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on the life and achievements of eighteenth-century inventor Henry Cort. Please email site controller Eric
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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.
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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.
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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.
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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.
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RELATED TOPICS Cort’s promotion
efforts 1783-6 Shropshire
and Staffordshire ironmasters |
henrycort.net
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