Iron, Steam & Money

by Roger Osborne

  Plot reported an attempt by an ironmaker called Blewston to smelt iron with coke at Wednesbury in 1677, but ‘experience, that great baffler of speculation, shewed it would not be’.6 Another contemporary writer lamented, ‘Oh if this coal could be so charked as to make iron melt out of the stone, as it maketh it in smiths’ forges to be wrought into bars.’7 By the late 1600s, coke was being used in smelting lead and copper and in the glass industry, but the great problem remained: how to get it to work in smelting iron without imparting impurities into the finished product.

  Abraham Darby was well placed to make the crucial leap as he had experience of both the brewing and the metalworking industries. The crucial element in the process was sulphur. Coking does not reduce the sulphur in coal, so using coke in a blast furnace produced sulphur-rich iron which was of no use. Darby must have known about the low sulphur content of Shropshire coal (around 0.5 per cent). As one historian of the industry has noted: ‘This was unusual in Britain; some coals have high sulphur, some make good but unreactive coke, but some of the Shropshire seams produced good coke with high reactivity and low sulphur.’8

  At Coalbrookdale, where the Severn and Coalbrook both cut deep gorges through the Middle Coal Measures, as well as the overlying ironstone, there had been extensive mining since the earliest times. In addition, the Severn was the industrial waterway of the iron, coal and wheat trades, and crucially the river gave access to water power to drive the blast-furnace bellows. All in all, if Darby was looking to develop coke as a fuel for blast furnaces, then Coalbrookdale must have looked a very good prospect. According to the following letter, Darby moved there in 1709, but he actually fired up the blast furnace using coke in January of that year, so it is likely he moved in 1708 or earlier; by 1711 he had certainly given up all his interests in Bristol and focussed on his Shropshire operation. This account written in 1753 by his daughter-in-law Abiah Darby reveals the inventor’s persistence:

  It was my husband’s father, whose name he bore [Abraham Darby I] who was the first that set foot on the Brass Works at or near Bristol that attempted to mould and cast Iron pots etc., in sand instead of Loam . . . in which he succeeded. The first attempt was tryed at an Air Furnace in Bristol. About the year 1709 he came to Shropshire at Coalbrookdale, and with other partners took a lease of the works which consisted of an old Blast Furnace and some Forges. He here cast Iron Goods in sand out of the Blast Furnace that Blow’d with wood charcoal . . . Sometime after he suggested the thought that it might be practicable to smelt the Iron from the ore in the Blast Furnace with it Col; Upon this he try’d with raw coal as it came out of the mines, but it did not answer. He . . . had the Coal coak’d into Cynder as is done for drying Malt and then it succeeded to his satisfaction. But he found only one sort of Pit Coal would best suit . . . He then erected another Blast Furnace and enlarged the works.9

  Darby’s aim was to produce iron that would be good for casting in sand but not too hard for the forgers to work into bar iron. The coke achieved a higher temperature than the charcoal, giving a fluid iron good for casting thin hard metal but also suitable for thicker pieces to be worked. Its sulphur content has been estimated at 0.1 per cent but the presence of manganese meant that this was taken up as the innocuous manganese sulphide and therefore caused no harm to the iron.10 The silicon content increased sharply over charcoal iron, giving a softer grey cast iron that was better for working directly but difficult to process in a finery.

  As well as providing a much cheaper and more plentiful fuel, and reaching higher temperatures than charcoal, coke had other advantages. A blast furnace usually operated from twenty to thirty weeks, with the rest of the year spent gathering charcoal for the next blast. This downtime was usually in the summer when river levels were lower and water power for the bellows reduced. With the use of coke the break in the production cycle was much shorter; ironmakers just needed to remake the hearth, recondition the furnace and repair the bellows. Darby and other ironmasters shortened the downtime further by laying out ponds and weirs to store water for dry periods.

