Making Charcoal in an Earth Mound — Pyrolysis in a Covered Burn

Select dense hardwood species — oak, beech, maple, or ash produce the best charcoal due to their high carbon density. Split the wood into uniform pieces roughly 5-10 cm in diameter. Stack the wood vertically in a dome-shaped mound approximately 1-1.5 metres in diameter and 1 metre tall, leaning pieces against a central stake. Leave a hollow chimney space at the centre for ignition. All wood must be well-seasoned and dry — green wood produces excessive smoke and yields poor-quality charcoal with high moisture content.

Cover the wood stack with a layer of green leaves, grass, or straw to prevent the earth covering from falling between the wood pieces. Then apply a layer of earth or clay 5-10 cm thick over the entire mound, packing it firmly. Leave the central chimney opening at the top uncovered, and poke 4-6 small air holes (about 5 cm diameter) around the base of the mound. These base holes allow limited air intake that sustains smouldering combustion while the earth seal restricts oxygen enough to prevent the wood from burning completely to ash.

Drop burning kindling down the central chimney to ignite the wood at the core of the mound. Once the fire is established (visible flames at the chimney and smoke from base holes), partially close the chimney with a flat stone, leaving a gap for smoke to escape. The burn must proceed slowly — if flames appear through the earth covering, patch those spots immediately with more earth. White or blue-grey smoke indicates active pyrolysis and water vapour being driven off. Yellow or brown smoke suggests incomplete combustion. The burn progresses outward from the centre over 6-10 hours.

As the burn progresses, the mound settles and may develop cracks in the earth covering. Patch all cracks promptly to maintain the oxygen-restricted environment. When smoke from the base holes transitions from thick white to thin blue-grey and finally becomes nearly transparent, the pyrolysis is nearing completion. At this point, seal all base holes and the chimney completely with earth. This stops all combustion and allows the charcoal to cool without burning to ash. The sealed mound needs 8-12 hours to cool completely.

Once the mound is fully cool to the touch, carefully break open the earth shell. Well-made charcoal is jet black throughout, lightweight, and produces a metallic ring when pieces are tapped together. If pieces show brown or grey interiors, they are only partially carbonised (called brands) and should be placed in the next burn. Expect a yield of roughly 20-25 percent by weight of the original wood — 100 kg of oak produces about 20-25 kg of charcoal. Good hardwood charcoal burns at temperatures exceeding 1100 degrees C with a forced air supply, making it the essential fuel for copper and bronze smelting.