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The era of charcoal smelting was already dying, partly because it was becoming unobtainable in the quantities needed for an expanding industry, and partly because it was such a soft material, it could not bear the weight of large amounts of ore and therefore was not suitable for use in larger furnaces.

Coal, in plentiful supply, was being used instead, but it contained undesirable impurities such as sulphur which marred the quality of the finished iron.

Darby was not the first man to use coke as a substitute. Nearly a century before, Dud Dudley (1599-1684), illegitimate son of the 5th Earl of Dudley, had experimented with it in his father's ironworks, but with only limited success. It was not until 1709 that Darby made it possible to use the process on an industrial scale.

The switch from charcoal to coal and coke meant production moved from the forests to the coalfields, with South Wales, Scotland, Staffordshire and Shropshire - the Darbies' base - becoming the key centres. Darby's works at Coalbrookdale on the River Severn began producing iron of the highest quality.

The Darbies were a short-lived family. Abraham Derby I died in 1717, aged about 39, and his son Abraham II, who was born in 1711, was only 52 when he died.

It was he who reputedly discovered how to make wrought (ie worked) iron from coke-smelted ore, but he left behind him no details of the process and it was left to Henry Cort (1740-1800) to patent between 1783 and 1785 a technique for puddling and rolling so that coke could be used for the production of refined bar-iron from pig-iron.

Puddling involved the decarbonisation of crude pig iron in a reverberatory furnace, where carbon from the iron ore combined with oxygen from the coke to produce a malleable or wrought iron - as opposed to cast iron - that could be rolled out in a rolling mill.

Cort, who was nicknamed The Great Finer, set up a foundry at Gosport, near Portsmouth, to supply iron for the Navy. Sadly, unknown to Cort, his main financial backer had embezzled Navy money in order to invest, and in the scandal that followed the discovery, Cort was bankrupted, forfeited his patents, and died virtually penniless, leaving his ideas to be exploited by other iron masters.

Darby's Coalbrookdale works experimented with Cort's system without adopting it, but they were achieving a national reputation for excellence, and produced many cast-iron boilers for Newcomen steam engines, which were in big demand for draining mines.

RICHARD REYNOLDS (left) a Bristol Quaker, managed the Darby company after the death of Abraham Darby II in 1763, and oversaw the development of a new technique for making wrought iron using coal instead of charcoal. Thomas Cort (right) took iron production to new heifghts before his spectacular fall from grace.. William Reynolds (1758-1803)

Abraham III lived for just 41 years from 1750 but he enhanced the company's reputation.

His major achievement was the construction of the world's first cast-iron bridge, a 100-foot structure that was prefabricated in the foundry in 1779 and spanned the Severn at a nearby spot now known as Ironbridge. The bridge is still in use today.

It was no surprise that Richard Trevithick used Coalbrookdale's expertise when he needed a company to fabricate the boilers for his high-pressure steam engines in the early years of the 19th century.

Iron became a major growth industry with a seemingly insatiable demand for the product from railway and shipbuilders, engine manufacturers, bridge builders and makers of domestic goods.

The business was given a great boost in 1856 when Sir Henry Bessemer ((1813-98), showed he could make steel - a high-grade alloy of iron - by forcing air into molten pig iron in his converter, removing the impurities.

But as with so many British inventions, home companies were slow to adopt the process. While they hesitated, American firms quickly saw the benefits and cashed in to make vast fortunes.