Although you thought this was a book about bridges, we are going to start at the heart of the matter with iron itself. We will begin with the history of iron making from pre-history up to Three-times-Great Grandfather's birth. It was in during his lifetime that major innovations in the ancient process of making iron came one after another throughout the 18th century.
We don't get too technical here so if that is your delight your love of detail might be disappointed. Nevertheless, even those of us who are not technically inclined will be quite able to understand just how this chain reaction of innovations in metallurgy affected an industrial process that had not changed during the previous perhaps 4000 years.
In ancient times, iron was considered a rare and valued metal simply because it was so difficult to refine and the manufacture of iron objects had to be laboriously fashioned by hand. Iron was prized for weapons and armor since its hardness means it holds a better cutting edge than articles of war made of bronze. Other uses that antiquity made of iron interest us here because some of them have to do with construction. The Greeks concealed wrought iron bars in the lintels of their marble doorways to reinforce the rather poor tensile strength of their stone beams (Engineering in History, Kirby et al p. 46). The Romans used iron pins and cleats to hold their stone blocks in place. Iron could not at that time be made on a scale large enough to be practical for other structural purposes.
About 1200 BC an Indo-European people, renowned for their metallurgy, migrated from Asia Minor through the Balkans and into Europe. The Celtic Hallstatt culture brought their inherited skill to Britain in the 8th century BC. They were over-run by successive waves of even more skilled iron-working Celtic tribes that were well settled in Britain by the time Caesar landed in 43 AD. Some of their iron work survives and it is exquisite.
Although iron is one of the most common elements on earth, it is difficult to convert from ore into a useful state. The impurities must first be completely removed in order to make it malleable, strong and hard. The melting point of iron is an extremely high 1540 degrees centigrade. For the first four or five thousand years of metallurgy, no furnace was able to achieve that degree of heat. The ancient furnaces could reach about 1200 C. which was sufficiently hot to melt some of the impurities which ran off as slag. But the remaining spongy mass or "bloom" of iron had to be laboriously manipulated by repeated reheating and hammering to remove the rest of the impurities. Iron workers world wide followed this ancient process until the Middle Ages when the monastic orders unexpectedly enter our story.
The Cistercian monks invented the blast furnace with bellows powered by a water wheel. Such intensely fanned flames were now able to achieve furnace temperatures high enough to greatly improve the smelting process, but they were not yet high enough to achieve molten iron. The first British blast furnace was built in the Weald of Sussex in 1491 and that area became the center of the iron industry during the Tudor dynasty.
Lend me your full attention now. Here is the technical part I warned you about. We are going to review the complex process which ancient man developed for refining iron and which his successors continued to follow. Your familiarity with these steps will later help you to appreciate the significance of the improvements to be made during Three-times-Great Grandfather's lifetime that will result in the creation of a "new" iron.
The first step in the smelting process began by roasting the ore for three days on an open hearth in a burning heap of ore, charcoal and limestone. The tall blast furnace meanwhile had been fired with charcoal for a week to achieve the requisite temperature. This annealed ore, more chunks of limestone or marl, and more charcoal were all three thrown into the top of the furnace to burn for 14 days while a water wheel-driven bellows fanned the fire from below to maintain the intense heat. The chemical reactions among the impurities in the iron, the oxygen in the air, the carbon in the charcoal and the calciferous limestone took place throughout the furnace as the whole mass slowly sank. Do take note of the enormous amount of charcoal required in this process as that will become very significant later in the story.
There were three end products of this firing: the solid impurities (or slag) floating on top of the semi-molten metal both of which flowed from the bottom of the furnace and the gaseous impurities which escaped through the flue. The semi-molten iron with the slag skimmed off was then 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 narrow channels resembled a sow nursing her piglets. As soon as the iron had blackened on top but was still viscous, the sow and piglets were broken apart to create smaller and more manageable pieces of the heavy iron.
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 poured into sand molds to form the desired product in a foundry, or it was fabricated into wrought iron objects at a forge by repeated heating and hammering. You will find it useful as the story unfolds to keep in mind the difference between a foundry (poured or cast iron) and a forge (reheated and hammered wrought iron).
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AUTHOR's NOTE: By popular demand from those readers whose scientific curiosity has not been satisfied by this layman's overview, I am noting the chemical distinctions among the substances mentioned in this story. You will not be quizzed on this and can skip it entirely if it makes you dizzy.
Iron ore occurs in three basic forms all over the world. a) As clay-ironstone nodules (siderite, iron carbonate); b) as inter-basaltic laterite iron ore or c) as bog iron ore (limonite, hydrated iron oxide).
In the blast furnace an oxygen reduction process is taking place involving carbon monoxide: Fe2 O2 + 3CO > 2Fe + 3CO2
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% up to 4% carbon. Wrought iron has less than 0.1% carbon but also has 1% to 2% slag. Steel lies in between as a solid solution of iron and 1.7% (plus or minus) carbon.
Of the three major ferrous metals, the most durable is cast iron; next is wrought iron; and the least durable is steel which means that modern bridges must be protected by paint.
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It was while Three-times-Great Grandfather was still an infant that the British iron industry began to experience some serious road bumps on its way to prosperity. The three major ones could be characterized as severe supply problems.
English iron ore had certain impurities which at that time could not yet be completely removed in the refining process. Consequently, Bar Iron made from English ore was inferior to that made from several foreign ores which did not contain those intrinsic impurities. Two thirds of the Bar Iron used by the British iron industry was imported - mainly from Sweden but also from Russia and America. During the turbulent 17th century, warfare periodically disrupted these sources. England's foreign policy decisions were affected by her dependence on this imported Bar Iron that was essential for manufacturing the ordnance and munitions required in her pursuit of war on various fronts.
