Ironbridge is a postcard perfect, chocolate box village on the outskirts of Telford. This Black Country setting, between Birmingham and the Welsh border, gives it a scenic and traditional English atmosphere. It lies on the edge of the Wrekin, dated to the Precambrian, among the oldest rock formations in England; and sits within a wooded gorge carved by the River Severn as it travels on its way to Bristol. Crossing this river is an iron bridge, a magnificent blue painted arc of metal that peaks in the middle and is massively overengineered. This was cast between 1779 and 1781, and it is from this that the town has gained its name. The roadsides are paved with a pale green-blue glassy material that is the slag from the bridges construction. This gorge was designated a UNESCO World Heritage site in 1986, due to the virtue that Ironbridge changed the world.
The Industrial Revolution was already gaining pace by 1779, Britain’s first modern canal having been constructed in 1761, and Britain's Agricultural Revolution had been taking place throughout the 18th century. The exodus from the villages to the towns was expanding and Britain was becoming a factory driven society. However, a pivot of the 19th century was iron, and it was from the experiments of three generations of the Abraham Darby family (Black Country quakers) with furnaces in Coalbrookdale that it became the material with which Brunel, Trevithick et al built the Victorian world. Darby and his descendants endeavored to create this technology, and when this was complete, they worked with Thomas Pritchard, to design and cast this magnificent structure. The engineering potential of iron was obvious. It is ironic that almost immediately after losing their most significant colonies, the British went on to develop the technology that would hand them The Empire.
From the beginning of the Iron Age, iron had been produced by the Bloomery Process. This involves smelting iron oxides (typically haematite and iron hydroxide bog ores) in a ceramic furnace filled with charcoal. Bellows are used to input air via tuyeres, causing the charcoal to burn to carbon monoxide. This produces reducing conditions in the furnace that convert the iron oxides to iron metal and carbon dioxide. Waste silicates preserved as gangue in the ores are turned to liquid slag which is typically released (tapped) from the furnace through an arch at the bottom. Variations in this technology had optimised it for different ores, varying abundances of charcoal and changes in the scale of the yield; In fact there is a stunning variation in furnace morphologies, but the basic technology had been the same for almost two millennia.
This technology was fundamentally solid state, producing a liquid slag but a solid metal bloom, and the metal produced usually includes trapped slag inclusions, weakening it. This leads to a need for smithing to produce usable wrought iron and is therefore labour intensive. The iron produced typically has a very low carbon content, and needs to be reprocessed to produce steel (such as by crucibles in Damascus Steel or Benjamin Huntsman’s technology).
The Blast Furnace was first developed in China in the 5th century BC. This technology involves using a larger amount of air to blow the furnace to a higher temperature, resulting in liquid iron. This liquid iron can be poured into molds producing pig iron. The first Western example of this technology was developed in Sweden in the 14th Century. These technologies both relied on charcoal as the fuel source, and due to the high quality of bloomery iron available at the time did not offer a significant improvement. These were still cottage industries performed on a small scale, much like bloomery smelting.
The pig iron produced has a very high carbon content, a result of the increased temperature and liquid state producing more reducing conditions. The resulting metal is brittle and unsmithable, but because it is liquid any slag inclusions will float out by buoyancy. It is notable that blast furnace slag is much glassier than bloomery slag; it cools faster from a higher temperature, preventing crystal formation.
The requirement for charcoal significantly limited iron manufacturing, leading to coppicing, and significant deforestation of Europe; woodland is too sensitive a resource to use as the cornerstone fuel of industry.
Abraham Darby I developed a new variant and larger structure of furnace. This was fuelled, by virtue of using a lower sulphur content coke (a form of baked coal), that was becoming mass excavated in Wales and Staffordshire. He developed large scale furnaces that produced a huge yield, producing pig iron that could then be poured. By a process of re-smelting this material cast iron could be produced, or reprocessed to produce steel.
It was cast iron that resulted in a revolution in iron production. With cast iron, objects could be manufactured on a large scale. Iron could be poured into set moulds, repeatedly producing the same item. This massively reduced the labour demands, removing the need for smithing in repeat manufacturing processes. Now you could repeatedly cast the same item, producing identical streetlamps, tools or utilities. Further it allowed the development of large items from steam engines to ironclads. At the same time Darby absorbed the lessons of industrialisation, this was a factory iron process with the huge yield that entailed. With this technology developed there was the potential to engineer revolutionary new constructions, The Eiffel Tower, The Clifton Suspension Bridge, The Great Eastern. Abraham Darby III developed Ironbridge, the first of these, outside the furnace his grandfather built, and from this small town came the rumble of the iron revolution that coated the world in iron, coal and smoke.
Once blast furnaces were developed the world changed, the industrial revolution was well underway and Ironbridge became the first modern engineered building. On the outskirts of Birmingham we find Britains least visited World Heritage Site, of far more significance than Canterbury Cathedral.