Page 105 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The calculation of relative molar volumes (RMV) may be potentially of value, since it pro­
vides a measure of the increase in volume that occurs when a metal transforms to another prod­
uct, such as an oxide, during corrosion. The concept of relative molar volume, first used by
Pilling and Bedworth in 1923 (Evans i960), is defined as

^jyiy _ (MW of product /density of product)
(n) (AW of element/density of element)

where η is the number of metal atoms in the product, MW is the molecular weight of the prod­
uct, and ^ ^ i s the atomic weight of the element. For cuprite, the RMV is 1.67; for tenorite, 1.75.
Both of these relative molar volumes are small compared with other copper corrosion products.
Oxides often have quite low molar volumes, which can assist with the preservation of the shape
of the object during corrosion. This is one reason why cuprite is able to preserve epitactic infor­
mation, since the volume expansion on cuprite formation from the metal is not great.
Robbiola (1990) demonstrated by direct-current polarography and alternating-current
impedance studies that the oxidation film formed on a bronze containing 7.5% tin showed an
overvoltage for oxidation compared with that of pure copper. This implies that greater energy
needs to be applied to the system to create corrosion on the bronze surface as compared with
pure copper. Chase (1994) believes that this additional protective effect of the cuprite layer may
be a consequence of the incorporation of tin into the cuprite, as S n x C u ( 1 _ x ) 0 2 . This may be a
possibility since recent sol-gel work shows that mixed oxides of copper and tin can be prepared.
Fang and colleagues (i996), for example, made nanocrystalline powders of Sn0 2 -CuO and
characterized the product by X-ray diffraction.

Intentional cuprite patinas The existence of deliberately patinated copper alloys (discussed
in greater detail in CHAPTER 11) has been known from antiquity,
and the story of their extensive use by many different cultures is told by Craddock and Giumlia-
Mair (1993). Of particular interest is their description of the well-known Corinthian bronze
alloy, famous in the ancient world for its lustrous black patina.
Alloys with patinas that have been intentionally applied for aesthetic reasons often contain
small additions of gold, arsenic, or other metals that influence the patina's formation. For
example, Craddock and Giumlia-Mair discuss ancient Egyptian bronzes known as hsmn-km
that have a black patina. Some of these bronzes contain about 4% gold, 1% arsenic, and 1% sil­
ver; X-ray diffraction studies have determined that the principal patina component is, indeed,
cuprite. Metalworkers in ancient times discovered that it was possible to grow cuprite films of
mixed composition. Research by Murakami, Niiyama, and Kitada (i988) on Japanese shakudo
alloys (made from the earliest centuries C.E. up to the nineteenth century) confirmed the abil­
ity of cuprite to include other metallic components, such as gold. Shakudo alloys are generally

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