British Iron Bridges

History

Britain before the Roman conquest (43 AD) was covered by virtually impenetrable forest relieved by the occasional heath, wetland or moor. Overland travel was along the ancient tracks of the bare uplands such as the Icknield Way under the ridge of the Chilterns or the track on the North Downs later known as the “Pilgrim’s Way” to Canterbury. By far the most practical way to get around was by water along the coast and up or down the many rivers. The only bridges dating from that period were of the “clapper” type or simple wooden ones.

The Romans began building the first British roads to consolidate their conquest in 43 AD, but they invested in only a few bridges. At key crossings, like Cambridge, a number of roads from different directions converged there, just to take advantage of the reliable bridge crossing. Although a direct route from A to B might have been many miles shorter, convenience was trumped by the certainty of being able to cross the river at the bridge head. Fully half of the north/south Roman roads converged on the wooden bridge at London. Other important Roman bridges were located at Rochester and at Newcastle. (Trevelyan p. 19)

When the Roman Legions left Britain (410 AD), their roads and bridges were abandoned. The stone causeways subsided or were used as quarries. “From driving roads they declined into pack-horse tracks, finally disappearing for the most part to moor and plough-land.” (Trevelyan p. 46) The turmoil of the next 700 years as various indigenous and invading factions vied for control of Britain meant that travelers who valued their goods and their lives avoided overland travel whenever possible and fell back into the habit of going by sea from one coastal town to another or by barge up the navigable rivers. For almost 500 years, no bridges were built except those of the most temporary and rudimentary kind.

During the early Middle Ages, the religious orders took up the challenge of building bridges. The Church, a wealthy international organization with communication needs and widespread commercial enterprises, needed good roads just as much as had the Roman Empire. The Freres Pontifes aided travelers and provided lodging, ferry service, and even bridges. Saint Benezet, an extraordinary engineer, was sanctified for a number of miracles including building the famous bridge at Avignon which was completed in 1185 and was so well designed and constructed that even after over 800 years and epic floods, four of the original 22 arches are still standing. In England, the Cistercian order was the foremost but not the only religious order which, needing access to its many business enterprises and vast properties, included the building of roads and bridges in its charitable activities.

Wealthy merchants of 14th century England would occasionally heed the urging of “Piers Plowman” and finance the building of a local bridge in the same spirit as they would endow an alms house. The Bishop of Durham offered 40 days of indulgence to those who would finance the repairing of Botyton Bridge. (Engineering in History, Kirby et al p. 109)

No one knows [as of when this was written] the exact age of the old wooden London Bridge, but it was standing in 994 when Ethelred defended it against the Danes. In 1176 Peter of Colechurch, “priest and engineer” began work on its stone replacement. After his death, King John appointed the French engineer Brother Isembert as his replacement. This London Bridge was completed in 1209 and was well enough constructed and maintained to survive the next 600 years (despite several fires and reconfigurations of its superstructure) until it was again replaced in 1831. At that time the original Roman black oak pilings were discovered, perfectly preserved by their submersion and with their original wrought iron “shoe” of a type only the Romans could forge.

Even as late as the Tudor dynasty, there were still very few bridges and the Roman roads were in such terrible condition that transportation of those goods that did not go by water could only be carried by pack animals since the roads were impassable to wheeled vehicles in bad weather. Travelers had to go miles out of their way to cross at the only available bridge when floods made a convenient ford unsafe. Whole regions of Britain were virtually without roads at all, such as Scotland, Wales and Cornwall. People and goods there traveled overland by foot or by horse. As always, the preference was to travel by water.

Since the inland cities were dependent on the barge traffic that carried goods up and down the rivers in trans-shipment from the coastal port, they resisted the building of bridges whose piers would be an impediment to the waterways. When high water made the fords unusable, the ferryman would come to the rescue. Since no point in England is more than seventy miles from the coast (Trevelyan p.340), this transportation situation was reasonably satisfactory until the third quarter of the 17th century and there was little demand for new bridges until then.

With the peace that followed the Settlement Act of 1689, which closed the painful chapter of the Civil War, English tradesmen and merchants could look forward to prosperity at last. This Act “led not only to a new and wider liberty than had ever before been known in Britain, but to a renewed vigor and efficiency in the body politic and in the government of the Empire. … From the external weakness that had characterized England in the 17th century the country rose … to the acknowledged leadership of the world, in arms, colonies, and commerce, in political and religious freedom and intellectual vigour.” (Trevelyan p. 472-3) The next two centuries would be a golden age for business of all kinds.

There would be an explosion of road and bridge building throughout the British Isles to meet a new demand for the efficient and safe transportation of raw materials and manufactured goods. Industries that had been rural and local, suddenly would became national and international in scope. The industrial revolution would be born and in its early stages, it was based on iron.

One could make the case that it began with the building of the first substantial iron bridge. This first iron bridge launched a revolution in bridge building; spurred the concept of civil engineering as a distinct discipline; encouraged the development of innovative ways of working iron that led to its use in many new applications which in turn led to the steam engine, the textile factories, the rail roads and even ships being built entirely of iron.

CLAPPER BRIDGES (“Clapper” in Old Anglo Saxon means “heap of stones”)

TARR STEPS CLAPPER BRIDGE Exmoor nr Dulverton, Somerset HEW 863 SS868 321 N53o 5’ 37.7” W3o 48’ 28.1”

This bridge is of Bronze Age origin. The raking slabs upstream and downstream act as shear-waters and serve also to deflect debris up and over the stones during floods. It might well be part of the ancient pathways on the uplands as there is a pack horse track leading down to it from the tops from which you can hike in.

