Coke smelting of iron

node · earth · Coalbrookdale, Shropshire, England

The substitution of coke for charcoal in the iron blast furnace, achieved by Abraham Darby at Coalbrookdale in Shropshire in 1709. Raw coal cannot be used, because its sulphur passes into the metal and makes it brittle; coking drives off the volatiles first and leaves a strong, porous, largely sulphur-reduced fuel that also stands up to the weight of a tall furnace charge better than charcoal does. Darby's success was partly a matter of local geology — the Shropshire coals happened to be low in sulphur — and partly of his product, thin-walled cast cooking pots, for which the resulting iron was good enough when it would not yet have served for the best forge work. Adoption was therefore slow for half a century, and only became general once furnace practice and coal selection had been worked out more widely. Its significance is categorical rather than technical: it is the moment iron stops competing with agriculture for land.