Page 419 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

3. bronzes in which both the alpha phase and the eutectoid phase are present (represent­
ing most ancient bronzes in terms of components)
4. bronzes in which the alpha phase is extensively cored, and the eutectoid phase is
present
Most ancient alloys have tin contents less than 17%. At this level, the bronzes can be cold-
worked and annealed. f the tin content is between 17% and 19%, however, the alloy is unwork­
I
able; a film of the brittle delta phase coats the grain boundaries, causing the alloy to break into
pieces. At more than 19% tin, however, the bronze can be hot-worked. Bells and mirrors were
often made in antiquity of ternary tin bronzes consisting of around 20-25% tin, 2-10% lead,
and the rest copper. Alloys of this type were almost invariably cast. Many binary tin bronzes
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containing more than 17% tin are often found to be made with tin contents in the range of 1-
24% tin, and this approximately corresponds to the equilibrium value of the beta phase of the
bronze system. At a temperature greater than 586 °C, a bronze in the beta region is readily
I
worked. f allowed to cool slowly to room temperature, however, the bronze decomposes into
alpha+delta phase and is impossible to work.
One advantage of bronzes with tin contents between 21% and 24% is that the beta phase
can be retained by quenching the alloy from above 586 °C. This beta phase usually appears as
a martensitic or acicular structure. These quenched beta bronzes are very hard but a lot less
brittle than bronze of the same composition allowed to cool to room temperature. In this latter
case, the alloy would contain substantial regions of the alpha+delta eutectoid phase, would be
very brittle, and would not be able to be worked in the same way as the beta phase alloy.
Other than a few cast figurines, most artifacts with a beta bronze composition that were
made in ancient times were worked in the following manner: (1) the alloy was made as accu­
rately as the technology of the time allowed, (2) a blank was cast in the approximate form of the
desired object, and (3) the object was shaped by hot-working at about 650 °C. At the end of the
working process, the alloy was uniformly reheated to approximately the same temperature and
then rapidly quenched to preserve the high-temperature phase and to produce a martensitic
structure. Hammer marks and oxide scale were then removed by grinding with various grades
of abrasives, often on a simple lathe, and the object was polished. Any surface decoration was
cut with drills or an abrasive wheel before the object received its final polishing.

Tinned surfaces Tin would be applied to the surface of a bronze primarily for
decorative reasons. For food-preparation, cooking, and storage
purposes, coating a bronze alloy bowl or vessel with tin avoids copper dissolution from acidic
plant materials.
At high levels of tin, such as those encountered in tinned surfaces, there are three inter-
metallic phases of the copper-tin system that need to be taken into consideration:

A P P E N D I X A
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