In this case “near Dulverton” means about six miles to the northwest and it isn’t easy to find the extremely narrow road out of Dulverton which turns off Fore Street, one street to the north of and parallel to High Street. From the entrance/exit of the mid-town car park, take the street between them and ahead you will see the tiny sign saying Tarr Steps on the side of a house. When you turn off the country road to the Tarr Steps and cross the first cattle grid, you can park and walk over the hills to the bridge – about a mile or so. Or you can drive in a mile or so and park close by.

POST CLAPPER BRIDGE Dartmoor N50o 35’ 39.1” W3’ 54’ 40.1” This bridge was probably built in the 14th century.

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Coincidently with the period in the mid 1800’s when the British canal system was being superceded by the rail roads, the traditional iron foundries began to produce more steel than iron using the 1856 process invented by Sir Henry Bessemer (1813-1898). The Bessemer process and its later refinements were able now to produce steel in large quantities and at a fraction of the old price. Cast steel is superior to iron in tensile strength. This quality in the steel form of iron meant that bridge builders could now achieve the spectacular bridge spans that we take for granted today. Steel rapidly became the metal of choice for all heavy construction purposes from railroads to ship building to sky scrapers from the second half of the 19th century right up into our time.

The fact that our iron bridges enjoyed such a brief moment of fame as innovators should be a point of pride. It is a good indication of the creative and entrepreneurial energy which was harnessed to create them. Their very success spawned their successors. In the beginning, the Industrial Revolution was based on iron and these audacious and pioneering bridges of their time were the first conspicuous demonstration of iron’s potential.

Our bridges have the further distinction of being the result of one of the first collaborations between empirical experience and scientific discovery as applied to industrial problem-solving. This innovative approach was first put into practice in England in the 18th century and has proved so successful that we can no longer imagine problem solving any other way. This was the true “revolutionary” aspect of The Industrial Revolution. This is the legacy of our bridges.

Now that you appreciate the historical significance of the iron bridges, do take a second look at them from yet another prospective. For me, these old iron bridges still have the edge on modern bridges in several other respects: in their intimacy of scale, in their still airy grace and flair, in their retained sense of audacity for their time. I love their precedence: that they were the first and provide the link with our era and all the previous eras back to the Clapper bridges of the Bronze Age.

It is my hope that you will enjoy seeking them out as much as I have. As you become familiar with them, you will begin to spot them everywhere and to notice their children and grandchildren in more modern bridges. They become familiar landmarks and beloved old friends, still modestly and faithfully fulfilling their original purpose some 230 years later with an aura of patriarchal serenity.

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During the 18th century, even before the Iron Bridge at Coalbrookdale dramatically demonstrated the potential structural uses for iron, those in the iron trade had begun to explore new uses for their improved cast iron that now had the combined advantages of a material with great strength when under compression with now considerably more tensile strength as well. There seemed endless possibilities for its use as a substitute for other softer metals such as brass, copper and bronze as well as a substitute for wood in many applications.

For the first time, since the Iron Age succeeded the Bronze Age about 1200 BC, the military would not be the primary customer of the iron industry. The techniques perfected over centuries to fabricate weapons could now be used to produce domestic products of equal precision and consistency. Cannon must be cast with great skill as any fault or air pocket can cause it to explode in the gunner’s face when fired. The British Board of Ordnance was so stringent in its testing and specifications that the industry had learned to consistently produce weapons of very high quality castings. British guns and cannon were considered the best and most reliable in the world.

A foundry whose precision castings of cannon are able to reliably withstand enormous explosive pressure can also cast high pressure steam engine cylinders which must also safely and reliably contain enormous pressures.

The same kind of cross-over application in the iron industry occurred in ordinance. A critical consideration when making fire arms is to achieve precision in the bore hole which directly affects the accuracy of the fired projectile. The hole left after the casting could be drilled slightly larger with precise tolerance in the easily handled barrels of pistols and long guns than in the awkward very heavy cannon. John Wilkinson (1728-1808) discovered how to achieve the same precision while boring the much more awkward heavy cannon by rotating not the drill but the cannon itself around a fixed bit. This accuracy earned him many lucrative military contracts.

The early versions of the steam engine were not designed for high pressure, so any imprecise tolerances of the cylinder and the piston were not critical. James Watt (1736-1819), frustrated by this common fault in his efforts to build machines that could hold pressure, was directed to Wilkinson and his process for achieving very precise cannon bores (patent 1774). Together, the two men developed a process for boring steam engine cylinders and making parts with a tolerance of incredible accuracy and consistency for that time. Watt and his partner Matthew Boulton (1728-1809), who were designing and installing steam engines of all kinds for many industries in Britain and abroad, specified that all the parts must be supplied by Wilkinson until they set up their own foundry, the Soho Works, at Birmingham, in 1795.

Iron began to be substituted for wood in many different applications: from door lintels to gun carriages to decorative architectural elements to bedsteads. In the textile industry where the endemic dust and lint made fire a real hazard, “fire proof” factories were being built with iron posts and beams within brick or stone walls.

