Charcoal

Charcoal is what is left when wood is heated without enough air to burn it. The water goes off first, then the volatile organic fraction — tars, acids and gases, which are roughly three quarters of the original mass — and what remains is a porous carbon skeleton holding the shape of the wood it came from.

The point of doing this is threefold and each part mattered. It burns hotter, because there are no volatiles to burn away in a smoky flame before the carbon gets going. It burns cleaner, for the same reason. And it weighs about a quarter of what the wood weighed while carrying most of its energy, which in an age of animal transport decided whether a furnace could be supplied at all.

That last point is why ironworks were sited in woods rather than at the ore: it was cheaper to carry the ore to the charcoal than the charcoal to the ore.

Traditionally by earth kiln, and the craft is in the airflow rather than the fire. Cordwood is stacked into a dome around a central flue, covered with turf and earth to exclude air, and lit; the burn of a small fraction of the wood supplies the heat that pyrolyses the rest, and the collier controls the process by opening and closing vents for several days without sleeping properly.

Retorts do the same thing in a sealed vessel heated from outside, which is cleaner, faster, and recovers the volatile fraction instead of releasing it. Before the petrochemical industry that recovery was the point: wood distillation was the source of methanol — 'wood alcohol' — acetic acid and acetone.

Yield is around 20 to 25 per cent of the dry wood by mass in a good kiln and much less in a bad one, and higher temperatures give a purer, harder charcoal with less of the wood's volatile content left in it.

Activated carbon is charcoal taken further, with steam or chemical treatment opening the pore structure until a gram of it has the surface area of a tennis court — which is the whole basis of its use as an adsorbent in water treatment, air filtration and medicine.

Charcoal smelted every metal humanity had until the eighteenth century. Copper, bronze, iron, lead, silver — all of it, because charcoal is both the fuel that reaches the temperature and the reducing agent that pulls the oxygen off the ore.

It is also a third of gunpowder by weight and the fastest-burning component, and willow charcoal was specified for it because of how it burns.

Its limit was forest. An eighteenth-century blast furnace consumed the sustainable yield of several square miles of coppice, and by 1700 English ironmaking was constrained not by ore or by demand but by wood — which is the direct cause of Abraham Darby's coke smelting at Coalbrookdale in 1709. Coke made iron production independent of the growth rate of woodland, and the Industrial Revolution followed from an escape out of a fuel constraint rather than from a new appetite for iron.

Charcoal never disappeared. It remains the primary cooking fuel for hundreds of millions of people, and it is still specified where its purity matters — in silicon metal production, in some ferroalloys, and in charcoal-fired steelmaking in Brazil.

Two accounts, both true, and which applies depends entirely on where the wood came from.

Charcoal made from coppiced or plantation wood is close to carbon neutral over its cycle, because the carbon burned was taken from the air within the last few decades and the stump regrows. This is the traditional European practice and it sustained ironmaking for centuries on the same woods.

Charcoal made by cutting standing forest is not, and that is most of the world's production today. Urban charcoal demand in sub-Saharan Africa is a documented driver of deforestation around cities, and the earth kilns used are inefficient enough that much of the wood's energy is released as methane and particulates rather than captured as fuel. It is also a serious indoor air-quality problem where it is burned unvented.

Biochar is the same material made deliberately for a different purpose — buried rather than burned, as a soil amendment and a carbon store, since the carbon in charcoal is stable in soil for centuries. The evidence for its agronomic benefit is genuinely mixed and depends heavily on soil type; the carbon storage is real arithmetic.