Series editors' preface - Wood Tools

208 Conservation of Furniture
previous historical working of the metal
artefact concerned. Metal grains also display a
range of imperfections in their crystalline
arrangements that help to explain why they
are tough and not brittle, why they will stand
heavy loads, extend a little and then stop
extending and why they can endure shock
loads and stress reversal better than most nonmetallic
materials (Street and Alexander, 1979).
Metals can be shaped by a multitude of
methods (Maryon, 1971; Untracht, 1968).
However, these can be summarized as (i)
shaping from molten metal (e.g. casting), (ii)
shaping from hot solid, (iii) shaping from solid
metal, (iv) joining by mechanical methods or
by soldering, brazing or welding, (v) powder
metallurgy.
For further information on metallurgy, that
is the science and technology of metals, in
particular their extraction, heat treatment and
alloying, see Shrager (1949) and Tylecote
(1976).
Iron and steel
The fundamental process in the production of
iron is the reduction of iron ore by carbon
which may be represented as
FeO + C → Fe + CO
Iron Carbon Metal Oxides of
oxide carbon
The principal oxides of iron include haematite
(Fe 2O 3) and magnetite (Fe 3O 4). Though iron
ores are relatively rich in metal, they contain
silica which is almost impossible to melt and
which tends to combine with metallic
compounds when heated with them. In early
times, the ore was heated in a charcoal fire to
give a spongy mass or forgeable ‘bloom’ of
iron containing relatively little carbon. This
was further processed to produce the wrought
iron of which most historic ferrous furniture
hardware is composed.
In the fifteenth century, furnaces were
developed that used a forced blast of air to
obtain temperatures high enough to melt the
iron which could then be poured in liquid
form and cast. Much of the silica and other
impurities are removed during the process of
smelting by mixing limestone with the iron
ore. In the great heat of the blast furnace, lime
combines with silica to form a molten slag that
floats on top of the molten metal and can be
separated from it. The resulting iron is called
pig iron. Cast iron containing between 5 and
10% of other elements including carbon,
silicon, and manganese is produced by remelting
pig iron from the blast furnace. Cast iron
is the cheapest of all metals. It contains a
higher percentage (3–5%) of carbon than
wrought iron and is very brittle.
In 1709, Abraham Darby succeeded in
smelting iron with coke, leading to technical
improvements, cost savings and increased
availability that opened the door to the use of
iron on a much larger scale. In 1828, a Scot,
James Neilson, developed the use of a heated
blast of air leading to further increases in
efficiency and cost savings. A typical output of
a blast furnace in the 1780s was about 900
tons per year. By the 1980s it was common to
find amounts greater than this being produced
daily.
Wrought iron, which had a high reputation
for strength and reliability, is usually a very
pure iron mixed with an iron-rich glass (slag)
made from spongy iron blooms either
smelted directly in ‘bloomeries’ or refined
from cast iron in ‘fineries’. It was made on
an industrial scale by puddling. Pig iron
could be melted and refined by addition of
iron oxide and other substances to oxidize
and remove carbon, silicon and sulphur. As
impurities were removed, the melting point
of the iron rose to approach that of pure iron
(1500 °C). At this temperature the furnace
was unable to keep the metal molten and it
was removed as lumps. Iron, which in this
form is highly malleable, was hammered to
squeeze out slag and then hot rolled and
forged to the required shape. The ‘wrought’
of wrought iron actually refers to the process
of hammer-consolidating the semi-molten
blooms. This hammering resulted in
elongated ‘stringers’ of slag running longitudinally
in the stock as the iron was
compacted and excess slag squeezed out.
Slag stringers give wrought iron a fibrous
structure that looks very much like wood
grain in corroded specimens. Wrought iron is
a particular and distinctive material and not
an interchangeable term for ornamental
ironwork. Because wrought iron is somewhat
softer under the hammer and forge-welds
more easily than mild steel, it continued to