By the middle of the 18th century iron was being used to great advantage in several aspects of transportation. The wooden ways that had facilitated the movement of the heavy ore carts in the mines and in the coal and iron yards were now being replaced with iron rails which reduced the friction even more and lasted much longer. The ore carts were still pulled by ponies or even men and women in the mines and mills, but one far-sighted entrepreneur, Richard Trevithick (1723-1777), was experimenting with making a steam engine that would propel them along those iron rails. Soon the ore carts and their wheels were no longer of wood either, but of wrought, cast, and plate iron. These innovations look forward to the 19th century railroad era which lies outside both the time frame and subject matter of our story.

A large domestic and over-seas market developed toward the end of the 18th century for cast iron pipe to supply water and gas lines within the expanding cities and towns all over Europe. For a long time the British iron trade had this market to themselves due to the high reputation of their products.

Continuous improvements in the smelting and casting of iron meant that now delicate and precise iron gears and parts were being substituted for brass and bronze in all kinds of mechanical products from clocks to fine machine tools. Iron parts were cheaper, wore much better, and were just as precise. As the textile and other industries became more mechanized, their machinery and its parts were made of iron for the same reason.

As a structural material for building the boxes that carried the water and the barge boats in Thomas Telford’s new canal aqueducts to all manner of new kinds of bridge – arched, truss, cantilevered, suspension, tubular – iron was now accepted as the strongest, most economical and practical material. It was the material of choice in many structural applications for the next fifty years.

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The architect Thomas Pritchard (1723-1777) is credited with being the first person to design a bridge that would be built entirely of iron. In 1773 he discussed this novel idea with two of his friends in the iron business, John Wilkinson (1728-1808) and Abraham Darby III (1723-1777). The iron masters realized immediately that this project would provide the perfect opportunity to demonstrate the superior qualities of their new cast iron and its potential as a structural material of great strength, easy assembly, and modest cost. The site, across the Severn Gorge, was close to their foundries in Coalbrookdale.

Prichard's design was for a single arch bridge spanning some 120 feet that would be just as strong and durable as one built of stone. Equally important on this very narrow and busy stretch of the River Severn, the bridge would require no midstream piers. It would be built at a height that allowed tall masted ships to pass under it even at flood stage. It was to be cast in many pieces that would fit together like a puzzle with joints that locked in place for easy assembly by adopting many of the joinery principles of wood construction.

The site for the proposed bridge was a gorge so steep that ferry service was not possible for wheeled vehicles. The lack of a bridge in this developing area was a great inconvenience to the increasing road traffic which had to make long detours to cross the Severn elsewhere. The Commissioners were from the local gentry and clergy of Bosley and of Madeley Wood on the opposing banks. The Subscribers guaranteed the estimated cost of upwards of 3000 pounds sterling. They chose Thomas Pritchard's design, and they commissioned Abraham Darby III to build the bridge. The Commissioners' petition received the Royal Assent on the 25th of March 1776. After some initial disagreements over whether the bridge should indeed be built of iron and the accuracy of the estimated cost of construction as well as the legal interpretation of the wording of the Act, the Commissioners overcame their anxieties and began construction in November of 1777.

The approach roads were built immediately after the acquisition and demolition of some structures on the bankside. The stone abutments which were to receive the base of each arch were built next. Meanwhile the ribs were being cast in open sand molds, most likely at Darby's Upper Furnace complex at Coalbrookdale. The bridge consisted of more than 800 separate castings. Each rib of the arch was 70 feet long and weighed over five tons. A total of 378 tons of iron were used over all. This was an extraordinary amount given the capacity of the iron foundries of that time.

During the summer of 1779 the parts were brought to the site on barges. A wood scaffolding was erected as shown in a contemporary water color sketch by Elias Martin ".. which shows an elegant wooden framework, rather like a set of goal posts with the cross-bar supported by diagonal struts, which was used to raise the half-ribs from the deck of a vessel on the Severn River below.

"It appears that the base plates were fixed onto the masonry platforms (abutments) on either side of the river and the uprights inserted into the base plates. One of the main half-ribs was then inserted into the base plate, and then the other; the two halves being fixed with vertical and horizontal pins in a crown piece, a complex component that could only have been cast with great skill ...

"Once the main ribs were erected, a stable platform would have been completed. ... (This) may have been a series of (wooden) decks, erected on the main ribs from which other components, the middle and back ribs, the cross stays and braces, the deck bearers and the ornamental circles and ogees, could have been inserted. Most pieces were joined by dovetails, wedges, or shouldered joints according to wood-working practice" (Attributed to Shelley White of the Ironbridge Archaeological Unit: "The Iron Bridge" by Cossons and Trinder 2002 p. 21-28)

The bridge was essentially erected in three months without any serious accidents and without interrupting barge traffic on the Severn. The scaffolding was removed in November 1779. During 1780 stone abutments were built on each end beyond the bridge iron-work to support the approach roads to the bridge. The Iron Bridge was officially opened to traffic on New Year's Day, 1781.

The architect Thomas Prichard had died in 1777 and Abraham Darby III was almost ruined by the cost over-runs that he had guaranteed to cover. Yet from the very beginning the bridge caused a sensation and became a major tourist attraction. It more than fulfilled the hopes and the expectations of its proponents. Its success demonstrated definitively that iron was a practical, economical, and beautiful structural material.

Now that we have successfully circumnavigated such subjects as the way iron is made and how to build bridges that don't get washed away, it is time to tuck away all this information where you can easily retrieve it because we are going to take our final loop, this time around the subject of the building of new roads and bridges to meet the transportation needs of the still very youthful but vigorous Industrial Revolution.

