Illustration after an engraving of a Bessemer converter in use. The converter was used for making high-grade steel, using a process that is little changed today. Air is blown through a mixture of molten iron and charcoal. The end result is a pool of steel (very pure iron with a controlled carbon content), impurities collecting as a slag on the surface. The device was described by Henry Bessemer (1813-98) in London in 1856, and still bears his name despite the earlier independent discovery of the idea by William Kelly of Pittsburgh. This engraving first appeared in the "Illustrated London News" in 1856.