Page 55 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I SOIL HORIZONS Soil is normally considered to have three
horizons: The uppermost A horizon is a zone of intense leaching and chemical and biological
activity. The middle Β horizon often contains reprecipitated minerals and is less intensely
altered. The C horizon, at the base of the soil profile, contains partially altered and fresh
bedrock (Garrels and Mackenzie 1971). The rates of evaporation and percolation control the
structure and composition of each horizon. When evaporation exceeds percolation, the prod­
ucts of soil alteration become concentrated n the A horizon; in the opposite case, all altered
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materials, including copper corrosion products in some cases, may be completely leached away.
If evaporation and percolation rates are nearly equal or subject to seasonal fluctuations, much
altered material will be deposited in the Β horizon.
Clearly, even within one soil horizon, the possibility of encountering very different corro­
sive effects on copper alloys buried for long periods is quite likely. Soils that are very compact,
such as pure clay, limit the supply of oxygen and moisture; under these conditions the corrosion
of bronzes may be quite restricted, as long as sulfate-reducing bacteria are not active. In soils
with a mixture of clay, gravel, and sand, the different access to oxygen, moisture, and ground­
water in different regions may produce severe corrosion. Differential oxygen concentration cells
may act locally under these circumstances, with the oxygen-deficient regions becoming anodic
and the other areas cathodic, providing a more favorable condition for corrosion processes.
I CORROSION MECHANISMS I N COPPER ALLOYS The ability of
clay particles to act as ion-exchange media with other chemical species present in the envi­
ronment may assist in the dissolution of some alloying elements such as nickel, cobalt, lead, and
zinc from copper alloys; arsenic and antimony are often preserved to some extent as oxides. As
noted previously, however, some burials in clay may be very well preserved. Tin also oxidizes
but tends to remain as tin(IV) oxide. Stambolov (i985) speculates that the presence of any
hydrogen sulfide from biological activity would create a yellowish precipitate of tin (IV) sulfide,
which, after subsequent oxidation to sulfate, would decompose to form hydrated tin (IV) oxide,
also known as stannic acid. Stambolov (i985) explains how a corrosion crust of stannic acid
could retain the original shape of the bronze object:

Stannic acid is an amorphous mass of a very large specific surface. Hence its extraordinary
adsorption capacity; material transport through its bulk is also possible. Alkaline con­
ditions cause the release of hydrogen ions from the stannic acid and thus, f carried far
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enough will charge the particles equally negative. Due to mutual répulsion they might dis­
perse but, as the concentration of ammonia needed for this charge is unlikely to be found
in soils, such dispersion seldom occurs. Therefore the amount of stannic acid tends to
increase continuously with respect to the copper concentration of the bronze which, in con­
trast, due to greater solubility tends to migrate to the environment. Accordingly as the cor­
rosion proceeds, stannic acid by being immobile, maintains the initial form of the bronze
object. (Stambolov 1985:16-17)

C H A P T E R O N E
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