The Iron Bridge

The Ironbridge Gorge lies within the Coalbrookdale Coalfield, a region richly endowed with minerals, coal, iron ore, limestone, sand, and useful clays. During the reign of Queen Elizabeth I local landowners began to exploit coal on a large scale. After 1600, wooden railways were built to carry coal from the mines to the banks of the Severn. Potters, salt boilers, tobacco pipe makers, lead smelters, glassmakers, blacksmiths, rope makers, coopers and basket makers made their homes along the slopes of the Gorge. Human skills were added to the mineral resources of the district. By 1700 there were several furnaces and forges in the area. All employed water power to work their bellows and used charcoal as their fuel.

In 1708 the blast furnace in Coalbrookdale was leased by Abraham Darby, a Quaker pot founder, who began to make iron there in 1709 using coke as his fuel instead of the customary charcoal. This was a momentous development which ultimately made possible a vast increase in iron production in Britain, a part of that series of dramatic changes which historians have called the Industrial Revolution.

The expansion of the local iron trade began in 1755 with the construction of a blast furnace at Horsehay by Darby’s son, the second Abraham. The Darbys and other ironmasters fostered a remarkable series of innovations in the use of iron. From the 1720s iron cylinders were supplied for steam engines. In 1729 iron wheels for railway wagons were cast at Coalbrookdale. Iron rails were made in 1767. In 1787 John Wilkinson launched on the Severn an iron barge, the first of many local associations with iron ships.

In 1758, 400 vessels were trading between Gloucester and Welshpool, and within fifty years this number had doubled. By the 1750s, six or more ferry crossings operated in as many miles. They were essential for transporting raw materials across the river to the ironworks and other industries in the valley. The only other crossing was the medieval Buildwas Bridge, 3km upstream. The river was often too shallow in summer and in winter was too swift and high, so industry was often at the mercy of the river.

The proposal for a new bridge was inevitable, and Abraham Darby III, the ironmaster, was commissioned to build it. The building of the bridge was partly a public relations exercise, advertising the versatility of cast iron and the skills of Abraham Darby III and his Coalbrookdale Company, and the site chosen was also the most dramatic part of the Gorge. The Bridge was promoted by the 18th century equivalent of a media campaign. The paintings Darby commissioned to advertise it show nothing of the pollution of the Gorge, and well-dressed sightseers admire the Bridge, and a carriage travels across, although the picture was painted before the Bridge was ready to carry traffic.

 In 1773 Thomas Farnolls Pritchard, a Shrewsbury joiner turned architect, wrote to the local ironmaster and entrepreneur John Wilkinson, suggesting an iron bridge across the river. His 1775 design proposed a single arch, avoiding the need to build piers in the river, and thus causing no obstacle to boats. A group of local businessmen commissioned the ironmaster Abraham Darby III, who estimated its cost at £3,200.

In 1776 the Act of Parliament was passed, authorising the Bridge, and shares were issued to raise the money. Wilkinson later transferred his shares to Darby who, as the major shareholder, planned to build the bridge of iron. Work began in November 1777. Pritchard died that same year, never to see the Iron Bridge, but it is known that in 1779 his brother received nearly £40 from Darby as payment for ‘his late brother’s drawings and models’, although the final construction was a modification of Pritchard's original design. Darby’s Bridge was to be the first in the world to use cast iron structurally, and artists, writers, spies and engineers came from all over the world to marvel at ‘that most incomparable piece of architecture’.                                        

384 tonnes of iron would have been needed to building the Bridge, which would have taken three months continuous production in one of Darby’s furnaces. Remarkably, where the iron members for the Bridge were cast is not known for certain, but it is generally thought that it was cast at Darby’s furnace (1.6 miles away from the Bridge’s location), but as, at that time, the whole Gorge was called Coalbrookdale and only later became Ironbridge, they could have been cast at Horsehay (about 3miles away) or Bedlam Furnace which was only 500 yards downstream from the site and on the riverside. In 1801 one reverend gentleman learned that iron for the Iron Bridge had been "cast into the proper pieces in open sand upon the spot".

As the members were so large and heavy, a closer location is favoured as it would have been extremely difficult to transport large loads over unmade surfaces by horsepower alone. Whatever the case it must have been a tremendous feat to cast, transport and raise these enormous structures.

There is only one known image of the Bridge under construction, a painting by a Swedish artist Elias Martin. It shows a simple scaffold frame in use, with the ironwork virtually free-standing. When the Bridge was originally built in 1779, it was erected with the abutments in the initial stages. The structure was assembled with the use of many tried and tested ‘timber’ jointing techniques, such as dovetails, wedges and mortise & tennons which may account for the Bridge’s resistance to geological pressure because of the "give" in such joints. Many people have speculated and theorised over the years as to how the Bridge was actually constructed. In 2000 BBC Timewatch came to the Museum and erected a half-scale model of the Iron Bridge over the canal which runs through Blists Hill Victorian Town.

They used the same methods as indicated in Elias Martin’s painting and successfully installed three ribs. During this exercise, new discoveries were made which gave a wider picture as to how the engineers of 1779 would have managed such a construction. The final cost of building the Bridge was £6,000, but if Abraham Darby had wanted to build such a bridge today, he would have to raise in the region of £1.5 million.