Page 420 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

ι. the delta phase, Cu 31 Sn 8 , containing about 32.6% tin
2. the epsilon phase, Cu 3Sn, containing about 38.2% tin
3. the eta phase, Cu 6 Sn 5 , containing 61.0% tin

The epsilon phase that appears in these high-tin alloys is of importance in understanding
the microstructure of tinned surfaces. When tin is applied to a bronze, layers of both the eta and
the epsilon phases can develop, which is dependent on the time and temperature of the diffu­
sion process. Interdiffusion between bronze and molten tin develops in the following sequence:
surface tin, eta phase, epsilon phase, substrate bronze.
When viewed under the optical microscope, tin appears light colored and silvery; the
eta phase has a slightly more gray-blue color; the epsilon phase is the darkest gray blue, and
the delta phase is light blue. The range of microstructural features that can form on tinned sur­
faces is complicated; the eta phase, Cu 6 Sn 5 , is common and is often misidentified as tin. Fur­
ther details on this subject can be found in Meeks (i986).

Leaded tin bronzes Many tin bronzes are leaded. In low-tin bronzes, typically used
for castings, the lead does not alloy with the copper or the tin
and occurs as small globules throughout the metallic structure. Some gravity segregation may
cause the lead to settle into preferred regions in the casting, but generally the distribution of the
lead globules is random, with particle sizes that range from a few microns across to large glob­
ules of 30-200 μιη in diameter. With higher tin contents, the metallic structure of the bronze
may be difficult to determine by optical metallography because the structure may become very
fine-grained. Variations of the Widmanstätten structure are possible in quickly cooled bronzes.

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