Puddling Wrought Iron from Pig Iron — Burning Out the Carbon in a Reverberatory Furnace

Pig iron straight from the blast furnace is cheap and plentiful but nearly useless for anything that must bend or take a blow: its high carbon makes it brittle as glass. To turn it into the tough, malleable wrought iron that built the railways and bridges of the 1800s, the carbon has to be burned out. Henry Cort's puddling process, patented in 1784, did exactly that — at scale, with coal.

The trick is the reverberatory furnace, which keeps the fuel separate from the metal. Coal burns in a side firebox and the flame is drawn over a low bridge and reflected down off the furnace roof onto a hearth of pig iron, so the sulphurous coal smoke never touches the iron. A workman called a puddler stirs the molten bath through a small door with a long iron rabble, exposing every part of it to the air until the carbon oxidises away.

As the carbon leaves, something strange happens: the purer iron's melting point rises above the furnace temperature, so the metal grows stiff and pasty even in the heat. The puddler gathers it into spongy balls, which are then hammered and rolled to squeeze out the slag and consolidate the fibres into bars of wrought iron.