Page 81 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

are being determined in the parts-per-billion range with modern analytical instrumentation,
but the significance of these compounds to the deterioration of art objects remains to be demon­
strated at this concentration level. For copper alloys, at any rate, the compounds — except for
carboxylic acids and aldehydes—probably have minimal or nonexistent effects.

The marine environment The dissolution of copper in seawater was recognized in the
early nineteenth century as creating difficulties for shipping.
The English chemist Sir Humphry Davy (1778-I829) was requested by the British Royal Navy
board to investigate this problem, and he responded with his report in 1824. In it, he wrote that
when a piece of copper is placed in seawater, the first effects observed were

a yellow tarnish upon the copper, and a cloudiness in the water, which take place in two or
three hours; the hue of the cloudiness is first white; it gradually becomes green. In less than
a day a bluish-green precipitate appears at the bottom of the vessel, which constantly accu­
mulates; at the same time that the surface of the copper corrodes, appearing red in the water
and grass-green where it is in contact with the air. (Davy 1824:152)

Davy determined that the precipitate formed in the seawater was a hydrated submuriate of
copper (one of the copper trihydroxychlorides); he also showed that in environments deprived
of oxygen, no corrosion of copper would occur. This presages the protection of fragile metal
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antiquities by preservation in a closed container with oxygen scavengers, or by storage n a
nitrogen atmosphere, to arrest further corrosion. For protection of His Majesty's ships of war,
Davy recommended attaching an iron or zinc plate to the copper hull, based on his laboratory
experiments showing that copper immersed in seawater would not corrode f small zinc or iron
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plates were attached to it. This protection, by the provision of a sacrificial anodic metal, has con­
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tinued to be of value in preventing the corrosion of ships for almost the last SO years.
I THE COMPONENTS OF SEAWATER The most common com­
ponents of seawater, apart from the water itself, are chloride, sodium, sulfate, magnesium, cal­
cium, and potassium ions, which make up more than 99% of the salt content; important minor
components in both seawater and river water include silicon and iron. The salinity of water is
usually stated in parts per thousand (%o) of salt. An average value is 35 % 0; higher values are found
in such environments as the Mediterranean Sea (38.6 %o) or the Red Sea (41 %<>). The principal
dissolved gases in seawater are oxygen and carbon dioxide from photosynthetic reactions. The
equilibrium between dissolved carbon dioxide and carbonate or bicarbonate ions acts as
an effective and important buffering system. The pH of seawater is usually between 7.5 and
8.4. Oxygen-rich seawater is strongly oxidizing, whereas seawater depleted in oxygen may be
a reducing environment. Because of biological activity, however, copper alloys in a reducing
milieu will not necessarily be well preserved.

C H A P T E R O N E
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