Page 148 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

there will be an equilibrium between cuprite and nantokite. These anodic pits can maintain
charge neutrality by drawing in chloride ions from groundwater, resulting in a solution with
higher copper ion and chloride ion activity inside rather than outside the pits. f the pits erupt
I
and release copper and chloride ions, then a mixture of products could form, such as parata-
camite and atacamite, or conditions might favor the formation of only atacamite.
I THE PRESENCE OF NANTOKITE Nantokite is usually found
under the carbonate or soil-encrusted outer zone of the patina, sometimes concealed within
a cuprite crust or revealed at the base of pustules when they are shaved with a scalpel during
mechanical cleaning. An unusual occurrence of pale, waxy nantokite was observed on the sur­
face of some silver-copper alloy coins in the J. Paul Getty Museum collections. The coins would
periodically turn a powdery green until the nantokite was removed by mechanical cleaning (the
nantokite did not actually form within a corrosion pit but as a deposit over part of the corroded
surface). On some bronzes, especially from the Near East, nantokite may actually penetrate into
the metal object, occurring as deep-seated pockets or layers rather than forming only a surface
layer. These inclusions of nantokite imply very serious problems for the long-term stability of
such bronzes unless the relative humidity is kept below 45% for storage or display. That nan­
tokite may occur at such depth from the outer corrosion interface demonstrates the ability of
chloride ions to interact with the metal in primary corrosion processes; the chloride ions are
drawn toward the internal regions, balancing the charge for the dissolution of copper cations.
I IMPLICATIONS FOR PATINA AUTHENTICITY Lewin (l973)
argued, incorrectly, that the concentration of the complex copper ions in solution during natu­
ral corrosion processes in the soil would be low. With low concentrations, paratacamite is the
most favored product, and Lewin posited that the detection of a mixture of paratacamite and
atacamite in a patina implied that the patina could not have formed in a natural burial environ­
ment and must, therefore, be fake.
This argument suffered from a number of serious drawbacks. As already discussed, the for­
mation of the copper trihydroxychlorides is not straightforward and depends on a number of
factors. X-ray diffraction studies of several natural patinas examined by different laboratories
have established that all four isomers may be present and are not limited to paratacamite. In
addition, low concentrations of copper chloride ions have not been confirmed by in situ studies.
Giangrande (i987), for example, examined samples of corrosion products from several ancient
bronzes during conservation treatment at the Department of Conservation and Materials Sci­
ence, Institute of Archaeology, London. In the cases where the copper trihydroxychlorides were
identified, mixtures of paratacamite and atacamite were common.

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