The furnace inferno was the result of an exothermic reaction: a chemical reaction that releases energy – usually in the form of heat – during the oxidation of impurities. After the silicon impurities had been quietly consumed, the oxygen in the air current reacted with the carbon in the iron, releasing a huge amount of heat. This heat raised the temperature of the iron far beyond what a coal-fired furnace was then capable of, so Bessemer didn’t need to use external sources of heat. The hotter the iron became, the more impurities burned off, which made the iron hotter still, so it burned off
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