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During the same period that these technical innovations were transforming the iron industry during the 18th century, England was experiencing changes in agriculture, in finance and banking, in law, in politics, even in peoples’ way of thinking about progress and change. All these changes would converge by the end of the century to enable the revitalized iron industry to grow exponentially. Together, these changes added up to more than the sum of their parts because in the fortuna timing of their confluence they made possible the Industrial Revolution.

Capital was needed to finance the expansion of the new iron industry. It was now feasible as well as practical to concentrate all the various processes on one site and to vertically integrate the business. The iron masters also saw an advantage in owning their own coal mines and to controlling the distribution system with their own barges and ships. Machinery such as steam engines, larger furnaces, workmen’s housing, and transportation systems were now a requisite.

It was not the role of the existing banking system to finance private capital investment. The Bank of England’s historic role in the 1700’s continued to be simply the handling of large sums for governmental purposes, facilitating loans to and from other countries, and meeting the foreign exchange needs of international trade. The Bank of England was, indeed, “The Bank of London” and had no provincial branches and no mechanism for making loans to private enterprises or even to provide them specie in small denominations for payroll purposes. As yet there was no retail banking as we know it and there was no “exchange” to provide easy liquidity for debt instruments.

To meet their working capital needs on a day-to-day basis, the new industrial businessmen improvised by issuing their own paper money or “bills of exchange” and resorted to rotating paydays when coinage was short or even minting their own company coins. Basically, they created their own in-house banks and sold their bills of exchange at discount through City banks having a relationship with the Bank of England. (A number of today’s banking institutions, Lloyds and Barclays for example, began as such in-house iron company banks).

For financing their capital needs they improvised several methods. They would self-finance improvements through the retention of earnings, paying the partners interest on their share of the deferred distribution. They borrowed from each other and later from merchants in other businesses who were sophisticated in international lending such as the tea merchants who financed the iron industry’s growth in South Wales or the tobacco merchants who financed the industry in the Clyde Valley. They formed joint venture companies and sold their stock into the new and growing market for debt instruments.

Parliament was financing the Continental and American wars by issuing government-backed bonds called Consols. These proved to very popular investments both domestically and abroad. In addition, new investment opportunities such as joint stock companies and Turn Pike Trusts were being created. The increasingly prosperous and growing middle class, having no retail banks to receive their savings, was learning to put their money to work earning interest from these debt instruments. This new source of private capital was now available to the enterprising young business ventures.

In 1760 there had been no market for buying and selling debt instruments, but in 1803 the stock exchange was opened and by 1830 market liquidity was achieved for debt instruments of all kinds and the capital markets were thriving.

Private retail banking on the local level was becoming available in the early 1800’s, but because these banks were prohibited from selling stock to raise capital for reserves, and because there was no limited liability available to private banks whose partners were personally liable for all funds received, they often failed when faced with a liquidity crisis. The surviving banks, however, received the savings from and made loans to the prospering middle class and small businesses.

The infusion of all this new money looking for investment opportunities as well as the new fluidity of capital through the buying and selling of consols and shares in joint venture companies meant that interest rates continued to fall. By 1757 Consols were paying 3%. Interest rates fluctuated but remained relatively low through out the 1700’s. It was an ideal financial situation for the entrepreneurial businessman.

Other barriers to new ways of doing business were falling as well. From the mid 18th century, emerging business concerns in the iron and mining and textile industries were avoiding the strict medieval regulation of most trades by guilds and government restrictions simply by locating their new enterprises in places where they were not in effect such as Lancaster, Yorkshire, Wales and Scotland, as well as in unincorporated towns such as Birmingham and Manchester. The medieval prohibitions on workers leaving their own parish had rarely been enforced since the Civil War in the mid 17th century and by the 18th century the resulting mobility of the labor force made industrial expansion possible.

There was a legal incentive to innovation that had actually been in place for some time but was only now being used to full advantage. Chief Justice Coke’s rulings during the Tudor dynasty had elevated property rights and suppressed vested interests and privileged monopolies. The Statute of Monopolies of 1624 also contained provisions for a patent law. Inventors and entrepreneurs could have exclusive rights to their inventions for a set period of time, although piracy continued and patent lawsuits were common. Parliament, as an incentive to act in the public interest, offered awards for those who were willing to make their inventions available to all.

All these factors taken together helped make the Industrial Revolution possible. But the greatest contribution came from the men who were leading these new ventures and the new ways of thinking who had seen the possibilities and seized the opportunity. It is they that actually made it happen.

Although a few of the landed gentry were involved, for the most part these were ordinary men who came from the traditional trades of wheelwright, stone mason, instrument maker, pharmacist but who, self-educated in the new scientific ventures, became proficient in many “new” specialties known today as civil engineer, chemist, mathematician, and physicist. There were several reasons why this unprecedented collaboration came about between the pragmatic tradesmen and the university-educated beneficiaries of the “natural philosophy” experiments and teachings of Sir Francis Bacon (1561-1626) and of Sir Isaac Newton (1643-1727).

English society, exhausted by the religious wars of the previous two centuries, was willing, as the price of peace, to practice the religious tolerance now allowed under the Settlement Act of 1689. Many sects who dissented from the established Church of England such as the Puritans, the Quakers, the Methodists, the Presbyterians, Baptists and Congregationalists became leaders in the new industries and financial institutions. The Quaker families of Darby, Reynolds, Wilkinson, Lloyd, and Huntsman dominated the early iron industry.

