Bessemer process

The Bessemer process is an industrial method of turning molten pig iron into steel by blowing air through it. Patented by the English inventor Henry Bessemer in 1855, it was the first inexpensive way to make steel in large quantities.

History of the Bessemer process

Before the process existed, only cast iron and wrought iron could be made in industrial quantities. Cast iron holds 2 to 5 percent carbon and can be cast into shapes, but it is brittle. Wrought iron has very little carbon and is malleable but cannot be cast. Steel, with up to about 2 percent carbon, has the advantages of both, yet until the mid-1800s it was made only in small, expensive batches.

The usual method in Britain was cementation, in which bars of wrought iron were packed with charcoal in a stone box and heated for up to a week to make blister steel. It burned up to three tons of coke for each ton of steel, and the finished bars sold for 50 to 60 pounds a ton. In 1740 Benjamin Huntsman improved quality in Sheffield with crucible steel, but at a higher cost.

Henry Bessemer was born on January 19, 1813, in Charlton, Hertfordshire, and made his early fortune from a process for producing brass powder. He patented experimental mortar shells in 1854. Cast-iron gun barrels could not withstand them, which led him to look for a way of making strong steel in bulk. At a small works in St. Pancras, London, he found that air forced through molten pig iron removed carbon quickly, and that the reaction produced enough heat to keep the metal liquid without extra fuel.

He patented the forced-air method in 1855 and presented it at the 1856 meeting of the British Association in Cheltenham. Iron makers rushed to license it, but their first trials often produced brittle metal. Bessemer set up his own steelworks in Sheffield and kept refining the process. By the time his patent expired in 1870 he had reportedly earned more than a million pounds in royalties. He was knighted, became a fellow of the Royal Society in 1879, and died in 1898.

Who invented the Bessemer process?

The process bears Bessemer’s name, but the credit is contested. An American, William Kelly, had worked on a very similar air-boiling method for making steel. Some accounts say Kelly claimed to have found the process in 1851. Kelly kept his work secret, then rushed to patent it after Bessemer’s 1856 announcement and received a patent in 1857. He had early success defending the patent in the United States, but he was little known elsewhere, and his version had not been perfected.

Historians still disagree about whether the two men worked independently or whether one learned of the other’s idea. Grace’s Guide adds that blowing air through iron had been used outside Europe for hundreds of years, though never on an industrial scale.

Others fixed flaws in the process. In 1856 the chemist Robert Mushet found that adding spiegeleisen, an iron alloy rich in manganese, removed excess oxygen. Bessemer used the step from 1859 but never conceded that it was needed. A Swedish iron master, G. F. Goransson, showed that low-phosphorus ores gave less brittle steel. In 1878 Sidney Gilchrist Thomas and Percy Gilchrist found that a limestone or dolomite lining and added lime removed phosphorus, which opened the way to high-phosphorus ores.

How the Bessemer process works

The work happens in a converter, a large egg-shaped steel vessel lined with clay or dolomite. A converter held between 8 and 30 tons of molten iron, and a typical charge was about 15 tons. The vessel sits on trunnions so it can tilt to receive the iron, stand upright during the blow, and tip again to pour out steel. Air enters through channels called tuyeres in the bottom.

As air passes through the molten iron, it oxidizes impurities such as silicon, manganese and carbon. The oxides leave as gas or form a layer of slag. The oxidation also heats the metal, so the process needs no extra fuel. The blow takes about twenty minutes, and the operator judged its progress by the color of the flame at the converter’s mouth. Afterward, carbon and other alloying ingredients were added to reach the desired grade.

The chemistry is a direct use of oxygen to burn off carbon and other elements from iron, and the result is an alloy with the right balance of carbon.

Types of Bessemer process

Two main versions developed. The acid Bessemer process used a clay lining and worked when the raw iron contained little phosphorus. The basic Bessemer process, also called the Gilchrist-Thomas process, used a dolomite lining to deal with phosphorus-rich iron, and it allowed ores that had earlier been unusable.

Impact of the Bessemer process

The process cut the cost of steel sharply. The earliest converters produced steel at about 7 pounds a ton, although it was first priced near 40 pounds. Once it spread, steel and wrought iron cost about the same, and manufacturers moved to steel. Bessemer’s process also took about half the time of earlier methods for steel of similar quality, and it needed only the fuel used to melt the pig iron.

Cheap steel supported industrial and military needs and became a driving force of the Industrial Revolution. It also led to new alloys, including the tungsten steel developed by Mushet. Modern structural steel belongs to the same lineage.

Decline and legacy of the Bessemer process

The process had weaknesses. It was so fast, ten to twenty minutes a heat, that there was little time for chemical analysis or adjustment. It could not remove phosphorus well unless a basic lining was used, it could take only limited scrap, and some steel grades suffered from nitrogen picked up from the air blast.

The open-hearth furnace, developed in the 1860s, gradually replaced the converter because it recycled scrap better, handled larger batches and gave better control of quality. The last Bessemer converter closed in 1975. Modern steelmaking has moved to later methods, including the basic oxygen furnace. Surviving converters can be seen in museums, among them one at Kelham Island Museum in Sheffield.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue