The Bessemer process was used to manufacture steel from molten pig iron. The principle involved is that of oxidation of the impurities in the iron by the oxygen of air that is blown through the molten iron. The heat of oxidation raises the temperature of the mass, keeping it molten during operation. The process is carried on in a large, egg-shaped container called the Bessemer converter, which is made of steel and has a lining of silica and clay or of dolomite. The capacity ranges from eight to thirty tons of molten iron; the usual charge is fifteen or eighteen tons. At its narrow, upper end, it has an opening through which the iron to be treated is introduced and the finished product is poured out. The wide, bottom end has a number of perforations through which the air is forced upward into the converter during operation. The container is set on pivots so that it can be tilted to receive the charge, turned upright during the "blow," and inclined for pouring the molten steel after the operation is complete. As the air passes upward through the molten pig iron, impurities such as silicon, manganese, and carbon unite with the oxygen in the air to form oxides; the carbon monoxide burns off while the other impurities form slag. Dolomite is used as the converter lining when the phosphorus content is high; the process is then called basic Bessemer. The silica and clay lining is used in the acid Bessemer, in which phosphorus is not removed. In order to provide the elements necessary to give the steel the desired properties, another substance (often spiegeleisen, an iron-carbon-manganese alloy) is usually added to the molten metal after the oxidation is completed. The converter is then emptied into ladles, from which the steel is poured into molds; the slag is left behind.

The whole process is completed in fifteen to twenty minutes. The Kelham Island Museum at Sheffield maintains an early example of a Bessemer converter.

Prior to the Bessemer process, steel was expensive and difficult to produce. It was used largely for specialized cutting tools and fine swords. Henry Bessemer pioneered the steel industry. He made steel plentiful and relatively cheap to produce. Lighter, stronger, and more flexible than either cast iron or wrought iron, steel became a driving force behind the Second Industrial Revolution. It would be used for skyscrapers, bridges, weapons, machines, tools, and transportation. Steel became the basic component of the physical structure in modern urban industrial society. Although Bessemer's process would be supplanted by other steel-producing processes in the twentieth century, especially the oxygenization process of the 1950's, it nevertheless began the rapidly accelerating process of worldwide steel usage on a large scale.

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