Fining before Cort
Once blast furnaces are used to make iron, it becomes important to turn the brittle pigs produced by the furnace into a malleable form.
This change, called fining, involves removing impurities, particularly carbon.
In early fineries (detailed more substantially on several other websites), a stream of air is passed over a mixture of pig iron and charcoal. This may sound perverse, since the intention is to remove carbon impurity. But the charcoal is there as fuel: enough air is passed through the finery to burn up the carbon in both fuel and iron.
Such a process raises concerns about charcoal shortage (as does smelting). Hence attempts to find a substitute for charcoal.
Several methods are patented in the pre-Cort period for fining using coal instead of charcoal. Their main problem seems to be combating the effect of sulphur impurity in the coal.
The first successful process is devised by William Wood (1728) and developed by his sons Charles and John Wood.
Pig iron taken from the furnace was broken into small pieces (stamping) and placed into clay crucibles (pots) with a flux to absorb sulphur. These pots were heated in a coal-fired reverberatory furnace. The high temperature oxidized the carbon and broke the pots. The metal was removed from the furnace and re-heated in a coalfired chafery and consolidated under a forge hammer.
From Dr Joseph Gross’s description of Wood’s process. In Puddling in the iron works of Merthyr Tydfil
The process becomes known as “potting and stamping”.
Variants claimed in patents by John Roebuck (1763) and John Cockshutt (1771) appear to be non-viable, but one by Wright & Jesson of West Bromwich (1773) becomes widely adopted in Shropshire and the West Midlands.
Patent No. 1054 of 2nd December 1773 was in the name of John Wright and Richard Jesson of West Bromwich for a process in four stages. In the first, cast iron mixed with scale or cinder was heated in a normal bellows-operated finery but using pit coal “or coaks” instead of charcoal. In the second stage the product was taken out in lumps and beaten to plates under a large flat stamp and the plates were broken into small pieces under a round stamp. In the third stage, having removed the small particles, supposed to be sulphureous, the product was “cleansed of sulphurous matter” by washing in a rolling barrel. In the fourth and final stage, the washed residues were heated in a common air furnace “in pots or otherwise” with a fire made of pit coal “or coaks” and then shaped to bars using a common chafery.
From Mott/Singer, Henry Cort: The Great Finer
An alternative process patented by brothers Thomas and George Cranage at Coalbrookdale in Shropshire succeeds only at the first few attempts.
We are informed by Mr. Reynolds of Coed-du, a grandson of Richard Reynolds, that “on further trials many difficulties arose. The bottoms of the furnace were destroyed by the heat, and the quality of the iron varied”.
From Samuel Smiles, Lives of the Engineers; quoted in Mott/Singer Henry Cort: The Great Finer.
The Cranage process attempts to fine with coal, but without potting the iron. Although unsuccessful (as Reynolds’s account shows)l, it is the nearest of earlier methods to the one Cort adopts using a reverberatory furnace.