. Charcoal burn .
Making charcoal .
Wood burns at too low a temperature for purposes such as iron production, so charcoal was produced by the partial combustion of wood in low oxygen conditions.
Four units (by weight) of wood is needed to produce one unit of charcoal, so the process was typically performed on site in an area in which the soil had a high clay content.
A circular pit between 12-16 ft in diameter was dug to a depth of 18 inches. Starting at the centre, a vertically-placed pyramid of wood was built, expanded outward in concentric circles, and then capped with a low pyramid of wood. The construction was then covered with the removed soil and turf. Ventilation holes were placed every few feet around the base, and a fire-hole was left open at the top of the stack.
The stack (or clamp) was ignited by dumping charcoal embers down the fire-hole. It was imperative that the stack not burn too fiercely, so the burn must be watched for the duration of the burn. Any cracks in the stack must be repaired to ensure that air could not enter and cause the burn to blaze fiercely.
After three to four days, the smoke emitted by the stack would change colour from white to blue. Those guarding the burn would then block the ventilation holes to stifle the fire. After a further two or three days, the stack would have cooled sufficient for the mound to be opened. Water was kept on hand in case the stack reignited. The charcoal could then be sorted and bagged; the sacks were transported out, usually on donkeys.