The history of the iron industry is an integral part of the story of our iron bridges. Their historical significance lies in their being made possible by a great leap forward in technology that occurred in 1709 at the foundry of Abraham Darby, a Quaker iron master in the Severn Valley of Shropshire.
“ … the day in 1709 when the first Abraham Darby actually smelted iron ore with coal on a commercial scale was a day of revolution, not simply in metallurgy. It determined the subsequent history of Britain, and … of the world.” (Engineering in History, Kirby et al p. 191)
The consequences of this break-through in one of mankind’s oldest industries were eventually to launch our modern era. Subsequent innovations in the iron trade during the intervening 70 years resulted in Abraham Darby’s namesake grandson building the world’s first iron bridge at nearby Coalbrookdale in 1779.
Although iron is one of the most common elements on earth, it is technically difficult to process into a useful state. The impurities must first be removed and its molecular structure altered in a labor-intensive smelting and refining process at extremely high temperature. Subsequently, it must be reheated and then either fabricated by hand at the forge into wrought iron objects or as molten iron poured into sand molds to make cast iron products in the foundry. Each method requires different but specific qualities in the iron itself.
The art of metallurgy among the Indo-European peoples began in the third to second millennium BC, known as the Bronze Age. Peoples with a proficiency in the mining and working of metal migrated from Asia Minor through the Balkans and into Europe. The first extensive use of iron for weapons and horse gear was brought to the Trans Alpine region by a Celtic people known as the Urnfielders about 1000 BC. Their successors, known as the Hallstatt culture brought their own iron economy to Britain in the 8th century BC. They were over-run by successive waves of more skilled iron working Celtic tribes from the Marne and Middle Rhine (the LaTene people) and from France (the Belgae) who were well settled in Britain when Caesar landed in 43 AD.
The Iron Age (1200 BC to Roman times) metallurgist was not able to achieve the temperature of 1540 degrees Centigrade which is the melting point of iron. He heated the iron ore in his furnace to about 1200 C. at which most of the impurities melt and run off as slag. He would then hand forge the resulting solid iron “bloom” by repeated hammering and re-heating, thus mechanically removing most of the residual slag and forming the iron to the desired shape.
Historically, iron was always associated with the making of weapons. The Romans, who were skilled in metallurgy of all kinds, left no written record of their technical methods which evidently were passed orally from generation to generation of craftsmen. (Engineering in History, Kirby et al p. 91) The only use they made of iron in construction was as cleats and pins to hold stone blocks in place. The Greeks had concealed wrought iron bars to bring “the greater resistance of iron under tensile stress to the aid” of their stone beams. (Engineering in History, Kirby et al, p. 46)
Throughout history until Medieval times, ironstone nodules were the main source of iron. Either bog iron or calcinated limestone was added and the whole was roasted on an open hearth with charcoal so that the carbon dioxide was released. The result would be haematite (iron oxide) ore which can be easily smelted.
This process of smelting iron and converting it into a useful material changed very little for several thousand years until in the Middle Ages the Cistercian monks, who were famous for their metallurgical skills, developed the blast furnace whose water powered bellows could achieve temperatures high enough to greatly improve the smelting process. The first British blast furnace was built in the Weald of Sussex in 1491 and during the reign of the Tudors, this region was the primary iron manufacturing area in England.
The first step in the Cistercian smelting process began by annealing the ore on open ground in a heap of burning charcoal for three days as always. The blast furnace had been fired with charcoal for a week to get it to the requisite temperature. The annealed ore, chunks of limestone or marl, and more charcoal were fed into the top of the furnace and burned for 14 days while a water wheeldriven bellows fanned the fire from below maintaining the intense heat. The chemical reactions between the impurities in the iron, the oxygen in the air and the calciferous limestone took place throughout the furnace as the whole mass slowly sank.
The end product was molten metal with some floating solid impurities or slag which flowed from the bottom of the furnace and the gaseous impurities which escaped through the flue. The molten iron with the slag skimmed off was run into sand molds to make uniform chunks of cooling metal called Pig Iron, so named because the smaller sand molds attached to the large ones by troughs resemble piglets nursing a sow. As soon as the iron had blackened on top but was still viscous, the sow and pigs were broken free.
Pig Iron is very brittle due to its high carbon and silicon content. It must be further refined in a “Finery Forge” where it is repeatedly heated to a pasty consistency and then hammered to bring it to a malleable condition called Bar Iron.
Bar Iron was then either reheated and cast into sand molds of the final object in a foundry. Or it was fabricated into wrought iron objects in a forge by repeated heating and hammering. The forge had always been the primary source of iron products. Cast iron objects from the foundry were too brittle for most applications. Darby’s great break-through of 1779 would reverse this situation by the end of the 18th century. The potential for mass production by the cast iron process was enormous and would play a crucial role in the explosive growth of the British iron industry.
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TECHNICAL NOTE: Some technical facts might be of interest and of help to a layman’s understanding of the complexities involved. Iron ore occurs in three basic forms all over the world: as clay-ironstone modules (siderite, iron carbonate), as inter-basaltic laterite iron ore or as bog iron ore (limonite, hydrated iron oxide). The differentiation between the types of iron – cast, wrought and steel - depends basically on the carbon content after refinement. Pig or cast iron contains 2.5 > 4% carbon. Wrought iron has less than 0.1% carbon but also has 1>2% slag. Steel lies in between as a solid solution of iron and 1.7 +/- % carbon. Steel was known very early in the history of iron making but was very labor-intensive to make out of wrought iron by constant re-heating and hammering to create what was called ‘blister steel’. In 1740 Benjamin Huntsman invented the crucible process of creating blister steel but again in such small quantities that it was not practical for construction purposes although it was extremely hard and had more tensile strength than iron. It was prized for sword blades or small parts for instruments where its superiority over other metals for that particular application justified its very high cost. Outside of the scope of this story, Henry Bessemer (1813-1898) developed a process in 1856 for making steel in large quantities at a reasonable cost which meant that steel would eventually become the metal of choice for large construction projects such as the railroads, bridges, and sky scrapers built in the second half of the 19th century.
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