Making Steel in a Bessemer Converter — Mass Steel by Blowing Air Through Iron

Crucible steel was superb but slow, made a few kilograms at a time. In 1856 Henry Bessemer found a way to make steel by the tonne in about twenty minutes, and the price of steel collapsed from a luxury to a commodity. Rails, ships, bridges, and the first steel-framed buildings all became possible almost overnight.

His method sounds reckless: blow cold air straight up through a vessel of molten pig iron. The oxygen in the blast burns out the silicon, manganese, and carbon that make pig iron brittle — and because those reactions give off enormous heat, the metal does not cool but actually gets hotter, staying liquid with no added fuel. A volcano of flame and sparks roars from the converter's mouth while the carbon burns away.

The trick is knowing when to stop. The operator reads the flame, and the instant the carbon is gone, halts the blow and adds back a precise dose of carbon-rich spiegeleisen to hit the exact steel he wants. Tilt, pour, and there is a ladle of molten steel where minutes before there was crude pig iron.