T.S. Ashton observes: “Many explanations have been offered for this close association between industry and Dissent. It has been suggested that those who sought out new forms of worship would also naturally strike out new paths in secular fields. It has been argued that there is an intimate connection between the tenets peculiar to Nonconformity and the rules of conduct that lead to success in business. And it has been asserted that the exclusion of Dissenters from the universities, and from office in government and administration, forced many to seek an outlet for their abilities in industry and trade. There may be something in each of these contentions, but a simpler explanation lies in the fact that broadly speaking, the Nonconformists constituted the better educated section of the middle classes.” (The Industrial Revolution 1760-1830 TS Ashton p. 14-15)

Ashton then goes on to note the contribution of men educated in the dissenting primary academies and the University of Edinburgh and the University of Glasgow in Presbyterian Scotland who came south to England to found industries and new technologies such as James Watt, John Roebuck, Joseph Priestly, Matthew Boulton, Thomas Telford, John McAdam to name only the ones we shall meet.

Dissenting academies were established by nonconformist sects in Bristol, Manchester, Northampton and elsewhere which were open to students of all creeds. Their curriculum included not only religion but mathematics, history, geography and “natural philosophy” or science. They were incubators of scientific thought.

The spirit of discovery and invention was furthered by associations such as the Royal Society, the Society of Arts and the Lunar Society where like-minded men of all occupations and social classes came together to share scientific discoveries, improved methods of production, and new ideas in general. There were all kinds of opportunities for an enterprising man to educate himself in the new sciences and discoveries through free lectures. A best seller for 100 years after its publication in 1751 was ”The Tutor’s Assistant”, a mathematics textbook. Journals were available that disseminated the latest theories of special interest to those in any number of trades and agricultural practices. One could learn as much or more in the coffee houses of London, where many of these associations met, as at any of the Universities.

Implicit in this search for scientific understanding was its practical application. As A.E. Musson says of Dr. William Lewis (1708-1781) F.R.S. “His work marks the arrival of the professional chemist, applying scientific knowledge systematically to industrial problems.” (Science and Technology in the Industrial Revolution, Musson & Robinson p. 53) This new scientist was in partnership with the tradesman and craftsman, each hoping to learn something from the other. Since this was an unprecedented situation, skilled craftsmen from all over Europe came to London to learn and to work. A very modern spirit of confidence that anything was possible and that man could understand and control Nature was abroad in England.

Another phenomenon of the 18th century that was to benefit the growth of the young iron industry and all the other new enterprises such as the textile industry and the factory system in general was the exponential growth of the British population. This was not the result of a rise in the birth rate, which actually declined slightly. It was partly the result of a much lower death rate from disease as the importance of sanitation was better understood, but the greatest contributing factor was a much better diet for the population as a whole.

This was a direct result of the agricultural experiments and reforms based on scientific discoveries that had begun in the previous century. The ancient and inefficient practice of subsistence farming in the “open fields” where each family had a narrow strip of his own land among many other family strips, had gradually over the last two centuries been succeeded by the “enclosure farming” of large hedge-row fenced fields used as pasturage for cattle and sheep or the growing of grain crops. The waste or common lands were also being enclosed and brought into production for the first time resulting in an increase of acreage under cultivation. The efficiency of farming in large fields when added to the benefits of land drainage, of crop rotation, and of applying fertilizers were all resulting in a much higher yield.

The new practice of growing root crops like turnips as winter feed for livestock meant that they no longer had to be slaughtered in the fall for the lack of winter forage. As a result meat was now available to everyone year around. A well nourished population with a longer life expectancy meant that a larger work force no longer tied to agriculture was available to fill the coming demand for labor in many kinds of work. In fact, T.S. Ashton argues that it was because the Industrial Revolution occurred there first that Britain was able to successfully absorb this population growth and therefore was able to avoid the political and economic upheavals which were soon to be experienced by other nations whose populations were also increasing, but whose economies were not.

The nascent iron industry had in place just what it needed for expansion. Capital was available in various instruments at a low interest rate. The regulations and government restrictions on trade and business were being avoided by locating away from where they were in effect. Property rights and patents protected innovation for long enough to allow investors to recoup their investment. A scientifically educated and increasingly prosperous middle class was available to contribute to as well as to invest in that innovation. A century of improvements in the refining of iron itself, at ever lower cost, meant that the British iron industry was very competitive in world trade. The attitude of English society in general was favorable to progress, to expanding trade, to new ideas and discoveries. There was nothing to prevent expansion of the iron industry except the need to find new applications for the improved product.

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By the end of the 17th century, the growing British iron industry was facing several crises which threatened its prosperity. The resolution with which the industry faced these challenges sparked the innovations that took place in the 18th century, completely transforming the industry itself and launching the Industrial Revolution.

England had iron ore in abundance and at shallow depths. However, because it was not yet technically possible to remove all impurities, English Bar Iron was inferior to that made from certain ore bodies outside of Britain which had fewer impurities. In the early 1700s, 2/3 of the Bar Iron used by the British iron industry was imported mainly from Sweden but also from Russia and America. Political events periodically disrupted these sources and England frequently found her foreign policy affected by her dependence on these sources.

Smelting and refining the ore required enormous amounts of charcoal. Early on this was not a problem as England was heavily forested, but by the 17th century the iron industry and the Navy were in competition for the remaining woodlands. As charcoal shortages became acute and the price rose alarmingly.

