It is possible to produce alloys from ores. Archaeological bronze is an alloy
of copper and tin and the alloys were made by mixing tin ore, (cassiterite,
SnO 2 ) with pure copper, covering the mixture with a layer of charcoal and
heating to approximately 800°C. Liquid alloy was tapped from the furnace.
Brass is an alloy of copper and zinc and the manufacture of this alloy was
achieved by taking pieces of copper and mixing them with calamine
(ZnCO 3 ), a zinc ore and charcoal and placing in a crucible. The crucible was
heated to 950–1000°C to reduce the calamine to zinc vapour which dissolved in
the pure solid copper. The temperature of the crucible was raised to melt the
alloy, the temperature being dependent on the amount of zinc dissolved in the
alloy. A 20% zinc alloy had a melting point of 1000°C, while a 30% alloy was
slightly lower at 904°C
Silver is extracted from sulfide bearing ores of lead and copper. To be viable,
the silver content should be in the order of 0.033%. During the production of
“pure” lead, all the silver present in the ore, will have dissolved in the lead.
The cupellation process was used to recover the silver from the lead. The sil-
ver–lead alloy was heated in hearths with bone ash to a temperature of 1000°C
(melting point silver 960°C) and air blown over the surface. The lead, together
with any other base metal, was oxidised while the silver remained unaffected.
The oxidised lead (PbO – litharge) was skimmed off and a small button of
metallic silver was left in the hearth.
1.2 Metallurgy
In the solid state, metals are crystalline, i.e.the atoms are arranged in a regular
three-dimensional pattern with cubic structures being the most common. This
accounts for the excellent mechanical properties of metals such as ductility and
toughness. Ceramics and glasses have extremely complicated crystal shapes
and, as a result, are very hard and brittle at room temperature. Due to their
crystal structure, it is possible to form alloys of two or more metals and this can
result in a considerable improvement in certain mechanical properties such as
strength and hardness.
Pure metals are easy to shape by hammering, beating or bending because of
their softness and ductility. Mechanical working of metals is what blacksmiths,
goldsmiths and silversmiths have been doing for centuries. If this process is
carried out at room temperature, it is called cold working and the hardness is
increased, but the ductility is decreased by this process. There is a limit to the
amount of change of dimensions that the metal can withstand by cold working,
as eventually the metal will be so hard that it will crack if any further defor-
mation is carried out. Cold working was thus a very simple way to harden a
pure metal but the increase would not be sufficient to manufacture an axe
head, for example. Metals could have their ductility restored by reheating them
128 Chapter 6