Steel before Bessemer
Steel is iron with a controlled, small amount of carbon, typically well under two percent. It is stronger and more versatile than cast iron, which has a lot of carbon and is brittle, and than wrought iron, which has very little and is soft. Before the mid-nineteenth century, steel was made in small batches by methods such as cementation (heating bars of wrought iron in contact with charcoal for days) or in crucibles. It was expensive, so it was used for tools, springs and blades, not for bridges or rails.
The core idea
Pig iron from a blast furnace contains several percent carbon plus silicon and other elements. To turn it into steel, most of the carbon must be removed. Henry Bessemer, an English inventor, announced in 1856 a method that did this by blowing air through molten pig iron in a large pear-shaped vessel, the converter. Oxygen in the air reacted with carbon and silicon in the iron. The reactions release heat, so the metal stayed liquid without extra fuel, which was a surprise to many observers. A converter could turn a charge of iron into steel in roughly twenty minutes or so, compared with days for older methods.
The burn-off was dramatic, with a roaring flame from the mouth of the converter, and the operator judged by the color of the flame when the carbon was nearly gone.
Teething problems and rival claims
Early attempts by other ironmasters to use Bessemer’s licensed method gave poor results, and for a while the process looked unreliable. Two issues mattered:
- Oxygen left in the steel. Blowing too long left the metal brittle. The metallurgist Robert Mushet found that adding a manganese-rich iron alloy (spiegeleisen) removed oxygen and put back some carbon in a controlled amount. His role is often considered essential, though his financial rewards were limited.
- Phosphorus. Many iron ores, such as those widely found in continental Europe and parts of Britain, contained phosphorus, which made the steel brittle. Bessemer’s acidic converter lining could not remove it, so the process worked best with low-phosphorus ores.
The American William Kelly also claimed to have discovered a similar air-blowing method in the 1850s, and he was granted a US patent. Like many nineteenth-century inventions, the question of who deserved the credit led to disputes, and the details are still debated.
The rivals
Bessemer steel was soon joined by other processes:
- Open-hearth (Siemens-Martin). Developed in the 1860s by Carl Wilhelm Siemens and Pierre-Émile Martin and others, this process melted iron and scrap in a shallow furnace heated by regenerative preheating of gas and air. It was slower than Bessemer’s but easier to control and could use scrap steel. It became dominant in much of the world for many decades.
- The basic process (Thomas-Gilchrist). In the late 1870s, Sidney Gilchrist Thomas and his cousin Percy Gilchrist found that lining the furnace with a basic material, such as limestone or dolomite, drew phosphorus into the slag. This allowed phosphorus-rich ores to be used and was especially valuable in Germany and other parts of continental Europe. The slag could also be used as fertilizer, a point worth noting for industrial chemistry.
- Basic oxygen steelmaking. In the early 1950s, steelworks in Austria, at Linz and Donawitz, began blowing pure oxygen instead of air through molten iron. This process, sometimes known as the LD process, was faster and produced cleaner steel, and in the second half of the twentieth century it largely replaced open-hearth furnaces.
- Electric arc furnaces, which melt scrap with electric arcs and are an important route for recycling steel today.
Effects on industry
Cheaper steel changed what could be built. Steel rails lasted much longer than iron ones, which helped railroads carry heavier traffic. Ships increasingly used steel plate. Later, steel frames made tall buildings possible, and steel bars reinforced concrete. Exact price changes varied by product and place, but there is broad agreement that prices fell sharply in the decades after 1856 and that annual output rose enormously.
Cheap steel also affected where industry grew: places with access to ore, coal and transport, such as Pittsburgh, the Ruhr and Sheffield, became major centers.
Limits of the story
It is easy to turn Bessemer into the single hero of steel. In reality, the process depended on a good blast furnace supply of pig iron, Mushet’s deoxidizing practice, better refractory linings and later the basic process. Each step solved a specific problem, and together they produced the steel industry.