Engineering places: Ironbridge
Image credit: Dreamstime
In celebration of IET@150, we look at feats of engineering from around the world.
It’s always a tall order to say exactly where anything ‘began’, but few places can claim a more pivotal role in the Industrial Revolution than Coalbrookdale, on the River Severn in Shropshire. At the end of the last Ice Age, as the glaciers melted, the ancient Lake Lapworth burst its banks, carving a deep scar across local deposits of iron ore, coal, limestone and fireclay, and in the process laid bare all the ingredients needed for an engineering masterpiece – the Iron Bridge.
Ironmaking had been a way of life in Coalbrookdale since the early medieval period, but the arrival of Quaker entrepreneur Abraham Darby I in 1708 led to a step change in technology. In April 1707, he had patented a new technique for casting cooking pots in sand. A year later he leased a derelict blast furnace in the valley and began work.
Iron casting was problematic, mainly because iron could only be produced in small, semi-liquid quantities by firing ore and charcoal together – otherwise the weight of ore crushed the fuel and put out the furnace. Coke from coal was known of but hadn’t often been used, as coal tended to have a high sulphur content that prevented the coking process from working efficiently. However, Darby realised that in Coalbrookdale the coal had a very low sulphur content. He began experimenting with coke blast furnaces that could produce large quantities of fully liquid iron.
The result was revolutionary. His iron was cheaper, better and easier to form into larger and more complex items. By 1715, his works were producing 81 tonnes of iron a year. After Darby’s early death, aged just 38, his company continued, casting its first steam engine pipes in 1718 and its first engine cylinder in 1722. The machine age was dawning.
When Abraham Darby II joined the company in 1728, he proved as innovative as his father. He introduced a steam engine in 1742 to recirculate the water supply, and improved production of bar iron, as well as buying up the rich natural resources of the area to be used in the nascent Industrial Revolution. Despite the family’s strong Quaker beliefs, this included building a cannon-casting facility.
With the death of Abraham Darby II in 1763, aged 51, his partners continued the business, which became the most important foundry site in England, manufacturing specialist ironwork for James Watt among others and casting some of the first iron tramway rails in 1767.
In 1768 the third Abraham Darby joined the company as a partner at the age of 18. It would be under his supervision that the greatest monument to the New Iron Age – the Iron Bridge – would be built on his doorstep.
Although the bridge is always associated with the name Darby, it was not a Darby who designed, or even suggested it. Thomas Farnolls Pritchard was an architect and interior designer who in 1769 had moved to Eyton on Severn to take up farming. Four years later he suggested to his friend and local ironmaster John Wilkinson that a new bridge across the Severn Gorge, between Broseley and Madeley, would improve business between these two increasingly important industrial villages. The gorge was dangerous, prone to flooding and landslides, but perhaps his design for a hugely strong and flexible iron bridge could solve the problem? He had asked the right man, as Wilkinson revelled in the nickname ‘Iron-mad’ and had made his fortune inventing perhaps the world’s first true machine tool – a boring device for cannon barrels and later a similar tool for engine cylinders. He formed a committee to pursue the Iron Bridge project, using Pritchard’s own plans.
It took three drafts before a design was agreed upon but in March 1776 an Act of Parliament was passed for the construction of the bridge, the casting and construction to be supervised by Abraham Darby III. To say the design was novel would be an understatement. Iron structures of this size had never been attempted before and the use of mortise and tenon joints and dovetails show the bridge’s debt to the carpenters, who were having to learn how to build in a new medium. Pritchard died soon after construction began, but over the next two years, the single-span 30-metre bridge rose over the gorge, consuming 385 tonnes of ironwork in 1,700 individual cast components, the largest weighing over 5.5 tonnes.
By December 1780, the bridge was finished and it opened to toll traffic on 1 January 1781, earning a handsome 8 per cent return on investment. As well as being a vital trade link, the bridge soon became a tourist attraction, drawing engineers from across Europe as well as other visitors.
However, the bridge was not without its problems, although these had as much to do with the ever-moving sides of the gorge as with the design. In December 1784, cracks were found in one of the stone land arches, and repairs were carried out several times before the end of the century, as fears that the walls of the gorge were pressing in and ‘squeezing’ the bridge were realised. Despite these problems, the huge strength of the bridge meant it was the only one over the Severn to survive the great floods of 1795.
The Darby foundry also continued to flourish, with Abraham III receiving a gold medal for his work on the bridge, although he too died young, aged only 39, so had little chance to see the effects his family’s work had on the coming industrial age.
The bridge itself served as both a focus for the growing industrial town of Ironbridge and an inspiration to later bridge builders, including Thomas Telford and Isambard Kingdom Brunel. It continued to take road toll traffic for over 150 years, until in 1934 it was designated an Ancient Monument, marking its transition from a part of the nation’s infrastructure to a part of her historical fabric.
Having recently been refurbished, it now stands in its original red-brown colour over the Severn Gorge – the gateway to the Industrial revolution.
Timeline: The Iron Bridge
1709: Abraham Darby, a former brass founder, discovers that coal from Coalbrookdale can be transformed into coke and used to smelt iron. He rebuilds the Coalbrookdale site as Europe’s first successful coke blast furnace.
c. 1754: The works experiments with the application of coke pig iron to the production of bar iron, marking a great expansion in coke ironmaking.
1767: The company produces cast-iron rails for railways.
1773: Shrewsbury architect Thomas Farnolls Pritchard suggests using the latest developments in iron-casting to build the world’s first iron bridge.
1775: A subscription raises funds of £3,000 to £4,000 for the bridge project. Abraham Darby III is appointed treasurer.
1776: Parliament approves the bill to build a single-span bridge.
1777: All iron for the bridge is cast at the Darby Coalbrookdale furnace. Pritchard dies a month after construction begins.
1 January 1781: The bridge opens to traffic. The final cost is around £6,000 – nearly twice the initial estimate.
July 1783: A 32m wall is built to shore up the river’s north bank.
December 1784: The bridge’s great weight prevents damage in serious flooding, but cracks are found in the stone land arch on the south side.
February 1795: The Iron Bridge is the only bridge on the River Severn to survive a severe flood. The medieval bridge at Buildwas is replaced with a cast iron bridge by Thomas Telford, which uses half the iron of the Iron Bridge despite being longer.
1812: Mathematician and bridge specialist Charles Hutton says the construction is “very bad” and will not last long because of cracks in the stone. Further strengthening is carried out.
24 August 1902: A 9m length of parapet collapses into the river.
July 1903: A 250kg section of deck plate falls from the bridge.
1934: The bridge is closed to vehicles and designated an Ancient Monument.
1973: A reinforced concrete strut is inserted to brace the two abutments against continuing movement of the gorge sides.
2017: English Heritage begins a complete renovation of the bridge, completed in 2018.
See more about the IET@150 at theiet.org/about/iet-150-anniversary/
Sign up to the E&T News e-mail to get great stories like this delivered to your inbox every day.