  Darby’s technique was revolutionary but it did not spread rapidly, partly because it was a method that had to be learned, rather than a device. We must also allow for the possibility that, while in general terms the invention was a real breakthrough, for individual ironmasters with established chains of fuel supply, it may not have been of much interest. We should also remember that, before the Industrial Revolution of the late eighteenth century created a set of conditions in which innovations could thrive, inventors were relatively isolated.

  Nevertheless Darby taught the technique to his employees and the firm expanded out of Coalbrookdale, taking on furnaces in Cheshire and at Dolgellau. To begin with, the ironmasters who most successfully made the change to coke were within striking distance of the Shropshire coalfield with its low-sulphur coal. Once Darby had shown that using coked coal in a blast furnace was possible, any ironmaster could adopt the technique, using his own ingenuity to make it work. Darby died in 1717 but he founded a dynasty of ironworkers, and the Coalbrookdale Company, run by his son and then grandson (Abraham Darby II and III) continued at the forefront of innovations in this crucial industry.

  Coke technology transformed the production of cast iron. In 1720 20,500 tons of pig iron and 14,900 tons of bar iron were produced in England and Wales by charcoal furnaces, with only 400 tons of cast iron produced by coke furnaces; by 1788 the ratio had been reversed, as 76,000 tons of cast and bar iron were produced in coke furnaces, while only 2,500 tons of cast iron were produced using charcoal; by 1806 there were just eleven blast furnaces using charcoal while 162 used coke.11

  Cast iron was needed for the making of cannon, particularly during the Seven Years War (1756–63), a conflict that cemented Britain’s place at the centre of a global trading network. Along with the Darbys, the best-known ironmaker of the eighteenth century was John Wilkinson, who built furnaces at Bersham and Broseley. Both the Darbys and the Wilkinsons began to make high-quality cast iron by taking the iron from the blast furnace and smelting it again in a reverberatory or air furnace. This is a device that separates the fuel from the iron mix (either ore or roughly made cast iron) rather than heating both together. The chemical reaction that produces the right composition of iron is brought about by letting the heat and fumes from the fuel pass through the iron mix.

  The ability to make plentiful cast iron helped to feed the boom in machinery that came with the inventions of Hargreaves, Crompton and Watt in the late eighteenth century. Abraham Darby II was the first to make cast-iron cylinders for Newcomen engines, which were much bigger and stronger than the previous brass cylinders. In the next generation John Wilkinson took this a stage further by making a device for boring out cylinders with extreme accuracy, rather than casting them. This not only improved the performance of artillery; it also, crucially, enabled James Watt to build steam engines to his desired high specification. For many years Boulton & Watt pressed their customers to use only John Wilkinson cylinders in their engines.

  The improvement of Newcomen engines by Smeaton and others, together with the new Watt engines, gave a further impetus to the ironmasters as the iron and steam industries fell into a productive symbiosis. As early as 1742 Abraham Darby II brought a Newcomen engine to Coalbrookdale to pump water to keep the blast-furnace bellows working at all times. Then, in 1768, John Smeaton patented an engine that drove blowing cylinders, which could produce a blast of air direct from a steam engine without need of waterwheel or bellows. Wilkinson built working blowers which were installed at Broseley and at Dowlais in 1776; they became so effective that by the early 1800s some ironmakers were able to use coal instead of coke. These changes enabled the production of cast iron in Britain to increase from about 20,000 tons in 1720 to 250,000 tons in 1800.

  The cast-iron bridge which Abraham Darby III, grandson of the original Abraham, built over the Severn at Coalbrookdale in 1779 is one of the wonders of the modern world and remains a symbol of C
oalbrookdale’s place at the heart of the Industrial Revolution.

  16. Henry Cort and Cheap Iron

  WHILE SUBSEQUENT GENERATIONS of Darbys and others improved on Abraham Darby’s innovation, one more major obstacle stood in the way of cheap, plentiful iron. While coke could be used to produce cast iron, making wrought iron or bar iron of the kind that could be worked in a chafery forge still needed large quantities of charcoal. In fact, the finery-forging process used more fuel than the smelting – to make one ton of bar iron required 1.5 tons of pig iron and around 1.2 tons of charcoal. In the most common process the cast iron was heated over charcoal and then beaten with hammers (some worked by water power in the bigger forges). Charcoal remained the only option because heating and hammering iron over coal would introduce impurities that degraded the iron.

