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29 October 2014

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You are in: Shropshire > History > Industrial Heritage > Solved - The mystery of Ironbridge



Solved - The mystery of Ironbridge

Ironbridge's status as the first structure built using cast iron is well-documented. But until relatively recently a mystery still surrounded the structure - how the bridge was built.

There are, for example, no eyewitness accounts of the bridge being built - and certainly no plans have survived.

However, recent discoveries and research and experiments have gone a long way to uncovering the mystery, challenging a few assumptions along the way.

Ironbridge in 1780(Ironbridge Gorge Museums Trust)

Ironbridge in 1780, by William Williams

The trail began with the discovery - in Stockholm, of all places - of a small watercolour sketch showing the construction of the bridge in 1779 - the only picture known that shows the bridge being built.

This was followed up by the construction of a half-size model replicating the bridge as it appeared in the watercolour to see if it would work.

The model construction required the services of expert iron founder Nigel Downs, engineers, other technical experts, historic advisers and a squadron of Royal Engineers to do the heavy lifting.

Using the newly re-discovered painting as a guide, they built a half scale replica of the bridge at Blists Hill Victorian Town in order to test the construction theory.

 Ironbridge Gorge Museum Trust carried out a detailed archeological, historical and photographic survey of the original bridge, while English Heritage built a virtual 3-D model of the famous structure.

Their findings were filmed for the BBC's Timewatch documentary series and screened on January 11, 2002.

David De Haan

David De Haan

The extraordinary story of how the mystery was unravelled began in 1997 when David de Haan, deputy director of the Ironbridge Gorge Museum Trust and programme director of the Ironbridge Institute, received news of the 1779 Swedish sketch.

The sketch, reproduced on the BBC Timewatch website, clearly shows the bridge under construction - but not in the way historians had believed it had been built.

River traffic

Up until recently it had been assumed that the bridge had been built from either bank, with the inner supports tilted across the river. This would have allowed river traffic to continue unimpeded during construction.

But the picture clearly showed sections of the bridge being raised from a barge, allowing the spans to be winched into place.

Bedlam furnaces - where bridge pieces were cast

The bridge was cast at Bedlam furnaces

It contradicted everything historians had assumed about the bridge, and it was even considered that the picture could have been a fake as no other existed.

So the half-scale model was built in order to see if the bridge could have been constructed in the way depicted in the watercolour.

The team discovered that it was possible, and by examining the original bridge they discovered some more clues.

Conventional wisdom had it that the bridge parts were cast as a series of pre-fabricated parts at the Coalbrookdale foundry a mile and a half away from the site and then assembled.

Individually cast

But the team found that each part was individually cast to fit at the Bedlam furnace less than 500 yards from the bridge. Interestingly the bridge was fitted together like a jigsaw - that is using methods more usually used to joint wood rather than metal.

Bill Blake, a surveyor who worked on the project, says it seems likely that that the components were individually made and cast to order.

Project engineer Jamie Hillier with bridge section

Engineer Jamie Hillier and bridge cast

"The way it went together as far as we can tell is that they started with an idea, they started with a measured and set out part, fitted it, and then measured the gaps and cast the parts to fit the gaps afterwards."

The team was successfully able to assemble the bridge, going some way to solving the mystery.

Mr de Haan said: "What it proves is that the painting shows a very realistic method of constructing the bridge that could work and was in all probability the method used."

Now only one mystery remains in the Ironbridge story. The Swedish watercolour sketch had apparently been torn from a book which could well have contained similar sketches. It had been drawn by a Swedish artist who lived in London for 12 years and travelled Britain drawing what he saw.

Nobody knows what has happened to the rest of the book, but perhaps the other sketches still exist somewhere.

Jamie Hillier with bridge in construction

The design was based on Swedish painting

And they would surely prove the experts right.

  • Ironbridge is made of no less than 482 main castings, but if you count the railings and deck facings, that figure rises to a staggering 1,736. The bridge weighs 378 tons.

  • The Iron Bridge was designed for Abraham Darby III by Shrewsbury architect Shrewsbury architect Thomas Farnolls Pritchard.

  • Remarkably, nobody was injured during the construction process - a feat almost unheard of even in modern major civil engineering projects.

  • It took only three months to build the bridge during the summer of 1779, but work on the approach roads took another two years.

  • The bridge wasn't welded or bolted together like metal bridges are these days. Instead it was fitted together using a complex system of joints. Each section was custom cast as the engineers went along.

  • It was the end result of three generations of extraordinary achievements by the Darby family. Abraham Darby I developed the use of coke in iron making, his son pioneered the manufacture of cast iron, and his grandson, Abraham Darby III built the bridge.

  • Abraham Darby III was just 29 when the bridge was built. It went way over budget and he died leaving massive debts just ten years later.

  • While may of the Coalbrookdale ironmasters had their portraits painted, none exist of the Darbys because they were Quakers and considered it to be a sign of vanity.

last updated: 30/04/2008 at 13:30
created: 08/04/2005

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