The Bessemer process made steel far cheaper to mass-produce. Before the 1850s, steel was valuable but expensive because it required slow, small-scale methods such as crucible production. Bessemer’s process transformed molten pig iron by blasting air through it, oxidizing excess carbon and other impurities.
Henry Bessemer patented the process in 1856, and a Bessemer converter could turn several tons of iron into steel in roughly 10–20 minutes. Earlier methods could take at least a day of heating, stirring, and reheating. The dramatic fall in production costs helped steel replace wrought iron in rails, bridges, machinery, and eventually building construction.
A common mix-up is calling the product “iron.” Pig iron was the starting material, while the process produced steel by controlling the carbon left after oxidation. Another important refinement was the basic Bessemer, or Gilchrist–Thomas, process, which allowed treatment of phosphorus-rich ores. Steel rails also lasted far longer than iron rails, accelerating railway expansion.