Page 405 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

A long-term outdoor trial of the polymer system initiated by the Fraunhofer Institute and
the Public Works Department of Dublin City Council is being carried out on a large-scale pub­
lic bronze sculpture in Dublin, Ireland, under the direction of the city's Office of Public Works.
The sculpture was coated with Ormocer in 1996, and it will take a number of years before its
effectiveness can be evaluated.
There is concern about the reversibility of these polymer systems. Over long periods of
time, oligomer molecules may be leached from the polymer, and subsequent cross-linking may
make the coating very difficult to remove. The bilayer systems can be removed with ethyl ace­
tate, but the monolayer coatings require more aggressive solvents such as methylene chloride.
Despite these limitations, there may be no adequate alternative to these coatings for the long-
term maintenance of outdoor bronzes.
Another recent research effort to attempt to improve the way in which surface coatings are
employed for the protection of outdoor bronzes was initiated by Brostoff and de la Rie (1997),
who began a series of trials of different resin treatments for outdoor bronzes based on the indus­
trial model of applying a primer, main coat, and then top coat. The primer promotes adhesion
of the main coat to the substrate metal; different metals may require different primers to cre­
ate an effective chemical bond. The main coat provides most of the protection from the envi­
ronment, and the top coat is partly a sacrificial layer. A common approach to this industrial
model already in use in conservation is to apply BTA as the primer, Incralac as the main coat,
and a wax top coat.
A good example of this treatment method is described in the study by Marabelli and Napoli­
tano (1991) on outdoor bronze statues in Rome. Laboratory tests by the Istituto Centrale del
Restauro and the Selenia Company investigated the protective properties of multilayer treat­
ments consisting of a blend of natural and synthetic waxes applied over acrylic coatings. The
performance of these multilayer systems was compared with that of the acrylic coatings alone.
Samples were exposed to a sequence of accelerated weathering tests, including a forty-five-day
humidity cycle, exposure to uv radiation, and a one-hundred-hour acidified salt spray test. The
acrylic-wax coatings gave a superior performance, and the best results were obtained with a
double layer of Incralac followed by Reswax WH containing added benzotriazole.
Taking another approach to the problem of stabilization, Brostoff and de la Rie (1997) are
evaluating organosilane coupling agents, corrosion inhibitors and their derivatives, and other
materials such as acrylics. They are investigating main coats of both thermoplastic and thermo­
setting polymers, such as acrylics and acrylic urethanes, with wax as a possible top coat.

Problems with coatings The combined effects of ultraviolet radiation, pollutants, snow,
sun, rain, wind, and particulate matter present an extremely dif­
ficult set of problems for outdoor bronzes. Currently there is no accepted coating for these
objects that does not require considerable maintenance to provide long-term protection of

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