Charcoal

Seasoning first. Fresh-cut wood is 45–60% water by weight. Every litre of that has to be boiled off inside the clamp before carbonisation starts, using wood that would otherwise have become charcoal. Season wood under cover for 6–12 months to below about 25% moisture. Wet wood gives a lower yield, a longer burn, and more half-carbonised pieces.

Species. Dense hardwoods — oak, beech, hornbeam, ash, hickory, many acacias and eucalyptus — give dense, strong charcoal that holds its shape under the weight of a tall furnace charge. That strength matters in a bloomery, where a crushed charcoal bed chokes the air blast. Softwoods — pine, spruce — give light, friable charcoal that lights easily and burns fast; many smiths prefer it for the forge, and it is poor for smelting.

Size. Billets of roughly uniform size carbonise evenly. Split anything thicker than about 15 cm; cut lengths to fit the method, commonly 0.5–1 m for a clamp. A pile of mixed sizes finishes with the small pieces burnt to ash and the large ones still raw at the core.

Where it comes from. Charcoal making stripped forests across Europe, the Middle East and now much of Africa. Coppicing — cutting broadleaf trees at the base and letting them regrow as multiple poles — produces charcoal-sized wood on a 7–20 year rotation indefinitely from the same stools. It is how charcoal burning was sustained for centuries.

Every method does the same thing — heats wood with too little air to burn it — and differs in how well it controls the air, and how much of the wood is burned to heat the rest.

Nobody can see inside a clamp. The smoke from each vent tells you what is happening in the wood beneath it, and managing the vents by smoke colour is the whole skill of charcoal burning.

The burn spreads from where it was lit — usually the top of the central flue — downward and outward. The collier opens vents around the base and progressively closes each as the smoke from it changes.

Draw in the morning, so any flare-up is visible and there is a whole day to deal with it. Open one side, rake the charcoal out, and have water and wet earth ready for pieces that begin to glow.

Sort as you draw:

• Good charcoal is black with a glossy fracture, lighter than the wood, and rings with a metallic clink when two pieces are struck together.
• 'Brands' — pieces still brown inside, that smoke when lit — are under-carbonised. Set them aside for the next burn; they are what makes a forge fire smoky.
• Fines and dust are usable in a smithing forge and as a soil amendment, and poor in a furnace, where they block the air.

Let it breathe before bagging. Freshly made charcoal adsorbs oxygen and water vapour, and that process releases heat. Charcoal bagged or heaped straight from the kiln can heat itself and ignite hours later, inside the store. Spread it out and leave it exposed to air for at least 24–48 hours before bagging.

Store it dry. Damp charcoal is harder to light and burns smokily. It does not rot and keeps for years under cover.

• Carbon monoxide. A clamp produces enormous volumes of it throughout the burn and cooling, and so do bags of warm charcoal in a closed space. Smoke from the vents disperses outdoors; the danger is any enclosed or sheltered space downwind — a hut, a tent, a hollow. And charcoal burned for warmth indoors is the classic CO death: it gives almost no smoke to warn anyone. See the carbon monoxide section of the fire guide.
• Falling into the clamp. A collier climbing on a clamp to seal a leak can break through the cover into a bed of burning charcoal. Never stand on a clamp — use a long-handled shovel from the ground, or a board laid across to spread the weight.
• Self-heating in storage — see above.
• Forest fire. Clear the ground around a clamp to bare soil for several metres, keep water on site, and never leave a burn unwatched in dry or windy weather.
• Burns while drawing. Heavy leather gloves and boots, and water within reach.

The reason to make charcoal is almost always the next step.