An ancient cooking pot in the collection of the Ironbridge Gorge Museum Trust looks like being the oldest known coke iron casting in the western world. Dr Richard Williams, who carried out the research, explains more
“Abraham Darby’s genius was more commercial than technical. He actually first smelted iron with coke, as opposed to charcoal from wood, for just one application.
© Courtesy Ironbridge Gorge Museum Trust
Without using coke to smelt iron, there would have been no Industrial Revolution: the supply of wood was simply not extensive enough. It has previously been assumed that Abraham Darby invented the process because wood was already becoming increasingly scarce and coke was therefore generally more economical. But it was, in reality, all about cooking pots.
We have now shown how Abraham Darby was the first man to make a profitable business from smelting iron with coke rather than charcoal. He saw an opportunity that no-one else did, applied for a patent to protect it and got on with creating the business to exploit it.
The key is a 300-year-old unique cast iron pot, dated 1714, in the collection of the Ironbridge Gorge Museum Trust. I wondered how it had been cast in order to have exactly the right metallurgical structure.
I saw the relationship between the only patent that Abraham Darby filed - about moulding such pots in sand - and his modified blast furnace. It had been previously thought that the two inventions were entirely independent, but Darby’s patent would only work if the liquid iron he used to pour into his moulds was made with coke. It would not have worked with charcoal, which was previously universally used.
© Helen Simonsson / Wikimedia Commons
Darby’s new process was much cheaper than the competitive one, which was most effectively practised on the continent, with a consequently large importation of pots into England.
On the continent they used charcoal but, in order to get the right structure in their iron, they had to pour their metal into moulds that were very hot, thus being obliged to use an expensive moulding process where the sand grains were bound together with clay, the so-called loam process. Darby’s patent specifically said that he was going to use no clay and his moulds could therefore not be heated.
To make most castings, the composition of the iron had to be such that a grey structure resulted rather than a white one. This was much more difficult when the casting was thin, as with a pot, because the metal cooled more quickly than with a thicker casting.
The iron had to be high in silicon to come out grey – something that was very difficult to achieve using charcoal. But coke did it much more easily and this Abraham Darby already knew, from the work of others before him.
He clearly knew it some years before he first set out to make iron himself, because his patent was published in April 1707 and he did not start his coke blast furnace until the end of 1708.
It has not previously been realised – at least in the UK – that moulds used to be regularly heated. I could find no reference to it in the English language. There are, however, many references to it in the French encyclopaedias published in the second half of the 18th century, of which the Encyclopédie of Diderot is the most famous.
I examined a number of 17th and 18th century pots made with the loam process at the Maison de Metallurgie in Liège. All pots bear characteristic markings that establish how they were made. The pot in the Ironbridge Gorge Museums, dated just six years after Darby’s first blast furnace came on stream, must have been cast using an iron made with coke.
© Wikimedia Commons
With no-one else known to be making coke iron at the time, it could only be a genuine Abraham Darby product – the oldest known coke iron casting in the western world.
To begin with, coke iron was only of economic use for the manufacture of cooking pots, but the profit from this activity allowed him and his descendants the time to develop the coke blast furnace for all the other applications became suitable for.
His first furnace produced just four tons per week. In the world today, more than one billion tons of iron comes out of coke-fired blast furnaces each year.”
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