A reliable source of swiftly running water was essential for the power to run the bellows, the hammers and the slitting machines. In times of drought the works had to pump water back up to reservoirs to be used again. When the rivers froze, the works had to close down.

Over the previous century, the British iron industry had initially responded to these challenges by specialization and by dispersal. Small enterprises conducting one aspect of the process were located near their most needed resource and in turn supplied their product to another company specializing in the next step in fabrication. For instance, the wooded Forest of Dean area specialized in charcoal smelting and supplied Pig Iron to forges and foundries. Black smiths would locate near a source of the coal which they used to fire their forge and would supply the immediate neighborhood with Bar Iron slit into small rods suitable for home industries such as nail making.

However, the most urgent problem by far was the industry’s voracious appetite for wood to make the charcoal which was essential to the smelting and refining process known at that time. Parliament, concerned about the rapid disappearance of a resource vital to the Navy, passed several Acts preventing the cutting of woodlands to make charcoal. The rising price of charcoal meant the price of British iron products was not competitive with imported iron wares. It was imperative that the iron industry find a way to use an alternative fuel such as peat or coal, both of which were plentiful.

Peat is very flammable and burns quickly and dangerously hot. The most promising fuel seemed to be coke (which is coal burned in an oxygen deprived atmosphere to concentrate its carbon), but coke contributed its own impurities - such as sulphur - which produced inferior Pig Iron. One could not use brittle coke Pig Iron to cast such things as plowshares and anchors which must be able to weather hard knocks. Worse, it seemed impossible to turn coke Pig Iron into the malleable Bar Iron needed to make wrought iron at the forge.

Abraham Darby of Shropshire approached the problem by concentrating on finding a better way of making coke in order to first eliminate the impurities in the fuel itself. Using the “sweet” clod coal of the district, he finally succeeded in making commercially acceptable Pig Iron using coke in his blast furnace at Coalbrookdale in 1709. He also built a larger blast furnace which achieved a higher temperature over a longer period of time thereby achieving a more complete fusion of the carbon coke and the iron. This produced a purer, more liquid metal that could fill all the tiny cavities of a mold and resulted in a superior casting.

By the 1750’s new coke furnaces were being put into blast throughout the country. Higher temperatures were being achieved by greatly accelerating the blast of air fanning the fires at first with Smearton’s blowing cylinders and later in the 1760’s by James Watt’s even more successful double acting steam engine to accomplish a more complete fusion. Coke Pig Iron was now good enough for all general purposes either as wrought or cast iron. However, charcoal was still used in the last step to create Bar Iron.

In 1784 Henry Cort introduced his “puddling” process which involved heating the Pig Iron in a hot air Reverberatory Furnace (which kept the iron separate from and unpolluted by the coke) that could maintain the molten iron at welding temperature while it was being stirred with paddles before being passed through rollers. The combination of the high temperatures and the physical manipulation of the iron eliminated all the dross and altered the molecular structure without the use of charcoal. The resulting iron was superior to Swedish Bar Iron and suitable for all purposes except for making steel. It was a faster process and cheaper than charcoal Bar Iron by half since it used only coke. This was a break-through of huge significance.

“… (Cort’s) discovery was one of the outstanding events in the history of technology”. (The Industrial Revolution, T.S.Ashton p. 55)

Ashton lists the beneficial consequences: it brought the forge and the foundry sides of the industry together in one vertically integrated business; it liberated Britain from her dependence on foreign Bar Iron; it led to an expeditious growth of the British iron industry; ” …and there was hardly an activity – from agriculture to ship-building, from engineering to weaving – that did not experience the animating effects of cheap iron”. (Ibid p. 54-55)

By the 1790’s the British iron industry had solved its power problems by employing Watt’s steam engines to neutralize the fickleness of water power; it had solved the critical charcoal shortage by developing ways to use plentiful coke as fuel; it had eliminated the need to import Bar Iron by developing a method to refine British ores to an even superior level of quality and it had done all this while greatly lowering the manufacturing costs. The industry was poised for an explosion of innovation and expansion. The Darby and Wilkinson clans and their industry colleagues were of the mind that there was nothing that could not now be made of iron.

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The history of the iron industry is an integral part of the story of our iron bridges. Their historical significance lies in their being made possible by a great leap forward in technology that occurred in 1709 at the foundry of Abraham Darby, a Quaker iron master in the Severn Valley of Shropshire.

“ … the day in 1709 when the first Abraham Darby actually smelted iron ore with coal on a commercial scale was a day of revolution, not simply in metallurgy. It determined the subsequent history of Britain, and … of the world.” (Engineering in History, Kirby et al p. 191)

The consequences of this break-through in one of mankind’s oldest industries were eventually to launch our modern era. Subsequent innovations in the iron trade during the intervening 70 years resulted in Abraham Darby’s namesake grandson building the world’s first iron bridge at nearby Coalbrookdale in 1779.

Although iron is one of the most common elements on earth, it is technically difficult to process into a useful state. The impurities must first be removed and its molecular structure altered in a labor-intensive smelting and refining process at extremely high temperature. Subsequently, it must be reheated and then either fabricated by hand at the forge into wrought iron objects or as molten iron poured into sand molds to make cast iron products in the foundry. Each method requires different but specific qualities in the iron itself.

