Page 376 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

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are carried out under binocular magnification of io to x40. The object is usually illuminated
by a fiber-optic light source to avoid undue heating of the object during cleaning. Prior to treat­
ment, ethanol or water is sometimes applied to the surface to moisten the soil or corrosion to
facilitate removal. Some of the fine tools used during cleaning include glass-fiber brushes, paint­
ing brushes, dental picks, a pin held in a pin vice, wooden carving tools or sticks, and small cam­
era bellows to blow away dust.
Before attempting overall mechanical cleaning of an object, it is essential that some explor­
atory cleaning be done to evaluate the extent to which a desirable result can be achieved for dis­
play purposes. This work may also be necessary to assess the mechanical stability of the object,
whether it is totally mineralized or whether some metallic core remains. The surface is investi­
gated at this stage to ascertain the presence of any unusual corrosion features, surface finishes,
or mineralized remains.
During the actual cleaning, no special problems are usually encountered when removing
soil or earthy minerals to reveal the outermost layers of the bronze patina. The real problems
begin when attempting to expose the object's original surface, which may be preserved in a
cuprite layer below outer, sometimes swollen, covering layers of basic copper carbonates and
basic chlorides. These layers are often quite hard, and the cuprite layer itself may be either very
compact or sugary, which cannot be gauged without prior exploratory cleaning.
A dramatic example of a bronze before and after mechanical cleaning is the victorious
youth, also known as the "Getty bronze athlete," in the collections of the J. Paul Getty Museum.
PLATE 82 shows the statue with heavy encrustation from marine burial, before mechanical
cleaning and conservation; PLATE 83 shows the same statue after treatment. Another example
is a small bronze bust of a Roman lady in the collections of the J. Paul Getty Museum. PLATE 84
shows the surface of the bronze extensively attacked by erupted pustules of bronze disease;
PLATE 85 shows the bust after mechanical cleaning.
I PROBLEMS WITH MECHANICAL CLEANING In deciding on a
cleaning method, it is difficult to generalize from experience gained with bronzes from one part
of the world and apply it to those from other regions; in some cases no comparison is possible.
For example, ancient South American bronzes are likely to be made of copper-arsenic alloys
that have a good cuprite layer but no developed passive-tin-oxide layer that could serve as a sur­
face to clean. Some of the finely preserved patinas in tin bronzes from the Old World are due to
the retention of tin oxides within the patina, whereas with many South American copper alloys,
the alloying with arsenic and the different burial conditions may produce an entirely different
surface, making the cuprite layer difficult to follow. Even f this layer is followed skillfully by
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the conservator, signs of surface scraping with the scalpel will be invariably evident under the
binocular microscope. In some cases, this surface layer is so close to the bare metal that it would
not be advisable to reveal the cuprite layer at all. More in keeping with the integrity of these

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