- Ironworking in the Celtic World -
in a simple furnace. However, in the same way that the addition of tin to copper
produces an alloy with a lower melting point than that of pure copper, so the addition
of between 2 per cent and 5 per cent carbon produces an alloy, cast iron, which may
melt at a temperature as low as c. I250°C, which was within the capabilities of the
furnaces used in the later Iron Age. Since iron is smelted with charcoal, a pure form of
carbon, this alloying process will occur spontaneously in the furnace if the conditions
are right, and there can be little doubt that cast iron will have been produced
from time to time in the Iron Age. Indeed, had the will been there, it could probably
have been made on a regular basis in the later Iron Age. But cast iron has two great
disadvantages as a raw material: it is extremely brittle, and, unlike bronze, it cannot be
hot-or cold-worked; defects so fundamental that Celtic technology found no use for
it. Instead, what was utilized was wrought iron (in modern terminology, low-carbon
steel), an alloy containing less than 0.5 per cent carbon, which lacks the defects of cast
iron. At the temperature at which wrought iron is formed (c.800°C) it is so intermixed
with the siliceous residue of the ore as to be almost unworkable. To free it from this
residue it is necessary to raise the temperature of the furnace to about I 200°C, when
some of the iron reacts with the silica to form a molten slag which runs off leaving a
spongy 'bloom' containing a high proportion of wrought iron. At no point does the
metal itself melt.
For such a bloom to be used it has to be freed from the slag intermixed with the
iron, and then forged into shape. As this cannot be done by hammering the cold
bloom it has to be made red-hot and then hammered on an anvil, a technique alien
to the early bronzesmith accustomed to casting and cold-working technique.
Not only would such smiths have been unfamiliar with this aspect of working hot
metals, but they would also have lacked the toolkit, most notably the tongs and long-
handled, heavy hammers, needed to process red-hot blocks of metal; a lack which
would have prevented them from utilizing any wrought iron which they produced
either by accident or as a result of the experimental smelting of iron ore, a process
which must surely have occurred from time to time.
The critical change came late in the Middle Bronze Age with the development
of the techniques of working sheet-bronze. Although bronze can be cold-worked,
prolonged hammering causes it to harden and become brittle before eventually
fracturing; a problem which prevented the development of sheet-bronze for many
centuries after the use of the cast metal had become commonplace. The solution lies
in annealing the metal, a process whereby bronze which has been stressed by
hammering is restored to malleability by heating to red-heat and then cooling. The
development of this process, and the consequent production of sheet-bronze, will
have accustomed the bronzesmith to handling hot metals and must have led to the
development of the smith's hearth for reheating the metal, as well as tongs, heavy
hammers and a suitable anvil. From there it was but a short technological step to
working wrought iron.
Iron had been used in the Near East and Aegean region for some centuries before
it appeared in central and western Europe. In the middle years of the second millen-
nium Be the Hittites appear to have enjoyed a virtual monopoly of the production of
iron, although the amount which they actually produced seems to have been relatively
small. The conventional view is that it was the collapse of the Hittite empire in the
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