The art of metallurgy among the Indo-European peoples began in the third to second millennium BC, known as the Bronze Age. Peoples with a proficiency in the mining and working of metal migrated from Asia Minor through the Balkans and into Europe. The first extensive use of iron for weapons and horse gear was brought to the Trans Alpine region by a Celtic people known as the Urnfielders about 1000 BC. Their successors, known as the Hallstatt culture brought their own iron economy to Britain in the 8th century BC. They were over-run by successive waves of more skilled iron working Celtic tribes from the Marne and Middle Rhine (the LaTene people) and from France (the Belgae) who were well settled in Britain when Caesar landed in 43 AD.

The Iron Age (1200 BC to Roman times) metallurgist was not able to achieve the temperature of 1540 degrees Centigrade which is the melting point of iron. He heated the iron ore in his furnace to about 1200 C. at which most of the impurities melt and run off as slag. He would then hand forge the resulting solid iron “bloom” by repeated hammering and re-heating, thus mechanically removing most of the residual slag and forming the iron to the desired shape.

Historically, iron was always associated with the making of weapons. The Romans, who were skilled in metallurgy of all kinds, left no written record of their technical methods which evidently were passed orally from generation to generation of craftsmen. (Engineering in History, Kirby et al p. 91) The only use they made of iron in construction was as cleats and pins to hold stone blocks in place. The Greeks had concealed wrought iron bars to bring “the greater resistance of iron under tensile stress to the aid” of their stone beams. (Engineering in History, Kirby et al, p. 46)

Throughout history until Medieval times, ironstone nodules were the main source of iron. Either bog iron or calcinated limestone was added and the whole was roasted on an open hearth with charcoal so that the carbon dioxide was released. The result would be haematite (iron oxide) ore which can be easily smelted.

This process of smelting iron and converting it into a useful material changed very little for several thousand years until in the Middle Ages the Cistercian monks, who were famous for their metallurgical skills, developed the blast furnace whose water powered bellows could achieve temperatures high enough to greatly improve the smelting process. The first British blast furnace was built in the Weald of Sussex in 1491 and during the reign of the Tudors, this region was the primary iron manufacturing area in England.

The first step in the Cistercian smelting process began by annealing the ore on open ground in a heap of burning charcoal for three days as always. The blast furnace had been fired with charcoal for a week to get it to the requisite temperature. The annealed ore, chunks of limestone or marl, and more charcoal were fed into the top of the furnace and burned for 14 days while a water wheeldriven bellows fanned the fire from below maintaining the intense heat. The chemical reactions between the impurities in the iron, the oxygen in the air and the calciferous limestone took place throughout the furnace as the whole mass slowly sank.

The end product was molten metal with some floating solid impurities or slag which flowed from the bottom of the furnace and the gaseous impurities which escaped through the flue. The molten iron with the slag skimmed off was run into sand molds to make uniform chunks of cooling metal called Pig Iron, so named because the smaller sand molds attached to the large ones by troughs resemble piglets nursing a sow. As soon as the iron had blackened on top but was still viscous, the sow and pigs were broken free.

Pig Iron is very brittle due to its high carbon and silicon content. It must be further refined in a “Finery Forge” where it is repeatedly heated to a pasty consistency and then hammered to bring it to a malleable condition called Bar Iron.

Bar Iron was then either reheated and cast into sand molds of the final object in a foundry. Or it was fabricated into wrought iron objects in a forge by repeated heating and hammering. The forge had always been the primary source of iron products. Cast iron objects from the foundry were too brittle for most applications. Darby’s great break-through of 1779 would reverse this situation by the end of the 18th century. The potential for mass production by the cast iron process was enormous and would play a crucial role in the explosive growth of the British iron industry.

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TECHNICAL NOTE: Some technical facts might be of interest and of help to a layman’s understanding of the complexities involved. Iron ore occurs in three basic forms all over the world: as clay-ironstone modules (siderite, iron carbonate), as inter-basaltic laterite iron ore or as bog iron ore (limonite, hydrated iron oxide). The differentiation between the types of iron – cast, wrought and steel - depends basically on the carbon content after refinement. Pig or cast iron contains 2.5 > 4% carbon. Wrought iron has less than 0.1% carbon but also has 1>2% slag. Steel lies in between as a solid solution of iron and 1.7 +/- % carbon. Steel was known very early in the history of iron making but was very labor-intensive to make out of wrought iron by constant re-heating and hammering to create what was called ‘blister steel’. In 1740 Benjamin Huntsman invented the crucible process of creating blister steel but again in such small quantities that it was not practical for construction purposes although it was extremely hard and had more tensile strength than iron. It was prized for sword blades or small parts for instruments where its superiority over other metals for that particular application justified its very high cost. Outside of the scope of this story, Henry Bessemer (1813-1898) developed a process in 1856 for making steel in large quantities at a reasonable cost which meant that steel would eventually become the metal of choice for large construction projects such as the railroads, bridges, and sky scrapers built in the second half of the 19th century.

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It is hard to realize today, when we take for granted that one level or another of our government will provide new roads and bridges and be responsible for keeping them in good repair, that from the mid 17th through the 18th century the tremendous growth in transportation projects of all kinds - the new roads and bridges and canals and later the railroads - was financed and built by private capital.

Until the mid 17th century, the burden of maintaining and repairing the roads had been the obligation of the local rate payer. Each shire was responsible for the roads that crossed it. The assumption was that the traffic on those roads was local traffic. However, as prosperity and trade grew, so too did resentment at having to pay locally for the maintenance of a road now being used by strangers transporting goods to and from distant locations. Local resistance to now having to make repairs more frequently as well as there being no coordination of repairs along the length of the road from shire to shire, British roads continued to deteriorate.

