Charcoal

Green wood can be half water by weight, and a clamp spends most of its heat boiling that water off — giving less charcoal, and crumbly charcoal. Air-dry cordwood under cover for 6–12 months, until it is below about 20% moisture: bark loosening, radial cracks in the end grain, and a clear knock rather than a dull thud when two billets are struck together.

Cut billets about 1 m long and 5–15 cm across, splitting anything thicker. Uneven sizes carbonise unevenly — the small pieces burn to ash while the large ones stay raw. Hardwoods — oak, beech, ash, hornbeam — give dense charcoal for smelting; softwood charcoal is lighter and burns faster.

On level, cleared ground: drive a central stake or build a chimney of stacked billets about 30 cm across. Stand the billets on end, leaning slightly inward, packed tightly around it in rings to form a dome — tight packing leaves less air, which means less wood burning away to ash.

Cover the whole dome with a layer of leaves, straw or turf — this stops earth falling into the gaps — then 10–15 cm of damp earth or sand over everything, leaving the chimney open. Set small vent holes around the base that can be opened and blocked with a sod. See the charcoal clamp.

Drop hot embers down the chimney. When the core is well alight — smoke rising strongly from the chimney — cap the chimney and close most of the base vents. From now on air enters only where you let it, and the fire creeps outward through the stack by smouldering, not flaming.

Someone watches the clamp day and night for the whole burn — typically 3–10 days depending on size. Flames breaking through the cover mean too much air: seal that spot immediately with earth. Check the cover every few hours for cracks as the stack shrinks and settles.

The smoke tells you what is happening inside:

• Thick white smoke — steam. The wood is drying.
• Yellow-grey, acrid, heavy smoke — tars and volatiles are coming off. Carbonisation is under way.
• Thin, faint blue smoke, almost clear — the volatiles are gone. That part of the stack is charcoal.

Manage the burn by opening vents on the side lagging behind and closing those where the smoke has gone blue. When all of the smoke is thin blue, seal every vent and the chimney completely, and throw more earth on anywhere smoke still escapes.

Leave the sealed clamp to cool for 1–3 days, depending on size. Air on hot charcoal burns a week's work to ash in minutes.

Test before opening: push an arm into the earth cover at several points, or push in a stick and pull it out — it must be cool to the hand. Open one side first, with water and a shovel of earth ready. Any piece that glows goes into a sealed metal bin or gets smothered.

Good charcoal rings when two pieces are struck, is light for its bulk, breaks with a shiny, glassy fracture, and leaves black on the fingers without being greasy. Brown or wood-coloured pieces are under-burnt — rebuild them into the next clamp. Fines and dust go to filters, soil and forge-lighting.

Weigh the output against the wood in. Expect roughly 10–25% of the air-dry wood weight; below that, the clamp let in too much air. Budget the fuel for what it feeds: a single bloomery smelt can consume more charcoal by weight than ore.

Coppice the woodland on a rotation so the same ground yields poles forever — see woodworking for rotations. Divide the wood into as many plots as the rotation has years, and cut one plot a year.

Then do the arithmetic before committing to metallurgy: the charcoal each smelt needs, divided by the charcoal each clamp yields, divided by the wood per hectare per year. Groups that clear-fell for charcoal deforest their own valley and then cannot smelt at all — a documented historical failure. See the charcoal guide.