However, for various reasons, raw coal, just as it was mined, could not be used at that time in the blast furnace. The chief difficulty lay in the fact that coal, as found in the earth, contains impurities. It has been pointed out that iron unites easily with some elements. Sulphur is one of them and there is sulphur in coal. If sulphur gets into iron— even a very small amount—it makes the metal brittle (or hot short). Worse, sulphur makes iron brittle when it is hot, so if an ironworker hammered it, it would crumble and he would find it impossible to shape the metal at all.
Several people tried using coal for making iron and a few actually took out patents, but none was successful. Dud Dudley, a Midlander, not only took out a patent but also wrote a book about using coal in the blast furnace, and because he claimed to have been successful his name has gone on record as the first man to use coal for making iron. However, if he made iron at all (which he could have done) it could not have been useable.
The year 1709 marks the second great step forward in the history of iron after the introduction of the blast furnace in about 1500. It is not an exaggeration to say that the industrial revolution really became possible after that date, for in that year Abraham Darby succeeded in making iron in the blast furnace with mineral fuel—not, that is, with raw coal, but with coke. Darby did not invent coke—it was known and used before his time for a few purposes such as making malt for brewing—but he did invent the idea of using it in a blast furnace. Coke was made at the time by burning coal in large heaps until all the unwanted impurities had gone off in smoke, and then cooling it quickly with large quantities of water. It was a similar method to that used for making charcoal from wood: in both cases what was left behind was really carbon, an essential constituent of ironmaking. By a stroke of good luck the coal Darby used had a low sulphur content, most of which was burned off during coke making.
Darby was a maker of cast iron cooking pots who had learnt his trade in Birmingham and Bristol. In 1708 he took over a small charcoal blast furnace at Coalbrookdale in Shropshire and in 1709 he made his successful experiments there. Today the site is a museum of ironmaking (part of the Ironbridge Gorge Museum). Darby’s original furnace has not survived, but a later one, rebuilt in 1777, is more or less complete and it is possible to see the sort of equipment Darby used. There are also many examples of objects made from cast iron in the Darby works which, after nearly 280 years, is still in operation as a foundry. It no longer has any blast furnaces.
It took some time for Darby’s coke-smelting process to spread to other parts of the country, and there were very good reasons why development was slow. Coalbrookdale was a very remote place then and news of developments there leaked out slowly. Secondly, Darby was only interested in the process for his own use: he did not patent it, but neither did he publicize it. However, by 1788 there were 53 coke-fired blast furnaces in England and Wales and only 24 using charcoal. By early in the nineteenth century the last of the charcoal furnaces had stopped.
The Blast Furnace: Emerging Metal Tools