The 1662 Act of Parliament enabling the formation of joint venture companies to finance and build private toll roads would totally revolutionize transportation and communication in the British Isles. Finally the design, construction and maintenance of major new long distance roads could be coordinated and consistent throughout the length of the new turn pike. These companies issued stock and paid dividends to the investors, a factor which played a role in another innovation affecting our story. Gradually, during the 18th century, the country was crisscrossed by new hard surface toll roads that were soon crowded with Royal Mail and regular stage coaches, with post chaises and wagons. There was a pressing need for new bridges.

Simultaneously with the building of toll roads, private capital was also building a network of canals. Large landowners such as Francis Egerton, 3rd Duke of Bridgewater (1736-1803) or the proprietors of iron works and coal mines or the owners of pottery works, such Josiah Wedgwood (1730-1795), were “eager to develop the trade of the area from which they derived their personal incomes” by linking the various natural waterways, especially in the mining and industrial districts, for the transportation of bulk goods and raw materials by barge. (The Industrial Revolution, T.S. Ashton, p66)

The men whom they hired to build the canals (and the roads and the bridges), such as James Brindley (1716-1772) and Thomas Telford (1757-1834), John Loudon McAdam (1756-1836) were working under contract as talented, self-taught individuals in a role that we would today describe as consulting civil engineers. The scope and complexity of the projects they successfully undertook “were larger than anything previously undertaken short of a military campaign.” (Ibid p 66). Their individual stories are as fascinating as the projects they built.

These canals also created a need for new bridges both to cross the canals and, in fact, to carry the canal itself across valleys, rivers, and even across other canals. The limiting factor was as always the tensile strength of the material being used. In the case of the canal bridges or aqueducts, it had to bear the great weight of the canal water as well as the weight of the structure itself. This greatly increased the cost of their construction. The time was ripe for the introduction of a new material for building bridges.

Simultaneously with this need came its solution. Although iron had never before been considered a satisfactory material for building structures under tensile stress, the “new” iron being made at the foundries of the Severn Valley would prove to be just the material that was needed to build these bridges.

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When mankind became a creature of settled habits, he set about making sure that he could cross the local rivers safely by laying down flat stones in the local ford for secure footing.

If a particular crossing was used frequently enough to justify the effort, he would build a bridge making use of indigenous materials such as bamboo, wood or stone. During the next 7000 years of bridge building, only the most sophisticated civilizations thought of using man made materials such as Nebuchadnezzar’s (reigned 605-562 BC) use of fired bricks in his bridge across the Euphrates or the Romans' use of pozzuolana cement.

Wooden bridges are easily built or replaced. They do require constant maintenance as they are vulnerable to fire, dry rot and flood. A few of them were very ambitious structures. London bridge was built and rebuilt of wood from Roman times right up until King John (1167-1216) rebuilt it in stone. Caesar and Charlemagne each built famous wooden bridges over the Rhine but did so 800 years apart.

The earliest stone bridges were the “clapper” bridges built of gigantic slabs of stone raised above water level by resting them on top of large boulders serving as piers. A clapper bridge is heavy enough to survive in place during most floods. To keep the rushing water from eddying around the boulders and undermining them, elliptical stone shear waters were placed to divert the current around and away from the foundation rocks and to deflect flood debris safely up and over the bridge. Clapper bridges are easily rebuilt by reassembling the scattered stones after a disastrous flood.

Given the materials at hand which could not span a great distance, early bridges of any length required piers in midstream. Piers cause problems for bridge builders and for the boats using the waterway beneath the bridge. The pier's foundation is difficult to lay down securely under moving water. The piers are vulnerable to being undermined by the swirls and eddies created by their interruption of the current. Sometimes the riverbed soils cannot provide the stability needed to support the bridge whose weight will be concentrated at the location of the piers.

For the river traffic, piers are a hazard to navigation situated right in mid-channel. The shear-waters protecting the pier foundations increase the turbulence of the water for passing boats, especially in tidal creeks and rivers. Shipping must also have good clearance beneath the bridge at all water levels. It has been the goal of bridge builders through the ages to place the decking well above flood level and to have as few piers as possible .

To minimize mid-stream bridge piers, one must maximize the span of the bridge between its supports. That span is limited by the tensile strength of the material being used to build the bridge. (In other words, what is the distance that the loaded bridge deck can it be unsupported before it begins to sag or fail?) Wood and stone have relatively poor tensile strength. Over the ages, mankind invented various ways of increasing the tensile strength of the few natural materials he had at hand in order to lengthen the span of his bridges.

The Romans and the Chinese invented slightly different versions of the stone arch and various cultures devised the cantilever, both of which redistribute a good deal of the load at the center of the span onto the supporting piers. Rope or hand wrought iron chains were used in suspension bridges.

Wooden truss construction, which is based on the principle that the sides of a triangle are mutually self-supporting, was recommended by Leonardo Da Vinci (1452-1519) who noted below a sketch of one “This bridge is unbreakable.”

In Europe by the end of the 17th century engineers felt that they had maximized the potential for greater spans using these traditional materials. A new material was needed for a new age. During the next century, British iron masters gradually developed a new process for refining iron much more economically and on a large scale. The resulting product had greater tensile strength than the 'old' iron, and had infinite versatility. The first substantial all iron bridge was opened in the Severn Gorge at Coalbrookdale in 1797.

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