Iron smelting and refining required enormous amounts of charcoal: 560 pounds of wood were required to refine one pound of iron. In the olden days when England had been heavily wooded, this had never been a problem. Now in the late 17th century, those ancient forests were depleted. The iron industry found itself competing for the remaining woodlands with the British Navy whose ship yards required a great deal of prime timber. Parliament, conducting several wars at the time, naturally felt that the Navy's need was the more urgent and passed Acts restricting the cutting of coppices to make charcoal. This led to severe shortages and the price of charcoal rose alarmingly.
A reliable source of swiftly running water was essential for providing the power to run the bellows, the hammers, and the slitting machines. In times of drought the works had to operate horse-driven pumps to return the water to high reservoirs for reuse. When the rivers froze, the works had to close down completely. During the extremely severe winters experienced during the 1600's this happened frequently.
The iron industry solved some of its minor supply problems through specialization and dispersal. Small enterprises conducting one aspect of the process were located near their most needed resource and supplied their product to another company which would then perform the next step in fabrication at a location near their own most needed resource. For instance, works in the wooded Forest of Dean specialized in charcoal smelting and supplied Pig Iron to forges and foundries. Blacksmiths operated near a source of coal which they used to fire their forges. Slitting mills located where there was sufficient water for their source of power and would in their turn supply the immediate neighborhood with Bar Iron slit into small rods suitable for home industries such as nail making.
The charcoal shortage was by far the most urgent and intractable challenge and here the iron industry responded boldly with ingenuity and innovation. It was imperative that a way be found to use some plentiful alternative fuel which had previously been found unsuitable for smelting such as peat or coal. Peat burned quickly and dangerously hot but could not sustain a steady and even temperature. Coal provided steady heat but not at the high temperatures required.
The most promising fuel seemed to be coke (which is coal burned in an oxygen-deprived atmosphere to concentrate it's carbon content since the resulting coke will burn much hotter than the original coal). However, the initial experiments with coke found that it contaminated the iron being refined with its own impurities, such as sulfur, thereby producing an inferior Pig Iron. Coke Pig Iron was very brittle and could not be used to cast such things as plowshares and anchors which must be able to weather hard knocks. Of equal frustration was the discovery that it was impossible to turn coke Pig Iron into the malleable Bar Iron needed to make wrought iron at the forge.
At last, in 1709, there was a significant breakthrough that would prove to be only the first in what would become a chain reaction of innovations in the iron industry. Quaker iron master Abraham Darby had been experimenting with coke as a potential fuel for his foundry in Shropshire. Darby's approach was to remove its impurities before the coke was to be used in the refining process. Using the "sweet" clod coal of his district, he finally succeeded in making commercially acceptable Pig Iron using coke alone to fuel his furnace. He also designed and built a larger blast furnace that could maintain 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 was able to fill all the tiny cavities of a mold to achieve a superior casting.
These superior castings were now fine enough to be substituted for wood and for the softer metals in everything from the structural members of fire-proof factories to the most precise machine parts. The coal fuel required was abundant so that the manufacturing cost was greatly reduced. The potential for mass production using the cast iron process would play a crucial role in the explosive growth of the iron industry which had until now been dependent on the labor-intensive and painstaking forge process of fabricating iron products.
" ... 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)
By the mid 18th century, innovations were coming thick and fast in the iron industry. 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. In 1712 Thomas Newcomen invented a coal-fueled steam engine to pump water from the coal mines. This "fire engine", when it was adapted to run the blast furnace bellows, meant that the water wheel could be retired. In the 1760's James Watt developed an even more efficient double-acting high pressure steam engine that was more powerful and reliable. The hotter furnace that resulted was able to produce Coke Pig Iron that was now good enough for all general purposes either as wrought or as cast iron. There still remained the inconvenient need for charcoal during the last step in making Bar Iron, but that too was soon to be overcome.
In 1784 Henry Cort introduced his "puddling" process which involved heating the Pig Iron in a hot air Reverberatory Furnace (designed to keep the iron separate from and unpolluted by the coke fuel) that maintained the molten iron at welding temperature while it was being stirred with paddles. The cooling iron was subsequently passed through rollers. The combination of the high temperatures and the physical manipulation of the iron eliminated all the impurities and altered the molecular structure without using charcoal. The resulting iron was superior to Swedish Bar Iron and suitable for all purposes except for making steel. This break-through was the capstone of all previous innovations in the iron industry.
"... (Cort's) discovery was one of the outstanding events in the history of technology." (The Industrial Revolution T.S. Ashton p.55) Ashton then lists the beneficial consequences of Cort's achievement: 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 eliminated its power shortages by adopting Watt's steam engines to neutralize the fickleness of water power; it had solved the critical charcoal shortage by finding a way to use plentiful coke as fuel; it had ended 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 significantly lowering the manufacturing costs. The Darbys and their industry colleagues were now of the mind that there was nothing that could not be improved by being made of iron and they set about to prove it.
When the new iron was found to be satisfactory in one application, that success would often lead to a further application thereby starting a chain reaction of invention with one initiative stimulating, even provoking, further innovation. Here is one example.
When James Watt (1736-1819) was designing his steam engine, he was frustrated by an inability to achieve fine tolerances between the cylinder and the piston precise enough to hold the steam under high pressure. He was directed to John Wilkinson (1728-1808) who had developed a technique for boring cannon with great precision (essential to the accuracy of the fired projectile). Wilkinson rotated not the drill, but the awkward and heavy cannon itself around a fixed bit. Watt and Wilkinson developed a process for boring steam engine cylinders and making parts with tolerances of incredible accuracy and consistency for their day. Watt and his partner Matthew Boulton (1728-1809), who were designing and installing steam engines of all kinds in Britain and abroad from their Birmingham works, specified that all the parts must by supplied by Wilkinson.