  So while sufficient cast iron could be produced in Britain, bar iron had to be imported in great quantities from Sweden and Russia.1 A long list of patents in the mid-eighteenth century shows how anxious British ironworkers were to make the leap from charcoal to coal or coke, but most were vague in their specifications and probably did not achieve much. Nevertheless it seems that some substitution of coal for charcoal had become possible. In 1737 the agent for Lord Foley – owner of vast estates in Worcestershire – told a House of Commons committee: ‘In some places along the River Stower in Worcestershire they sometimes use Wood only to draw it into Coop and Anchonies, which take up Four Cord only, and from thence they draw it into Bar with Stone Coal; but the greatest Part of Iron is drawn by Charcoal.’2

  In 1766 Richard Whitworth reported that the forge in Upton consumed eight tons of charcoal and five of pit coal per week, and that twelve forges in Shropshire and Cheshire produced fifty-two tons of bar iron using fifty-nine tons of charcoal and thirty-seven of pit coal.3 They were probably using a two-stage process with a reverberatory furnace (in which the fuel is kept separate from the iron) converting cast iron to bar iron, which could then be worked on a chafery forge. In one process, patented by the Carnage brothers at Coalbrookdale and Bridgnorth, grey iron of the type made by Darby was converted in a furnace where the flame was deflected by bricks. This was widely taken up and the resulting iron was good for making nails. As Richard Reynolds wrote in 1784: ‘The nail trade would have been lost to this country had it not been found practicable to make nails with iron made with pit coal. We have now another process to attempt, and that is to make bar iron with pit coal.’4

  By the time Reynolds was writing, Henry Cort, an iron-forger from Fareham in Hampshire, had just taken out a patent that described the solution to the problem. Cort was born in Lancaster in 1741, the son of a builder. In the 1760s he was working for the Royal Navy where his duties included finding reliable sources of high-quality wrought iron at a time when this was in short supply.5 In 1774 he inherited from his wife’s uncle an ironworks at Funtley near Gosport, along with contracts to supply iron goods to the navy. With a secure order book Cort was in a position to experiment with ways of making wrought iron using coal.

  Henry Cort’s first patent, granted in January 1783, was for a rolling machine for making bars and bolts; his second, taken out the following year, covers the crucial process of puddling. The patent reveals how much the processing of iron depended on a close understanding of the practicalities of each step. First, the forger needed to melt pig iron in a reverberatory furnace (or bring it molten from the blast furnace). Holes in the furnace walls allowed workmen to stir the melt with iron rods, which is the technique known as puddling:

  After the metal has been for some time in a dissolved state, an ebullition, effervescence, or such like intestine motion takes place, during the continuance of which a blueish flame or vapour is emitted; and during the remainder of the process the operation is continued (as occasion may require) of raking, separating, stirring, and spreading the whole about the furnace till it loses its fusibility, and is flourished or brought into nature. As soon as the iron is sufficiently in nature, it is to be collected together in lumps, called loops, of sizes suited to the intended uses.6

  Instead of being heated again and hammered in the finery forge, these loops were then – under the terms of his first patent – put between rollers at welding heat. Rolling at this stage, rather than at the end, pressed out all the impurities, leaving high-quality bar iron. As one of Cort’s friends described the process: ‘As the stirring of cream, instead of mixing and uniting the whole together, separates like particles to like, so it is with the Iron: what was at first melted comes out of the furnace in clotted lumps, about as soft as welding heat, with metallic parts and dross mixed together but not incorporated.’7 The rolling had the effect of pressing the ‘dross’ out of the iron.

