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This page is part of a website based
on the life and achievements of eighteenth-century inventor Henry Cort. Please email site controller Eric
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CORT’S PROCESSES IN IRON MANUFACTURE
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In 1800, Britain was leading the
World into a new industrial age. A major
factor in the country’s success was the technical revolution which had
transformed its iron industry during the preceding century. Two names stand out in this story – the Shropshire ironmaster Abraham Darby I, and Henry
Cort. From John Challen, “The
Impact of Henry Cort” in Eric Alexander (ed.), West Street Trail. |
In a blast furnace, a blast of air is passed
through a charge of iron ore, limestone and some form of carbon. This "smelting"
process is a chemical change: the molten iron produced is tapped from the
furnace as "pig" iron.
Because of impurities, pig iron cannot be
forged. But it has a low melting point,
so that it can be cast.
Since Cort's day there has been a tendency to
classify this iron chemically: pig iron is nowadays regarded as a form of
"cast iron".
The purer type of iron, used by blacksmiths in
Cort's day, is similarly called "wrought iron", whether or not it has
been forged.
The process of converting cast to wrought iron
involves removing carbon impurity, and is known as fining: Cort's
"puddling" process is an example.
Thus the only published biography of Cort
calls him "The Great Finer".
Steel, incidentally, is less pure than wrought
iron, but purer than cast iron. It can
be both forged and cast. But in Cort's
day there was no cheap process for making steel on a large scale.
In the early days of
ironmaking, before blast furnaces came into use, the smelting of iron ore
in "bloomeries" produced wrought iron: fining was not necessary. In the first blast furnaces, charcoal was
the form of carbon used for smelting.
Charcoal was also used as fuel in fineries.
In the eighteenth century, there were worries in
Britain about the consumption of wood in the production of charcoal for
smelting and fining: the Navy preferred its wrought iron to be imported rather
than British-made. Ironmasters found
they could smelt with coke instead of charcoal, but had problems using coke or
coal as fuel for fining.
When Cort started his experiments, the favourite
fining method was "potting and stamping" using
coal as fuel: it was a complex and expensive process, and the wrought iron
produced was of low quality for forging.
It was, however, improved at the same time that
Cort was developing his process, leading to an expansion in iron production
which was later falsely assumed to be due to large-scale adoption of puddling.
The puddling furnace is an adaptation of a type
already used for casting iron, known as a reverberatory or air furnace. Its features are a hearth where the fuel is
burnt, a firebridge where the charge is fined, and a chimney.
The sectional diagram shows the hearth on the right, the chimney
on the left. At the front (the
direction from which we are looking) are two doors. One is used to load fuel to the hearth, the other to load the charge
into the firebridge. The firebridge
door has a small covered aperture, through which the stirring pole can be
inserted.
In Cort's "dry" puddling process, cast
or "pig" iron is loaded into the firebridge. The hot gases melt the iron, then start to
remove the impurity.
As the iron becomes purer its melting point goes
up and it starts to solidify.
The purpose of stirring is to remove the coating
of solid iron, exposing more molten iron to the action of the gas. This goes on for an hour or so before fining
is complete: very hard work for the puddler who does
the stirring.
Cort obtained his starting material from the Navy
as iron ballast, cast iron items no longer needed. These had originally been made from charcoal-smelted pig. When Cort's process was tried with
coke-smelted pig, it worked at first: but impurities absent in charcoal-smelted
iron built up in the furnace and spoiled its performance.
It was years before ironmasters in South Wales
found a way round this problem. Their main improvement was preliminary
refining of the iron by heating, a process that had already been used in the
"potting and stamping" process.
Crawshay's other contribution was to maximise
throughput by feeding the output of four puddling furnaces in turn to a single
shingling hammer: the puddling operation took about four times as long as
shingling per charge. Initially he sent
the hammered "blooms" to Fontley or Rotherhithe for rolling.
When Crawshay installed his own rolling mill, he
found it could deal with the output from two shingling hammers. He therefore operated with eight puddling
furnaces of the twelve installed.
In a later improvement by Joseph Hall, introduced
in Tipton around 1816, iron coated with oxide was added to the charge. The presence of oxide helped to purify the
charge, and Hall found he did not need to refine the iron first. This variant of Cort's process was called
wet puddling, but the shape of the furnace remained the same.
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RELATED TOPICS Cort’s promotion
efforts 1783-6 Shropshire
and Staffordshire ironmasters |
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