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How We Got Here: Turning Points in Sustainability

Abraham Darby pioneered the use of coke instead of charcoal in iron smelting, freeing the British economy from its dependency on wood. However, he was a devout Quaker, and refused to have his portrait painted. This is one of his descendants, Abraham Darby IV, who was baptised as an Anglican in 1849.

The Discovery of Coke

Coal
Background Turning Point Impact Implications Learning Tools

Abraham Darby (1678-1717) was a devout Quaker. Conscientious and hardworking, he set up a brass foundry in the English town of Bristol before moving in 1708 to Coalbrookdale, Shropshire, where he purchased a derelict blast furnace. Darby mainly produced pots, pans, and kettles, but found himself increasingly constrained by a lack of wood supplies for charcoal. His thoughts turned to using coal as an alternative fuel.

Coal, however, was unsuitable for iron smelting since it contained many impurities—among them, sulphur. These impurities were passed along into the iron during the smelting process, resulting in an inferior product. Darby discovered that if coal was 'coked'—burned in large piles while starved of oxygen—the sulphur and other impurities would be removed. When coke is used as a fuel for iron smelting, the result is a higher quality iron.

How a Blast Furnace Works
How a Blast Furnace Works
Darby's experimentation was hard work. His workers laboured in shifts 24 hours per day for weeks, tipping baskets of coke, limestone, and iron ore into the furnace. The coke burned in contact with the ore, melting out the iron, which collected at the bottom of the furnace. The limestone combined with the impurities in the iron ore to form a 'slag' that was drained off. The molten iron was released at a temperature of 3500°F (1900°C). Large castings, such as cannons or bridge parts, were filled directly from the tap hole into open sand moulds. Small castings were filled with either small, one-man ladles or 'great-ladles', which needed two or three men to carry.

At night, the smelting and casting of the iron lit up the sky for miles. With tremendous temperatures being sustained for long periods of time, the inside of the furnace was actually burnt away. Major repairs were consequently required to the furnace in between each smelting 'campaign'. The environs of the foundries, illuminated by the molten metal, became hives of activity, drawing awe-struck visitors.

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Darby's hard work quickly yielded results: Coalbrookdale became an acclaimed centre of the iron industry. Darby's metalworks benefited from the close proximity of three of vital ingredients: iron ore, limestone, and high-quality coal with a low sulphur content. Everything from iron pots to cannon barrels were made there. Later, Coalbrookdale provided the first iron rails used on railways and also the components for the first civil-engineering work in the world employing cast iron, the Ironbridge in Shropshire (1777–79). From seemingly modest beginnings—hoping to avoid a crippling reliance on scarce charcoal fuel—Darby's furnace literally became the crucible the modern world.


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