Page 358 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Middle Bronze Age, dating to about 1000 B.C.E., and an urn from a grave site dating to the Early
Iron Age, around the second to third century C E . Some of these objects, which had been
entirely leached of copper, had such an osseous appearance that they were originally mistaken
for stained or indurated bone.
The sword blade retained a marker layer, or original surface, even when only hydrated tin
oxide remained. The urn, however, had a thin, dark green patina and an almost white tin-oxide
layer underneath this green surface, which retained the shape of the object. A representative set
of Geilmann's analyses are given in TABLES 11.1 and 11.2. Geilmann deduced that the absolute tin
content of these bronzes was unchanged during corrosion and that no arsenic or antimony was
lost. Iron and aluminum levels were increased in the patina layer from interactions with soil
minerals. Geilmann's data show that some objects retain a copper-rich patina, but the study also
revealed that other objects have copper leached from the surface zone until only hydrated tin
oxide remains. This hydrated tin-oxide layer is then quite stable and resistant to further attack
by soil groundwater. Geilmann writes:

[T]he formation of these patina in burial is ascribed to oxygen and carbon dioxide carried
by groundwaters. The salts present in soil solutions can influence the formation of the
patina and the nature of the corrosion, but on the whole are of minor importance. The dis­
solution of copper and other divalent ions is solely due to the free carbon dioxide in the
water and the progress of this dissolution is dependent on the concentration of carbon
I
dioxide in solution. f this concentration is high, as for example in humus or porous sandy
soils, then the patina is formed rapidly and the continuing dissolution of copper results in
an end product of pure tin oxide, whereas in other soils, such as clay, bronzes of the same
age may only be covered in a thin patina. In humus-rich sandy soils with a low lime con­
tent, there is no inhibiting effect on the decomposition of the patina to create a tin-oxide
surface, while in lime containing soils, where the soil solutions may contain calcium bicar­
bonate, this decomposition is inhibited. (Geilmann 1956:208)
Geilmann's experimental dissolution studies showed that copper would be lost from a lab­
oratory mixture of copper carbonate and tin oxide. In addition, salts of trivalent iron and alu­
i
minum may contribute to tin-oxide enrichment f they are present in the burial environment.
The copper carbonates are preferentially dissolved while iron or aluminum hydroxide is
precipitated; this accounts for the increase in these metallic ions in the patina. Analyses
of soil samples taken from the earth surrounding the buried object in this study showed the
fixation of most metals from the bronze with the exception of arsenic, antimony, and tin.
Geilmann proposed that cuprite is the first corrosion product to form, along with the corrosion
of both the copper and the tin content, followed by the dissolution of copper in the oxidation
zone of burial. Geilmann explains this as follows:

SOME ASPECTS OF BRONZE PATINAS
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