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Iron process innovations

A major change in the iron industries during the Industrial Revolution was the replacement of wood and other bio-fuels with coal; for a given amount of heat, mining coal required much less labour than cutting wood and converting it to charcoal, and coal was much more abundant than wood, supplies of which were becoming scarce before the enormous increase in iron production that took place in the late 18th century.

In 1709, Abraham Darby made progress using coke to fuel his blast furnaces at Coalbrookdale. However, the coke pig iron he made was not suitable for making wrought iron and was used mostly for the production of cast iron goods, such as pots and kettles. He had the advantage over his rivals in that his pots, cast by his patented process, were thinner and cheaper than theirs.

In 1750, coke had generally replaced charcoal in the smelting of copper and lead and was in widespread use in glass production. In the smelting and refining of iron, coal and coke produced inferior iron to that made with charcoal because of the coal's sulfur content. Low sulfur coals were known, but they still contained harmful amounts. Conversion of coal to coke only slightly reduces the sulfur content. A minority of coals are coking. Another factor limiting the iron industry before the Industrial Revolution was the scarcity of water power to power blast bellows. This limitation was overcome by the steam engine.

Use of coal in iron smelting started somewhat before the Industrial Revolution, based on innovations by Clement Clerke and others from 1678, using coal reverberatory furnaces known as cupolas. These were operated by the flames playing on the ore and charcoal or coke mixture, reducing the oxide to metal. This has the advantage that impurities (such as sulphur ash) in the coal do not migrate into the metal. This technology was applied to lead from 1678 and to copper from 1687. It was also applied to iron foundry work in the 1690s, but in this case the reverberatory furnace was known as an air furnace. (The foundry cupola is a different, and later, innovation.)

Coke pig iron was hardly used to produce wrought iron until 1755–56, when Darby's son Abraham Darby II built furnaces at Horsehay and Ketley where low sulfur coal was available (and not far from Coalbrookdale). These furnaces were equipped with water-powered bellows, the water being pumped by Newcomen steam engines. The Newcomen engines were not attached directly to the blowing cylinders because the engines alone could not produce a steady air blast. Abraham Darby III installed similar steam-pumped, water-powered blowing cylinders at the Dale Company when he took control in 1768. The Dale Company used several Newcomen engines to drain its mines and made parts for engines which it sold throughout the country.

Steam engines made the use of higher-pressure and volume blast practical; however, the leather used in bellows was expensive to replace. In 1757, ironmaster John Wilkinson patented a hydraulic powered blowing engine for blast furnaces. The blowing cylinder for blast furnaces was introduced in 1760 and the first blowing cylinder made of cast iron is believed to be the one used at Carrington in 1768 that was designed by John Smeaton.

Cast iron cylinders for use with a piston were difficult to manufacture; the cylinders had to be free of holes and had to be machined smooth and straight to remove any warping. James Watt had great difficulty trying to have a cylinder made for his first steam engine. In 1774 Wilkinson invented a precision boring machine for boring cylinders. After Wilkinson bored the first successful cylinder for a Boulton and Watt steam engine in 1776, he was given an exclusive contract for providing cylinders. After Watt developed a rotary steam engine in 1782, they were widely applied to blowing, hammering, rolling and slitting.

The solutions to the sulfur problem were the addition of sufficient limestone to the furnace to force sulfur into the slag as well as the use of low sulfur coal. The use of lime or limestone required higher furnace temperatures to form a free-flowing slag. The increased furnace temperature made possible by improved blowing also increased the capacity of blast furnaces and allowed for increased furnace height.

In addition to lower cost and greater availability, coke had other important advantages over charcoal in that it was harder and made the column of materials (iron ore, fuel, slag) flowing down the blast furnace more porous and did not crush in the much taller furnaces of the late 19th century.

As cast iron became cheaper and widely available, it began being a structural material for bridges and buildings. A famous early example is the Iron Bridge built in 1778 with cast iron produced by Abraham Darby III. However, most cast iron was converted to wrought iron. Conversion of cast iron had long been done in a finery forge. An improved refining process known as potting and stamping was developed, but this was superseded by Henry Cort 's puddling process. Cort developed two significant iron manufacturing processes: rolling in 1783 and puddling in 1784. Puddling produced a structural grade iron at a relatively low cost.

Puddling was a means of decarburizing molten pig iron by slow oxidation in a reverberatory furnace by manually stirring it with a long rod. The decarburized iron, having a higher melting point than cast iron, was raked into globs by the puddler. When the glob was large enough, the puddler would remove it. Puddling was backbreaking and extremely hot work. Few puddlers lived to be 40. Because puddling was done in a reverberatory furnace, coal or coke could be used as fuel. The puddling process continued to be used until the late 19th century when iron was being displaced by mild steel. Because puddling required human skill in sensing the iron globs, it was never successfully mechanised. Rolling was an important part of the puddling process because the grooved rollers expelled most of the molten slag and consolidated the mass of hot wrought iron. Rolling was 15 times faster at this than a trip hammer. A different use of rolling, which was done at lower temperatures than that for expelling slag, was in the production of iron sheets, and later structural shapes such as beams, angles, and rails.

The puddling process was improved in 1818 by Baldwyn Rogers, who replaced some of the sand lining on the reverberatory furnace bottom with iron oxide. In 1838 John Hall patented the use of roasted tap cinder (iron silicate) for the furnace bottom, greatly reducing the loss of iron through increased slag caused by a sand lined bottom. The tap cinder also tied up some phosphorus, but this was not understood at the time. Hall's process also used iron scale or rust which reacted with carbon in the molten iron. Hall's process, called wet puddling, reduced losses of iron with the slag from almost 50% to around 8%.

Puddling became widely used after 1800. Up to that time, British iron manufacturers had used considerable amounts of iron imported from Sweden and Russia to supplement domestic supplies. Because of the increased British production, imports began to decline in 1785, and by the 1790s Britain eliminated imports and became a net exporter of bar iron.

Hot blast, patented by the Scottish inventor James Beaumont Neilson in 1828, was the most important development of the 19th century for saving energy in making pig iron. By using preheated combustion air, the amount of fuel to make a unit of pig iron was reduced at first by between one-third using coke or two-thirds using coal; the efficiency gains continued as the technology improved. Hot blast also raised the operating temperature of furnaces, increasing their capacity. Using less coal or coke meant introducing fewer impurities into the pig iron. This meant that lower quality coal could be used in areas where coking coal was unavailable or too expensive; however, by the end of the 19th century transportation costs fell considerably.

Shortly before the Industrial Revolution, an improvement was made in the production of steel, which was an expensive commodity and used only where iron would not do, such as for cutting edge tools and for springs. Benjamin Huntsman developed his crucible steel technique in the 1740s. The raw material for this was blister steel, made by the cementation process. The supply of cheaper iron and steel aided a number of industries, such as those making nails, hinges, wire, and other hardware items. The development of machine tools allowed better working of iron, causing it to be increasingly used in the rapidly growing machinery and engine industries.