Structural steel

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Iron alloyed with a controlled small fraction of carbon, strong in both compression and tension and, unlike cast iron, ductile rather than brittle. Before the mid-19th century steel was a craft product made in small batches and priced accordingly; the change is metallurgical and economic rather than architectural. Henry Bessemer described his converter process to the British Association at Cheltenham in 1856, blowing air through molten pig iron to burn out carbon and silicon and using the heat of that oxidation itself rather than external fuel; the Siemens-Martin open-hearth process followed and dominated for decades because it handled scrap and gave better control. The result was structural steel at a fraction of its former cost and in large repeatable sections. Ductility is the property that matters for buildings: a steel frame deforms visibly before it fails, which is a different and much more forgiving failure mode than the sudden fracture of cast iron.