Charcoal

Charcoal is wood heated with so little air that it chars instead of burning, leaving a light, black fuel made mostly of carbon. Used by metalworkers since at least the fifth millennium BCE, and later made in earth-covered clamps and pits, it gave smiths the heat and the reducing gas they needed to win copper and iron from their ores.

What it is

The same wood before and after: charcoal shrinks and cracks but keeps the grain of the tree.

Charcoal is what remains of wood heated with too little air to burn. Water and most of the volatile matter leave as smoke, and a black, brittle, light solid stays behind, made largely of carbon.

Weight for weight, it holds roughly twice the heat of air-dried wood. It burns with almost no flame or smoke, and with a bellows it reaches temperatures a wood fire struggles to hold. In a furnace it also forms carbon monoxide, the gas that takes oxygen out of metal ores. For some six thousand years it was the fuel of copper, bronze and iron.

The plate shows the earth-covered clamp that European colliers built in the woods until the twentieth century: a dome of wood stacked around a central chimney, wrapped in leaves and earth, and left to smoulder for days.

How it works

Heated wood first dries at 100 °C or a little below, then warms to about 280 °C. Both steps take energy, supplied by burning a small part of the charge. Above about 280 °C the wood breaks down on its own and gives off heat. Water, acids, tars and flammable gases escape, and charcoal is left.

A pit kiln works the same way: the wood chars in a hole in the ground under leaves and earth.

Without outside heat the reaction peaks at about 400 °C, and the charcoal keeps around 30% of tarry matter. Taken to 500 °C, it reaches about 86% fixed carbon, but heat costs yield: oven-dry wood gives about 42% of its weight as charcoal at 300 °C and 33% at 500 °C, not counting the wood burned for heat.

The earth cover acts as the furnace wall. It admits only the air the collier lets in through holes at the foot. A little wood burns, uses up that oxygen, and its heat cooks the rest. In a clamp the fire is lit in the central chimney and spreads slowly outward through the pile to the foot. The smoke tells how far it has gone: dense and white while the wood dries, bluish later, almost clear when a zone is done. As the wood shrinks the mound sinks and the cover cracks, and every crack must be closed at once, or the charcoal burns to ash.

How to build

This is the outline of a small clamp. You need a few cubic metres of seasoned hardwood such as oak, beech or hazel, split so that no piece is thicker than about 20 cm, plus straw or leaves, sandy soil or loam, a shovel, a rake and about 100 litres of water.

Prepare the hearth. Clear a level, well-drained circle about 6 m across down to bare earth, away from overhanging branches and buildings.

Stack around the chimney. Drive three or four stakes in the centre to frame a shaft. Lay a grid of small logs on the ground so that gas can pass under the pile. Stand the billets upright against the shaft, leaning slightly inward and packed tight, and fill the gaps with small wood. A village mound is typically about 4 m across and 1 to 1.5 m high.

Cover it. Lay leaves, straw or turf over the wood, then 10 to 20 cm of earth, mixed with charcoal dust if you have some. Avoid sticky clay, which cracks as it dries. Leave six to ten holes around the foot.

Light and tend. Drop a shovel of embers down the chimney and feed them with dry kindling until the core of the pile is alight. When thick white smoke rises steadily, partly close the top. From then on someone watches the mound day and night, sealing cracks with damp earth, closing any vent that glows red and opening new ones on the side that lags. A small clamp can finish in a day or two; traditional stacks of medium size took twelve to fourteen days.

Seal, cool and open. When the smoke turns thin and blue, close every hole and add earth. Wait until the mound is cold right through, which takes two or three days for a small one, then open it a section at a time, rake the charcoal out and smother any smouldering piece with sand or a little water. Hot charcoal can catch fire again in the open air, so keep watching.

Safety. A clamp can start a forest fire: check the local rules on open fires, keep the hearth bare, and have water, sand and a shovel ready. The smoke is rich in carbon monoxide, which has no smell and kills. Stay upwind, never lean over a vent or the chimney, and never burn, cool or store warm charcoal in a closed space. Never climb onto the mound: as the wood shrinks, the cover can give way over a glowing hollow and cause deep burns. Wear leather gloves and boots and keep children away.

History and evolution

Charcoal is as old as the use of fire: every ancient hearth leaves some, and archaeologists date hearths by it. At the Chauvet cave in France, painters drew animals with pine charcoal about 37,000 to 34,000 years ago.

Metal created the first large demand for it, although nobody knows when people first made it on purpose. In the fifth millennium BCE, smelters in the southern Levant and on the Iranian plateau heated copper ore with charcoal in crucibles and small furnaces. At Horvat Beter, near Beersheba, the ore came from Wadi Faynan, over 100 km away. An estimate published in 1982, drawn from later records and experiments, puts copper at 20 to 40 kg of charcoal per kilogram of metal. At 100 kg of charcoal for a 5 kg smelt, and seven parts of wood for one of charcoal, that is some 700 kg of wood. In China the oldest charcoal kilns found so far, at Xiwubi in Shanxi, lay beside copper-smelting remains of the Erlitou period, around 1900 to 1600 BCE.

Colliers lived beside their clamps in huts of poles and turf, so the burn was never left alone.

Iron, by the same estimate, needed no more than 10 kg of charcoal per kilogram, forging included, but it was made in far larger amounts, and charcoal burning became a woodland trade. In England’s Weald and in the Carranza valley of the Basque Country, colliers supplied ironworks and forges for centuries, working from spring to autumn and living with their families in huts beside the mounds.

By the eighteenth century an English furnace needed about eight tonnes of wood, turned into two tonnes of charcoal, for each tonne of pig iron. In 1709 Abraham Darby smelted iron with coke made from coal at Coalbrookdale, and coke slowly took over. Charcoal survived elsewhere: in 2011 it still made about 30% of Brazil’s pig iron, against less than 1% worldwide, from eucalyptus plantations but also from native forest. It also remains the barbecue fuel, the artist’s willow stick and, once activated, the carbon in filters.