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1 A Brief History
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.
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Fe2 03
Iron Ore |
+ |
3C
Carbon |
> |
2Fe Iron |
+ |
3CO Carbon Monoxide |
Smelting of iron first developed around 2000 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. |
| Wrought iron is too soft to hold a good cutting edge, but around 1400 BC it
was discovered 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. 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. |
| 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 re-melt 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.
NB: More information on iron is available in the Commercial Topic |
| GROWTH IN IRON
PRODUCTION: (Cast & Wrought Iron, approximate tonnages)
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| 1720 |
35,000 tons per year |
99% using charcoal, 1% coke |
| 1796 |
250,000 tons per year |
6% using charcoal, 94% coke |
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| 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 |
| |
Wrought iron |
Steel |
| 1870 |
3,000,000 |
250,000 |
| 1900 |
250,000 |
5,000,000 |
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