  Puddling furnace: In this version the fuel is separated from the iron ore. The rods are used to stir the liquefying ore so that clods of iron are formed. The clods are then run between rollers, which squeeze out the remaining slag.

  This process used techniques that were already in use – rollers had been used for making rods or bars for decades and puddling had also been tried elsewhere. But Cort’s combination of puddling and rolling produced high-quality iron far more efficiently than other methods. His iron was as good as the Swedish oregrund and, while the old method of using steam-operated hammers to drive out impurities produced a ton of iron in twelve hours, Cort could produce fifteen tons in the same time by rolling.

  Cort’s ironworks were run in financial partnership with Admiral Adam Jellicoe, whose son Samuel was a working partner. Once Cort’s process was established, Jellicoe Snr got the Navy Board to declare that they would only purchase iron made in this way and, over a number of years, the Jellicoes invested £50,000 into the business. Unknown to Cort, however, much of this money had come from public funds through the admiral’s position as Deputy Paymaster of Seamen’s Wages. Cort had given Jellicoe the rights to his patents as security and, when the fraud was uncovered, Cort’s patents were confiscated by the government and he was made bankrupt.

  It may well have been difficult for Cort to enforce his patents in any case; his was a new process, not a new machine, and ironmasters could claim that each part of the process had been in use before. However, as the importance of his discovery became apparent, other ironmakers lobbied on his behalf and he was paid a state pension of £200 from 1794 until his death in 1800.

  Cort showed a remarkable faith in his own ability to solve the problem of producing bar iron with coke. In 1779, a full five years before his second patent, he had consulted Matthew Boulton about a steam engine, probably to pump water to drive his forge and slitting mills, and in 1782 he visited James Watt. As Watt wrote to Boulton on 14 December:

  We had a visit today from a Mr Cort of Gosport who says he has a forge there and has found some grand secret in the making of Iron, by which he can make double the quantity at the same expense and in the same time as usual. He says he wants some kind of Engine but could not tell what, wants some of us to call on him, and says he has some correspondence with you on the subject. He seems a simple good-natured man but not very knowing.

  Cort wrote to Watt six months later, by which time he had secured his first patent and was no doubt working on the process on his patent:

  When I did myself the pleasure to call on You at Soho – We had some conversation on the Subject of Iron – I intimated I had solicited a Patent for my invention of Manufactg Iron – on an improved method – wch have obtained and You were kind enough to say You would mention me to Mr Wilkinson. I have therefore taken the liberty of enclosing a Lre to that Gent and I will be obliged to You to forward him – amongt other things I profess to make Ordinary Iron – Tough – by a short and simple process.

  When you can say anything of the forge to be worked with Steam – I will thank you to communicate the same to me – excuse this trouble – I will do as much or more for You.

  In contrast to the slow spread of Darby’s technique, iron-makers immediately latched on to Cort’s new method. Iron-m
aking in South Wales in particular was fuelled by the process with Richard Crawshay, one of Wales’ biggest iron-makers, increasing capacity from 500 tons of bar iron in 1787 to 10,000 tons by 1812. The huge expansion in iron production brought a matching demand for power to run rollers, furnaces, forges and mills, which was supplied by steam engines with rotary power. Better steam engines also enabled deeper mining for iron ore and coal. Faster production of bar iron using Cort’s process drove up demand for cast iron, which was its raw material: output of cast iron rose from 68,000 tons in 1788 to 250,000 tons in 1806. The ready availability of good quality iron meant that machines of all kinds could be produced more easily, further boosting industrial production in textile factories, for instance.

  Last but not least, freed from the need for wood and water, which had dispersed the industry and separated the furnaces from the forges, iron-making could now be integrated on single sites on coalfields and iron fields. Huge investments were made in plants where furnaces, forges and rolling and slitting mills all worked in line. Parts for mules, looms and carding machines, hoists, bridges, buildings and eventually railways were now churned out in their millions. British iron-making took a central role in the revolution that was to take over the world.

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  Henry Cort in The Times