Page 418 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Low-tin and The tin bronzes can be divided into two categories, the low-tin
high-tin bronze bronzes and the high-tin bronzes. Low-tin bronze contains less
than 17% tin, the maximum theoretical limit of the solubility of
tin in the copper-rich solid solution. In practice, the limit is closer to 14%, although it is rare to
find a bronze with this tin content existing as a homogeneous single-phase alloy.
When a tin bronze is cast, the alloy is extensively segregated, usually with cored dendritic
growth and an infill of the alpha+delta eutectoid surrounding the dendritic arms. The cen­
ter of the dendritic arms is rich in copper, which has the higher melting point; the successive
growth of the arms results in the deposition of more tin. At low tin contents, for example,
between 2% and 5%, it may be possible for all the tin to be absorbed into the dendritic growth.
This varies considerably, depending on the cooling rate of the bronze and on the kind of casting
involved. f the cooling rate is very slow, there is a greater chance of reaching equilibrium, and
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the amount of the interdendritic delta phase will be greatly reduced or will disappear entirely.
In antique castings with a reasonable amount of tin content, around 10%, complete absorption
of the delta phase is highly unusual, and the dendrites will generally be surrounded by a matrix
of the alpha+delta eutectoid.
As the tin content increases, the proportion of the interdendritic eutectoid also increases.
If a homogeneous copper-tin alloy is worked by hammering and annealing, then the typical
features seen in face-centered cubic metals will develop, namely, annealing twins, strain lines,
progressively finer grains as a result of the working and flattened grains f left in the worked
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condition. The same features will develop f the alloy is two phased, although the eutectoid is
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moderately brittle and may be broken up to some extent. The usual microstructure shows the
presence of small islands of the alpha+delta eutectoid between the recrystallized grains of the
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alpha solid solution. f coring in the original cast ingot was pronounced, then this may be car­
ried over in the worked alloy as a faint or "ghost" dendritic pattern that is superimposed on the
recrystallized grains. When a bronze section is etched with ferric chloride, this difference in
alloy composition due to coring may be apparent only as vague differences in shading of the
alloy; a dendritic outline of the shading may be difficult to see. Only with experience in exam­
ining bronzes is it possible to differentiate between uneven etching and uneven coloring of a
specimen's surface due to coring.
Apart from complications introduced by other alloying elements, such as zinc, the features
seen in most low-tin bronzes are the following:

1. homogeneous bronzes in which all the tin has dissolved with the copper, and no cor­
ing or residual cast features are displayed
2. cored bronzes in which there is an unequal distribution of copper and tin, and the
eutectoid phase is absent

SOME ASPECTS OF H E CHEMISTRY O F COPPER AND BRONZE
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