- Chapter Seventeen -
Unlike bronze, wrought iron is only slightly hardened by hammering; only by
alloying it with another element is it possible to produce a markedly harder metal.
In a few cases such alloys are produced naturally from ores which contain suitable
impurities; phosphorus and manganese are the commonest. Of these, phosphorus
is the more common and is found in some British ores, but it has the disadvantage
of making the iron brittle as well as harder. Despite this, many iron age artefacts do
contain appreciable quantities of phosphorus. Manganese alloys have no such dis-
advantages, but manganese-rich ores are less common. However, they are found in
Austria, and a reference in Pliny's Natural History (xXXIV.145) shows that the iron
produced there was exceptionally highly regarded.
The commonest alloy is with carbon, of course, and the percentage of carbon has
a profound effect on the physical properties of the metal. Wrought iron, with less
than 0.5 per cent carbon, is almost unaffected, while cast iron, with more than 1.5 per
cent carbon, is so brittle as to be unforgeable. Between these two lies steel, an alloy
which is not only harder than wrought iron, but one whose properties can be
controlled by heat treatment to produce a metal which is easily forged when red-hot
but extremely hard and resilient when cold.
Until relatively recent times the only way of producing carbon steel was by
carburization, where iron is heated with carbon, normally in the form of charcoal,
for many hours during which the carbon diffuses into the iron and alloys with it.
Unfortunately, it does not diffuse very far into the iron and the end-product usually
is not pure steel, but a bar of wrought iron with a coating of steel. To what extent
this process was practised by the Celtic smith is still an open question, although
current opinion is that some steel was produced by controlled carburization. To
some extent this process occurs naturally in the smith's hearth, but the coating of
steel so formed is too thin to be of great value, although in an object which was
repeatedly heated during forging the cumulative amount of steel formed could be
appreciable. However, the conditions necessary for the production of steel exist in
the smelting furnace itself, for the iron which is produced at the beginning of a smelt
remains in contact with the burning charcoal for several hours before the process is
ended, during which time some of it may be converted into steel. An experienced
smith would be quite capable of differentiating those parts of a bloom which were
steel from those which were wrought iron by their relative hardnesses, and the two
could be separated easily.
The use of untreated steel produces a stronger and harder artefact than does
wrought iron, but to utilize its properties to the full it needs to be quenched and
tempered. In quenching, the steel is raised to white-heat and then rapidly cooled by
being plunged into water or some other liquid. With pure steel this produces a very
hard and brittle material, so hard and brittle that these qualities are usually modified
by tempering. This involves reheating the metal and allowing it to cool gradually; the
higher the temperature reached in the reheating, the softer the metal becomes. By
combining quenching and tempering a smith can control the hardness of steel with
great precision. In reality the Celtic smith hardly ever used pure steel but rather a
mixture of wrought iron and steel. The effect of quenching this is to harden and
embrittle the steel while leaving the wrought iron unaffected. Tempering such a piece
is unnecessary for the brittleness of the steel is effectively counterbalanced by the