moves towards them, as smoke and steam leaves by the two chimneys. Every hour the chimneys are moved to the other vents to encourage an even spread of flame across the bottom of the kiln.
Its important to ensure air is only entering the kiln by the vents, so clay is packed around the base of the kiln, and around the lid/kiln junction. At some point Richard decides that the temperature and smoke colour are right, and the kiln is completely sealed by removing the chimneys and filling all four vents with packed clay.
I think the heat is sufficient to drive off the volatiles, and they may not burn up completely, but they will condense on the sides of the kiln and leave the wood as charcoal. But I am a little (lots) uncertain about the exact process. Anyway after two days we gathered to open the cool kiln. Breaking away the clay and carefully lifting the lid, all the while listening for the tinkling noise that means the charcoal is reigniting as the air enters. If this had happened we’d have quickly replaced the lid and the clay seals. But the fire was out and we could examine our charcoal. In all about 70 kilos of charcoal and twice that of charred wood (which will go into the next burn).