Page 52 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

because the reaction is not kinetically favorable (Bockris 1971). This is particularly obvious in
the case of tenorite, CuO, which has a region of stability shown on some of the diagrams but,
in practice, is rarely encountered. The product usually found in contact with copper metal is
cuprite. Tenorite is not kinetically favored and is usually found only in burned burial environ­
ments. Corrosion or the growth of patina appears strongly influenced by the presence of cuprite
and by the growth of minerals that can occur from subsequent reactions with cuprite or with
copper ions. Whatever the precise argument for the thermodynamic rationale, the practical con­
sequence is that the region of stability of tenorite shown in Pourbaix diagrams rarely has any
practical validity in predicting the appropriate phase to be formed in a specific environment. It
is difficult to model the kinetic phenomena that may influence the deterioration of copper alloys
over very long periods of time, and therefore less work has been done on this subject.

The burial environment Burial in soil or in barrows, tombs, or cemeteries is the most
common event in the life of most bronze artifacts, which, after
excavation, suffer the indignity of being catalogued, packed, conserved, and displayed or
stored. They are rarely discarded. An outer layer incorporating soil minerals, quartz grains,
associated burial material, organic residues, and so on, may be present on top of the corrosion
products derived from the copper alloy. Burial, often over hundreds or thousands of years, may
produce corrosion effects that cannot easily be modeled or accurately predicted from a break­
down of a typical set of soil parameters, such as moisture content, pH, percent air voids, bulk
density, chloride-ion content, soil type, degree of aeration, calcium content, bicarbonate-ion
activity, cation exchange capacity, or other factors. Multivariate statistical analysis is often used
in soil studies to try to correlate these variables with observed effects because the parameters
influencing corrosion are so numerous and the data so complex (Miller, Foss, and Wolf i98i).
I SOIL PROPERTIES Practically all bronzes buried in soil form a
cuprite crust that is adjacent to the metal and overlaid with malachite, but exactly how corro­
sion processes can differ so markedly from one burial to another is often far from clear. Corro­
sion in a particular soil is often attributable to several soil properties that interact to make the
soil more or less corrosive to buried copper. Many of the soil properties responsible for corro-
sivity are the same as those used to divide the soil continuum into definable groups, such as
sandy loam, dense clay, silt, humus-rich stony soil, and so on. The National Bureau of Standards
(NBS) and the now-defunct British Non-Ferrous Metals Research Association (BNFMRA) have
published evaluations of the burial corrosion of copper in terms of corrosion rates, as shown in
TABLE 1.1. These empirical results are from corrosion tests on unalloyed copper in the soil car­
ried out in the United States by Romanoff (1957), in the United Kingdom by Shrier (1977), and
in Sweden by Camitz and Vinka (1992). The tests covered exposure periods of fourteen, five to
ten, and seven years, respectively. The average rate of attack in most soils was found to be 0.05-
3.9 μιη per year, but for the most corrosive soils, rates as high as 35 μπι per year were recorded.

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