Page 357 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I CORROSION MECHANISMS AND MATERIALS Chinese mirrors
were usually made in a high-tin alloy to produce a highly reflective, silver-colored surface.
Many Chinese textual descriptions of these mirrors imply that they were purposely manufac­
tured to be bright and shiny. Some excavated mirrors retain vestiges of this silver-colored sur­
face, which is simply the color of the polished metal. During corrosion, the alpha-phase solid
solution and the alpha component of the eutectoid present the common mode of attack, fre­
quently engulfing the lead globules in the metal, to a depth that varies from 25 to 200 μπι. The
delta phase remains uncorroded and retains or imparts a silver-colored surface appearance. The
black-surfaced mirrors show the same kind of corroded surface with the important difference
that the delta phase of the eutectoid is also corroded, often to a depth of only a few micrometers,
but this is sufficient to produce a black coloration. It is interesting to note that very little cuprite
is seen in the corrosion crusts of these mirrors. Soil minerals are absorbed into the corroded sur­
face layer, which, in addition to some cassiterite or stannous oxides, may include silicon, alumi­
num, iron, and sulfur.
All three major alloying elements suffer mineralization in the surface layers of these mir­
rors; there is a severe depletion of copper, which is mostly leached away into the surrounding
soil. The tin and lead are converted to oxides or, in the case of lead, possibly to carbonates. Low
concentrations of silicon, aluminum, iron, phosphorus, chlorine, potassium, and calcium ions
are also found, readily explained by exchange with the surrounding soil minerals. Hydrous tin
oxides, which may form part of the patina, are essentially gel-like on initial formation and act
as efficient ion-exchange media for ions from the immediate burial environment. 20
I BURIAL ENVIRONMENT AND PATINA Collins (l934) found a
Han dynasty mirror that was only partially buried in the typical Chinese loess soil. The buried
part had a blue-black patina while the exposed part remained silver colored, suggesting that
in most circumstances the burial environment is the critical factor in determining the nature of
the surviving patina. Sun and colleagues (1992) showed that humic acids can have an important
influence over the corrosion process in helping the formation of the black surface. No reference
is made in this discussion, however, to the important early work published by Geilmann (i956),
whose results are directly applicable to the Chinese bronze mirrors considered here. Geilmann
carried out a detailed examination of the corrosion of bronzes buried in the sandy soils of
German Bronze Age tumuli, employing wet chemical analysis to investigate the elemental or
element-oxide concentrations of twelve bronzes, many of which were reduced to a residue of
stannic oxide. Geilmann analyzed objects that were completely corroded, samples in an
advanced state of deterioration, and samples of patina. He also studied the dissolution of copper
from the patina, the fixation of metallic constituents that had been dissolved from the patina or
bronze, and the process of corrosion of bronzes in the absence of chloride ions. In an examina­
tion of objects in an advanced state of corrosion, Geilmann included a sword blade from the

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