Page 54 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

the Getty Conservation Institute Museum Research Laboratory as part of a technological study
of the material (Scott 1999). These copper fragments date from about 4000 B.C.E., making
them six thousand years old. They are smelted copper, relatively pure, with only a few parts per
million of typical elemental impurities, such as silver, arsenic, gold, antimony, and lead; yet the
corrosion penetration through the copper averages only about 30 μιη. The corrosion consists of
a thin layer of cuprite and a thinner layer of malachite. This patina translates into a corrosion
rate of 0.005 μπι per year, and in some cases, less than that. Even in the most benign soils, this
is a substantially lower rate than would be inferred from the recent experimental work. One
possible explanation is that, in undisturbed contexts, the rate of corrosion of unalloyed copper
progressively decreases as the cuprite layer develops and impedes further attack on the copper,
slowing the corrosion processes to levels far below those experimentally predicted, particularly
for soils low in chloride-ion content.
Groundwater flowing through igneous bedrock accumulates insignificant quantities of
chloride ion but may gain appreciable concentrations of Si0 2 , F~, Na , HC0 3 ~, and Ca .
++
+
Such groundwater may be slightly acidic. In sedimentary and metamorphic rocks, the ions pres­
ent may include substantial Cl~, 0 4 , Ca , C 0 3 , Fe 2 , A l 3 , and N0 3 ~. Here the water
S
=
+
+
++
=
may be neutral or alkaline (Baas Becking, Kaplan, and Moore i960). Plants and bacteria may
contribute about 550-2750 ppm of chloride ions in regions of dense vegetation. Chloride ions in
groundwater may also be derived from the atmosphere, from evaporites such as halite, and from
the decomposition of certain micas.
Highly organic soils or soils over calcareous bedrock have high carbon dioxide contents and
may be chemically very aggressive because the carbon dioxide may react with water to form car­
bonic acid (Garrels 1954), which may attack metals directly and prevent the formation of a pro­
tective film on the metal surface (Wilkins and Jenks 1948). Calcareous soils may also act in a
quite benign fashion, however, especially f carbon dioxide and water produce the soluble cal­
i
cium bicarbonate. This may act to protect the bronze from corrosion: since calcium bicarbon­
ate is a salt of a weak acid, its aqueous solution is alkaline, and by binding with carbon dioxide,
it prevents the extensive dissolution of copper (I) ions (Geilmann 1956). At values of pH > 8,
calcium bicarbonate precipitates as carbonate, and, in subsequent acidic conditions, this may
dissolve instead of copper (II) compounds. The overall pH in dilute natural groundwater is prin­
cipally controlled by this CaC0 3 — H 2 0 — C 0 2 equilibrium (Garrels and Christ 1965).
The groundwater in soils may have a significant phosphate content, which may be delete­
rious to buried bronzes, although this is frequently controlled by the aluminum content of
the soil (Lindsay 1979). The phosphate activity may also be dependent on the speciation
of the phosphorous. For example, the chemical nature of dissolved phosphate may be prin­
cipally P0 4 ~ at pH > 12 or H 2 P0 4 ~, between pH 2 and 7.5, so even in this case, it is not
3
possible to predict all the effects of soil phosphate content simply by knowing how much phos­
phate is in the soil.

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