The Open Door Web Site : History : The Industrial Revolution : Iron and Steel Manufacture

The iron industry began in forested areas since trees were necessary to make the fuel, charcoal. It was cheaper to move iron to the iron works than to move the vast amounts of charcoal needed. When ironworking and shipbuilding caused the forests to shrink rapidly, it became necessary to search for an alternative fuel. Iron was made by smelting iron ore or heating the ore up to melting point. The liquid iron was then cast into ingots, called pigs. The pig iron could then either be reheated until it was molten and cast into moulds, or heated and hammered into bars of wrought iron. Of the two, wrought iron was more malleable and less brittle. Attempts had been made to use coal in the smelting process, but the sulphur in the coal produced an iron which was too brittle for use.

In 1709, an ironmaster in Coalbrookdale, Abraham Darby I, succeeded in producing cast iron using coal. He discovered a process whereby coal was first turned into coke. When coal is turned into coke most of the sulphur is lost as sulphurous gases. The coke could then be used in the smelting process to produce iron. Darby kept his discovery a secret and passed it on only to the next generation of Darbys. His son, Abraham Darby II, and his grandson, Abraham Darby III, eventually perfected his method.

Because they kept the secret, the idea of smelting iron using coke did not become widespread until the second half of the 18th century. The Darby's method of producing iron could only be used for cast iron. The search was still on for a better and cheaper method of producing both wrought iron and steel. Until that time, steel had been very expensive to produce and its uses were limited.

It was Henry Cort who, in 1783, discovered an economic method of producing wrought iron. His 'puddling furnace' produced molten iron that could be rolled straight away, while it was still soft, into rails for railways, pipes, or even sheet iron for shipbuilding.

20th Century Iron and Steel Production

Iron is the fourth most common metal in the earth's crust. It makes up 5% of its weight. Iron occurs naturally in a variety of ores in sedimentary rocks:

Ore	Chemical Name
iron pyrites (or fool's gold)	iron II sulphide
heamatite	iron III oxide
limonite or goetite ("bog ore")	hydrated iron oxide (same composition as rust)
magnetite	iron II oxide and iron III oxide
ironstone	iron II carbonate

Iron pyrites, or fool's gold, cannot be used to make iron because of its high sulphur content which makes the iron too brittle.

Although the early iron industry used "bog ore" to obtain iron, ironstone is the most common iron ore and it is extracted from open cast (surface) sites in England, from the River Humber to the River Severn.

To obtain iron from ironstone the ore is first roasted with coal. This process is called sintering. Sintering drives off impurities, such as water, carbon dioxide, sulphur dioxide and arsenic compounds. It leaves a sinter which is mainly granules of magnetite (an oxide of iron).

The magnetite is then reduced in the blast furnace. The sinter is mixed with high grade coke and limestone (calcium carbonate). Hot air at 2 atmospheres pressure, is blasted into the furnace, creating temperatures of up to 1900°C. The iron ore reacts with carbon monoxide in a reduction reaction producing iron and carbon dioxide. Any impurities fuse with the limestone to form a sludge which sinks to the bottom of the furnace. The molten iron, known as pig iron, lies on top of the sludge and can be run off. If the pig iron is re-melted and poured into moulds, it sets as cast iron.

Cast iron is brittle which makes it impractical for some uses. However, it does have a high compression strength and can be heated with air and hammered to produce wrought iron. Hammering cast iron into wrought iron was a long process.

To be converted into steel, the pig iron has to be melted in the presence of oxygen to remove any remaining impurities. Then an alloy of iron, manganese and carbon, is added. The result is a tremendous display of explosive sparks which shoot out of the converter. The carbon converts the iron into steel. High carbon steels are extremely strong and durable.

Iron was first extracted from its ores over 5000 years ago. Until the 18th century, charcoal was used as the reducing agent. By the early 18th century, charcoal was in short supply and had become expensive. It took 200 acres of forest to supply one iron works for one year, and iron was in demand.

Abraham Darby I owned an iron works at Coalbrookdale in Shropshire. His iron works made everything from domestic pots to the huge iron cylinders needed for Newcomen's steam engine. In 1709, when he was 31 years old, Darby developed a new process for smelting iron. This new process made pig iron, and it used coke instead of charcoal. The demand for coke increased, as did the demand for Newcomen's steam engines since they were used to pump water out of coal mines. Although coke was the cheaper option, it took another 50 years before it completely replaced charcoal.

Benjamin Huntsman, a 36 year old clockmaker, made steel, in small quantities, as early as 1740. He did not "discover" steel, however. In 334 B.C., Aristotle had described Damascus steel which had been used to make swords. Huntsman made steel by putting molten iron into earthenware crucibles and then heating it, while excluding air at the same time.

In 1762, Matthew Boulton set up the Soho Manufactory in Birmingham. His factory made iron which was transformed into useful articles, such as buckles and bolts. What made Boulton's factory so special was that it was large and situated near the Midlands' coalfields. Most of the other iron works at that time were small affairs and built close to forests, since they still depended on charcoal.

Henry Cort was from Lancaster in Lancashire. His work for the Navy took him to Plymouth. In 1775, after ten years in the west country, he retired from his naval job and bought a small ironworks just outside the city. His innovations in the iron industry earned him the name "Father of the Iron Trade". Cort invented a new process to make wrought iron. His method was called the "puddling process". He also developed a rolling mill to produce wrought iron bars. He patented his inventions in 1783. In Cort's process, the melted pig iron was heated with air and iron ore. The resulting pasty metal was then hammered to remove some of the impurities (or slag). To make iron bars, the molten metal was passed through grooved rollers. As a result of Cort's method, wrought iron production increased by 400% over the next twenty years. Unfortunately, Cort lost his patent when his business partner was discovered to have financed the project using stolen money. Cort went bankrupt and lived the rest of his life on a small pension.

Henry Bessemer was a self-educated man who came from Hertfordshire in England. In 1856, he developed a "basic oxygen converter" to change pig iron into steel. In 1879, Bessemer received a knighthood and a fellowship in the Royal Society for his contribution to the iron and steel industries. Bessemer's process was only suitable for British iron ore, since the ore did not contain much phosphorous. It was not until 1879, that the more advanced Percy Gilchrist and S.G.Thomas method, which was suitable for phosphoric ores as found in Europe, was adopted by the continental steel makers, such as Alfred Krupp in Germany.

After 1770, iron (and later, steel), replaced wood as the material for making industrial machines and tools. In 1806, the annual production of pig iron had reached 272000 tons, which was a 200% increase over 18 years.