Puddling (coal-fired wrought iron)

Puddling is the finery forge's job done with coal. Like the finery, it takes brittle, high-carbon pig iron and burns the carbon out by oxidation to make forgeable wrought iron. The problem it solves is that you cannot simply fire a finery with coal — coal's sulfur would soak into the iron and ruin it. Henry Cort's answer was the reverberatory furnace: burn the coal in one chamber, put the iron on a separate hearth, and let a low arched roof reflect the flame's heat down onto the metal so the fuel never touches it. The iron melts, an iron-oxide lining feeds oxygen into the bath, and a workman — the puddler — stirs it with a long iron bar until the carbon burns away and the metal, now purer and higher-melting, stiffens into balls he can lift out.

Wrought iron was the structural metal of the early industrial world, and until 1784 every ton of it was refined in a charcoal finery, chaining the iron industry to the forests even after coke had freed the blast furnace. Puddling broke the last charcoal bottleneck: now both stages of ironmaking — smelting the ore and refining the iron — ran on coal. Cort paired it with a grooved rolling mill (patented 1783) that squeezed the puddled balls into bars far faster than hammering. Together they multiplied British wrought-iron output enormously within a generation and turned Britain from an importer of Swedish and Russian bar iron into the world's great iron exporter. The rails, plates, boilers, girders, and machinery of the first industrial century were, overwhelmingly, puddled iron.

Line the hearth with iron oxide, melt pig iron on it under a reflected coal flame, and stir hard as the oxide burns the carbon out. The bath boils, flickering with the blue flames of burning carbon monoxide, until the metal "comes to nature," balling up because removing carbon has driven its melting point back above the furnace's heat. The puddler gathers the pasty iron into several balls, which are shingled under a hammer and rolled into bars. (See recipe.) The whole art lived in the puddler's stir — a brutal, skilled labour that machines could not easily replace.

Cheap, abundant wrought iron built the age of iron infrastructure: railway track, iron ships and bridges, steam-boiler plate, and the framing of countless machines and buildings. Puddling also carried forward, at industrial scale, the central principle of steelmaking — refine iron by controlled oxidation — and set the output benchmark that Bessemer's converter (1856) and the open hearth (1865) had to beat. Those bulk-steel processes eventually made puddled wrought iron obsolete, but they were built on the ground it cleared.