Making Blister Steel by Cementation — Carburizing Wrought Iron into Steel

Wrought iron bends but will not hold an edge; cast iron holds an edge but shatters. Steel — iron with just the right sliver of carbon — does both, and for centuries the great problem of metalworking was how to make it reliably. The cementation process, used across Europe from the 1600s, was the first method to manufacture steel in quantity from ordinary wrought iron bars.

The idea is to add carbon to solid iron without ever melting it. Bars of wrought iron are packed in powdered charcoal inside a sealed stone chest and roasted for a week at a bright red heat. Over those days, carbon slowly creeps out of the charcoal and diffuses into the surface of the iron, turning the skin into steel. Gas trapped under the surface raises little blisters across the bars — which is why the product is called blister steel.

Blister steel is uneven, carbon-rich at the surface and soft in the core, so smiths refined it further: forge-welding and folding it into shear steel, or melting it in crucibles into perfectly uniform cast steel. But cementation was the foundation — the step that first made steel an industrial material rather than a rare treasure.