Coke smelting

Coke smelting is blast-furnace ironmaking with the fuel changed. Raw coal cannot simply replace charcoal in a furnace: its sulfur poisons the iron, making it crumble when hot-worked, and it is too weak to bear the weight of a tall furnace stack. Coke solves both problems. Bake coal in a heap or oven with limited air and its tar and volatile gases cook off, leaving a hard, porous, almost pure carbon that has shed most of its sulfur — just as charring wood makes charcoal. Coke burns hot and clean, and it is strong enough to hold up a burden many metres deep. Substitute it for charcoal and the blast furnace runs on a fuel dug from the ground instead of grown in a forest.

Charcoal was the hard ceiling on iron. Every ton of iron demanded acres of coppiced woodland, and as ironmaking grew it ate through the forests until fuel, not ore, was the binding constraint — Britain was importing bar iron from the still-forested Baltic. Coke cut that cord. Coal reserves dwarfed any woodland, so iron output could grow without limit, and furnaces freed from charcoal's fragility could be built ever taller and hotter. Abraham Darby's furnace at Coalbrookdale first made the switch work in 1709, but the deeper significance is systemic: coke smelting welded the iron industry to the coal industry, and that pairing — cheap coal feeding cheap iron — is the material foundation the entire Industrial Revolution was built on.

First turn coal into coke by baking off its volatiles and sulfur, then charge the blast furnace with coke, ore, and limestone and blow it hard. (See recipe.) Two properties of coke are the whole story. It is strong, so the furnace can be built tall enough to smelt far more iron per blast; and it is dense and slow to ignite, so it demands a more powerful, steadier air blast than charcoal — which is why coke ironmaking and the steam-powered blowing engine grew up together, each pushing the other larger.

Coke smelting made cast iron effectively unlimited, and unlimited cast iron is the raw material for everything downstream: the rails and rolling stock of the railways, the frames of factories, the cylinders of steam engines, and — once puddling, Bessemer's converter, and the open hearth learned to refine it in bulk — cheap steel. By binding iron to coal, coke also locked in the logic of the fossil-fuelled economy: dig carbon, burn it, make metal, build machines that burn more carbon. Nearly every iron object of the industrial age begins as coke-smelted pig iron.