I have just returned from the annual meeting of the Association for Industrial Archaeology. This year we were based in Telford, Shropshire. This is close to Coalbrookdale and we could seldom forget the famous cast iron bridge over the river Severn. My personal highlight was visiting a modern foundry to watch the production of complex iron castings using traditional methods. Pouring large volumes of white hot iron is rather exciting to watch even when observed from a safe distance.
In Britain between the Iron Age and the medieval period iron was produced in tall clay structures called bloomeries using charcoal as a fuel and wind, or hand worked bellows, to provide an air draught. Iron oxide ore was oxidised to metallic iron in what was essentially a solid state process. When you broke open the bloomery you obtained a mixture of elemental iron and a slag waste product. Smiths could consolidate this material by hammering, which removed the slag and gave you a bar of ‘wrought iron’. This could then be fashioned into tools or weapons and, with some difficulty, small amounts could be converted into steel to provide the sharp cutting edges of blades or axes.
The blast furnace was almost certainly a Chinese invention and was introduced to Britain from Europe in the late middle ages. The crucial difference from a bloomery was that the iron produced was fully molten and could be tapped off at regular intervals. Once ‘in blast’ the furnace was never allowed to become exhausted but was regularly charged with a mixture of iron ore, charcoal fuel, and crushed limestone which enabled the production of a liquid slag. This slag then floated on the iron and could also be tapped off. To maintain the high temperature the draught had to be provided by more than human muscles; bellows powered by water wheels were usual and much later steam powered blowing engines were developed.
Cast iron, the product of a blast furnace, was liquid so it could be run into sand moulds to produce iron ‘pigs’. Remelted and cleared of impurities it could be poured into more complex moulds to produce castings, a process already familiar from brass or bronze working. Unfortunately cast iron was essentially an alloy of iron with 2-4% carbon which made it brittle when cold. Cast iron could not be rolled or forged. It could be made into supporting columns, firebacks, or pots, but wrought iron was still needed for gates, rails, edged weapons and many other fabrications. Techniques were developed to convert cast iron to wrought iron by removing carbon and if this interests you search for ‘finery furnace’ or ‘puddling furnace’.
The production of charcoal for fuel required the coppicing of huge acreages of woodland. For a century or more there had been experimental use of coal, either alone or combined with charcoal, as a fuel for iron smelting. Coal could be used for black-smithing but it was found that most contained sulphur and phosphorous, which renders the iron produced in a blast furnace brittle and unusable. It was finally appreciated that coal could be converted into coke which provided an acceptable fuel for smelting. The name of Quaker ironmaster Abraham Darby I is associated with this discovery which took place in Coalbrookdale in 1709. The pig iron produced was cheaper, although not as good quality as that from charcoal fuel. The fact that vast quantities of wood were not now required meant that iron ore in new areas, Bradford being one, could be exploited. Darby’s original blast furnace can still be seen at Coalbrookdale museum, although it seems his grandson’s (Abraham Darby III) beautiful iron bridge was cast at a different foundry.
Abraham Darby I’s exploiting of coke could be said to have triggered the British industrial revolution and he lived long enough to witness the consequences of his creation. Technological improvements continued, notably the mass conversion of cast iron into steel by the Bessemer process. Low carbon (‘mild’) steel has effectively replaced wrought iron and the focus of the whole iron production industry has not only moved out of Coalbrookdale but almost entirely out of the Britain. But liquid cast iron being poured is still a beautiful, as well as an historical, sight.