4 Metals and Their Conservation
INTRODUCTION
The extraction and use of metals has a history going back into remote
antiquity. By 3900 Be copper working had reached an advanced level
in Egypt, Persia and Mesopotamia. According to the Old Testament,
Tubal-Cain, an early descendant of Cain, was the master of all
coppersmiths and blacksmiths (Genesis 4:22). It seems probable that
relatively pure copper (native) was first used, then bronze (an alloy
with tin) and later brass (an alloy with zinc). Copper was one of the
first metals to be used and, together with its alloys, was used in the
decoration of wood and stone; for making household articles such as
basins, ewers and mirrors; for musical instruments; for armour; and
for the manufacture of idols.
From the beginning, the metalsmiths' work was artistic as well as
being utilitarian, particular advantage being taken of the malleability
of the precious metals. Gold, for example, may be hammered or
rolled into foil much thinner than a human hair. It can be made to
transmit light.
The metals known in antiquity are few in number - apart from
gold and silver, there were copper, lead, tin and iron. They have been
alloyed with each other in various combinations, both by accident
and intentionally, from earliest times. The compositions of some of
the alloys used from antiquity to the present time are given in table
4.1. Henry Schliemann, the archaeologist who uncovered the ruins of
Mycenae, discovered a vessel full of mercury in a grave at Kuma,
dated to the sixteenth century Be. However, the metal is very scarce
as a mineral and even more so as the free metal, although it was
eagerly sought by mediaeval alchemists in their search for the
transmutation of base metals into gold.
Gold, silver, lead, copper and iron are sometimes found as native
metals (uncombined with other elements, as minerals), although
they, together with tin, may also be extracted from their ores,
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D. Burgess, Chemical Science and Conservation
© David Burgess 1990
Table 4.1 Composition by mass of various alloys
Principal metal Alloyed with
Copper Brasses: Zn, 10--50%
Bronzes: Sn, 5-20%
Bell metal: Sn, 20--25%
Coin bronze: Sn, 4-5%; Zn, 1-3%
Gun metal: Sn, c.1O%
German silver: Ni, 34%; Zn, 20%
Tin Britannia metal: 10--20% of any mixture of Sb, Cu,
Pb,Zn
Pewter: Pb, 10--20% (Sb instead of Sn in modem
pewter)
Solder: Pb, 50--70%
Iron Cast-iron and steels: C, 0.1-5%
Gold Electrum: Ag up to 25% (also occurs naturally, with
widely variable proportions)
Silver Coin silver: Cu, 5-25%
commonly oxides, sulphides and carbonates. In antiquity it is unlikely
that gold was extracted from its somewhat scarce ores; the native
metal can sometimes be found in veins of quartz and, when these
have weathered and disintegrated, tiny nuggets of the metal can
sometimes be found in streams. The native element often contains
silver and sometimes copper.
THE NATURE OF METALS
A pure metal may be represented as in Figure 4.1, which shows a
regular array (actually in three dimensions, of course) of positive ions
(M+), bound together by the readily mobile outer, or valence,
electrons. These electrons can move freely through the crystal, in any
direction, and form a kind of 'electron gas' (see Figure 4.1).
These highly mobile electrons account for the good electrical and
thermal conductivities of metals. Also, because the layers of similar
ions can slide over each other, metals may readily be deformed by
hammering or rolling (the property of malleability), and by extrusion
through dies to form wires (ductility). Table 4.2 shows the relative
order of malleability and of ductility for the metals mentioned earlier.
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