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Smelting of iron
Iron
is one of the commonest elements in the earth’s crust. It occurs in rocks, usually in combination
with oxygen (Fe203) giving them a noticeable red (haematite)
or yellow-brown (limonite) colour. The
chemical process for removing the oxygen is called smelting.
For
British iron workers, a freshly-smelted lump of iron is either a bloom or a
pig. Whoever thought up these names
deserves credit for imagination. They
may not have been the first names used, but they were the ones that stuck. For centuries.
The
bloom came to Britain first. It came
via Europe from the Middle East, where iron technology is supposed to have
started.
Wandering
through an iron-producing area – Ashdown Forest or the Forest of Dean, perhaps
– during the Iron Age, you might come across an early smelter (a
"bloomery") in a clearing in the woods: probably just a hollow in the
ground. You might see the workers load
it with iron ore and charcoal, then light the mixture and fan it using bellows.
If
you were a time-traveller from the twenty-first century, you might recognise
the reaction going on, similar to the one in a modern blast furnace. You might know enough chemistry to write a
balanced equation for it.
2Fe203 + 3C = 4Fe +
3CO2
You
would know that charcoal is a form of carbon, which takes the oxygen out of the
ore – "reduces" it, as the chemists say; that some of the charcoal
acts as fuel, reacting with oxygen in the air to produce the heat you need to
make the smelting reaction go; that impurities in the ore will form a layer of
slag. All facts of which the early
ironworkers are unaware: they just know that the process works.
You might expect the iron to come out
liquid, as it does in the blast furnace.
You would be wrong. The bloomery
is not hot enough to melt the iron, which emerges as a spongy lump – a
"bloom". The blacksmith will
be close by, to hammer the bloom while it is still hot: hammering drives out
most of the impurities and "consolidates" the metal into a small lump
(called bar iron for many centuries, before chemists adapted the name wrought
iron to describe it). The smith then
takes it to his forge – probably in the same clearing – to work it.
You
will have to travel some distance before you find the next bloomery. If they are too close together, they demand
more wood for charcoal than the local forest can supply.
Come
again a few centuries later: the main change you find is that the bloomery has
become more sophisticated. Now it is a
furnace about a metre high, and the bellows are less important. The bloom that comes out is bigger. Probably the clearing is bigger too, because
a bigger bloomery demands that more trees are cut down for making charcoal.
Fast
forward to the thirteenth century, where you may find an early blast furnace –
larger than a bloomery, but not so large as a modern blast furnace. Look for a fast-running stream in the
valley, a waterwheel driving the bellows that provides the blast.
The mixture going into the furnace is much
the same as in a bloomery, perhaps with the addition of limestone to help
remove impurities. But now the iron
comes out molten: with the extra heat in the furnace, it has absorbed carbon, lowering
its melting point. It runs down a
central channel and collects in side channels, where it solidifies. Someone with imagination thinks the cluster
of channels resembles a sow suckling a litter of pigs: hence the name "pig
iron".
This
type of iron, with its carbon impurity and lower melting point, is good for
casting. But it is brittle and does not
forge well. The blacksmith wants a
purer form. He can still get it from a
bloomery, but the size of the output from the blast furnace is so much greater
that he would rather take the pigs and purify ("fine") them.
As the demand for iron increases, the
supply of charcoal to smelt it cannot keep up.
A different material is needed to reduce the ore. Could coal be an alternative?
Not
quite, but coke is derived from coal and is (like charcoal) composed mostly of
carbon. The impurities in coke (mostly
silicon compounds) will, indeed, cause problems, as they contaminate the iron
produced and make it difficult to fine. But for casting, the products of the
coke-fuelled blast furnace are more than adequate: the problem of finding a
substitute for charcoal in smelting has been solved.
Credit
for this invention is usually given to Abraham Darby I in 1709, but recent research
suggests that Shadrach Fox achieved the breakthrough a little earlier. However that may be, it is under the Darby
family in their Shropshire works at Coalbrookdale
that the process becomes commercially viable.
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RELATED TOPICS Cort’s promotion
efforts 1783-6 Shropshire
and Staffordshire ironmasters |
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