Iron bridge

The Iron Bridge: A History
If the surrounding area were not bristling with museums and heritage signs, you might be forgiven for missing the significance of the Iron Bridge. It spans a very pleasant stretch of the river Severn in the middle of rural Shropshire, a 100ft-long arc of graceful cast iron, painted grey, supporting a 24ft-wide deck with two pavements. You might stop and look at theboard on the tollhouse wall, which tells you what the various charges used to be for crossing it, and then stroll up to the apex of the bridge where, inscribed in the centrepiece on either side in small white numerals, its date of construction would suddenly bring you up short – 1779.

Iron Bridge
© Cognitive Applications/Kaho Lee
Iron was the must-have building material of all the greatVictorian civil engineering projects. Think of Brunel’s suspension bridge in Clifton. Think of the railways. In 1779, though, William Wordsworth was still a boy, HMS Victory had only just entered active service, and the first eccentric wheeled contraptions called velocipedes – forerunners of the bicycle – were making their appearance on the streets of Paris.

Iron had been produced in Shropshiresince the time of Henry VIII. The exact combination of geological circumstances in the region favoured it. As well as iron ore in the soil, there was limestone, which had been used in agricultural contexts for 1,000 years, clod coal and refractory clay. That clay would turn out to be ideal for making furnace bricks, while the coal that would fire the furnaces lay obligingly to hand in seams exposed bythe gorge the river had cut through the land. It could be removed without the need for mining.

Prior to the 1700s, cast iron had been used mainly for tools and utensils. Cooking pots for both business and domestic use, blacksmiths’ tools and cauldrons were made of iron. Its use in the sugar-refining industry helped to make that process less costly and brought down the price of sugar. Iron hadnever been used for structural purposes, though.

An inscribed centrepiece reveals the date of construction
© Cognitive Applications/Kaho Lee
The river Severn had been used as a transport route since the 15th century. Goods on their way to Shrewsbury and Bristol, or going via the Midlands towards London, used the river, which meandered through the deep gorge originally cut by Ice Agemeltwater. Crossing points were essential. A couple of miles from where the Iron Bridge was built there was a wooden bridge, with a more substantial stone one a couple of miles in the other direction. At the precise spot where the Bridge now is, Abraham Darby and his family operated a small ferry.
The Darbys

Iron Bridge and Coalbrookdale industrial chimneys, painted by George Robertson c.1780
©Ironbridge Gore Museum Trust
Around the mid-18th century, the Darbys began thinking big. In 1709, old Abraham Darby (grandfather of the Bridge designer) had been responsible for perfecting the use of coke as a fuel in iron-smelting. The Darby plant encompassed three furnaces with chimneys tall enough to generate greater heat than had ever been known in iron production, making its manufacture herecheaper than could be achieved anywhere else. The volumes produced were therefore greater than anywhere else. Indeed, the family enjoyed a brief monopoly before others caught on to using the same techniques.

Abraham’s grandson, also Abraham, was 23 when he first proposed the idea of building a river bridge from iron. The idea was thought crazy at first. We now know that there is a structuraliron frame inside one of the towers of Salisbury Cathedral, but the tower itself is stone. Nobody had tried to build a load-bearing construction of iron before. It simply wasn’t an architectural material. The safety implications of the proposal were alarming. David de Haan, deputy director of the Ironbridge Gorge Museums Trust and the country’s leading expert on the Bridge, invites us to compare...