Working with Iron

Making Iron - A Brief History (Part II)

Iron-making remained a rural craft until development of the blast furnace probably in the Liege area of Belgium during 14th century. Fed by air-blast from water-powered bellows, temperatures up to 1150 degrees C could be achieved, sufficient to melt the iron, which was cast from the furnace into sand moulds to form finished products, or into blocks (called “pigs”) for conversion to wrought iron.

At these higher temperatures about 4.25% carbon combined with the iron, making it brittle. Much was therefore refined (in a “finery”) to produce the purer, softer, forgeable wrought iron which was considered much more useful than brittle cast iron.

Blast furnaces increased the availability of cast and wrought iron, but depended on charcoal as fuel. Shortages of timber and competition from other users made charcoal increasingly scarce in the seventeenth century. Coal could not be used due to the deleterious effect of its impurities on the iron. Then, in 1709 Abraham Darby used coke (purified coal) in his blast furnace at Coalbrookdale, Shropshire. Coke was found to support a larger charge of iron ore/limestone than charcoal did, and allowed blast-air to pass more freely, so blast furnaces could be made bigger and more efficient.

However charcoal was still needed in the fineries to convert pig iron to wrought iron, and shortages continued. (Coal could not be used as its sulphur content caused brittleness in the iron at high temperatures)

In 1784 Henry Cort developed a furnace at Funtley, Hampshire, where coal was burned separate from the pig iron, its heat being reflected, or reverberated off the roof. The charge was stirred (puddled) until almost all the carbon was burned out by combining with oxygen, then pulled from the furnace, hammered and rolled. After further heating, hammering and rolling it was finally rolled to a wide range of finished sections in grooved rolls also developed by Henry Cort.

John Wilkinson developed a steam-powered furnace blower in 1776, and in 1794 a cupola furnace to remelt pig iron with coke in foundries away from the blast-furnace site.

Thus, in the 18th century iron manufacture developed from a charcoal-dependent woodland craft into a coal-based industry. Freed from charcoal shortages, and fuelled by the increasing demand of the industrial revolution, the production of both cast and wrought iron grew dramatically in the 18th. century.

GROWTH IN IRON PRODUCTION: (Cast & Wrought Iron, approximate tonnages)

1720    35,000 T. per annum, 99% using charcoal, 1% coke.
1796    250,000  T. per annum, 6% using charcoal, 94% coke.

The nineteenth century saw many further developments and improvements, including the heavy steam hammer invented by James Nasmyth in 1839. In 1856 Henry Bessemer developed a method of blowing air through, rather than over, molten pig iron to oxidise away its carbon in a tilting converter. This reduced the conversion time to minutes from the hours required for Cort’s puddling process, and produced steel which was stronger than wrought iron. The process was further improved by the Siemens- Martin open hearth process in the 1860’s, and the cost of steel plummeted. 

The decline of wrought iron was then inevitable, and by 1900 its usage was small relative to that of steel, see table below. The last puddling furnace in the UK, Thomas Walmsley’s Atlas Forge in Bolton, Lancashire, finally closed in 1973.  Its furnace, shingling hammer and rolling mill are now preserved at the Ironbridge Gorge Museum in Shropshire, an area associated with some of the world’s most important developments in the manufacture and use of iron. 
DECLINE IN WROUGHT IRON PRODUCTION: Tons per annum
            Wrought Iron        Steel

1870      3,000,000        250,000
1900      250,000            5,000,000

To read Part I of this article, click here.

Making Iron - A Brief History (Part I)

Iron, the fourth most abundant element in the earth’s crust, occurs widely in ores with a metal content up to 60%. Iron has a strong affinity for oxygen, but fortunately carbon has an even stronger attraction for oxygen. At high temperature carbon will therefore combine with oxygen in the ore, leaving the metal behind, but much heat energy is required.

        Fe₂ 0₃       +                3C         >                 2Fe            +          3CO
        Iron Ore                  Carbon                      Metallic Iron           Carbon Monoxide
                                  (Charcoal/coke)                                              gas driven off

Smelting of iron first developed around 1000 BC - the Iron Age. By heating iron-ore in the charred embers of a fire blown by bellows, it was discovered that iron ore could be reduced to a spongy metallic bloom and hammered to consolidate and purify it. This was further refined by reheating and hammering, becoming usable worked or wrought iron. The furnace, a bloomery, was a small bowl-shaped hole in the ground lined with clay and blown by manually-powered bellows, achieving temperatures around 700 degrees C.

It was discovered, probably by accident, that the addition of limestone chips helped the purification process by combining with many of the ore’s impurities, forming a fluid slag.

Wrought iron is too soft to hold a good cutting edge. Around 1400 BC it was discovered, probably in Asia Minor, that reheating blades in carbon (charcoal) produced a harder and tougher surface that could be sharpened. The carbon combined with the implement’s surface forming iron carbide, or steel (Iron carbide is called “cementite”, which later gave its name to the cementation process for the production of steel). This steel surface could be heated and quenched in water to produce a hard edge.

Wrought iron production in bloomeries was small-scale and expensive, so in pre-industrial times it was used where its strength, hardness and malleability were essential, eg. in weapons, tools, security applications, (locks, window bars) wearing parts, (hinges, bearings, bell hangers and clappers, parts of machines such as pumps, wind/watermills, etc) fastenings (nails, rivets, collars,cramps,) and ornamentation. Because of its value it was also used as currency and jewellery, currency bars being bent round at their ends to prove that they did not crack, demonstrating their quality as usable iron, and therefore valuable.

Click here to read Part II of this article.

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