Page 102 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

these data apply to archaeological material, also found weaker absorption bands, probably due
c
to impurities, as well as a band at 3360 m from the presence of adsorbed water. Another band
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at lioo cm" was previously reported by Pasterniak (1959), who ascribed it to the presence of sil­
1
icon impurities in the cuprite lattice.
Natural cuprite patinas PRESENCE OF COPPER CRYSTALS Metallic copper is some­
times seen in the cuprite patina on a corroded bronze. One
mechanism for the deposition of copper is from the dissolution of cuprite inclusions within
the bronze or corrosion from the dissolution of cuprite crusts formed during burial. In some
bronzes, these redeposited copper crystals, usually twinned, occur together with a cuprite
region or cuprite inclusion from the original melt. The redeposited copper may occur together
with cuprite since, during corrosion, voids or intercrystalline regions of the grains may be con­
verted to cuprite at some depth in the alloy. The cuprite inclusions may, in some cases, also origi­
nate from oxidation during casting or from cuprite inclusions in the melt. As a result, there may
be partial alteration of these cuprite areas to redeposited copper. The cuprite layers that develop
as a result of subsequent corrosion may also be subject to reductive processes, with formation
in some areas of redeposited copper.
Davy, writing in 1826, may have been the first to observe such deposits in cuprite patinas.
He noted that "red crystals were found to be formed of octahedrons of the red oxide of copper,
intermixed with crystals of the same form of metallic copper. These crystals were most distinct
at the surface" (Davy 1826:56-57).
The prevalence of copper crystals on the surface of an object is often observed on ancient
bronzes, especially where the cuprite layer is exposed to view (i.e., not covered with a second­
ary layer or cleaned away). There is no doubt that this redeposited copper is an effect of corro­
sion and not of incomplete melting of the alloy. Understanding the genesis and formation of this
redeposition, however, requires further research. Chase (1994) notes that cuprite is the stable
phase in the presence of carbonate, chloride, and sulfur at pH 8 and at an oxidation potential of
0.1 V (Pourbaix 1973). f the pH or oxidation potential drops, the Pourbaix diagram for this sys­
I
tem moves into a region where copper metal is the stable phase. In natural corrosion, where
the processes may be very slow, some patinas show in cross section a uniform transformation
of cuprite to copper. Also, where inclusions or voids filled with cuprite below the surface have
undergone alteration, there may be a rim or outer zone where the cuprite has been transformed
into redeposited copper, some of these showing unaltered cuprite cores. This transformation
occurs when a bronze alloy contains lead that is present as small globules. A probable scenario
in this process is that corrosion of the lead globules leaves behind spaces that are then filled with
cuprite; in response to slowly changing burial conditions, this cuprite is then altered into rede­
posited copper.

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