Page 60 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The outdoor environment Corrosion events associated with outdoor exposure of artifacts
may appear to be somewhat daunting, as Plutarch (46-120 C.E.)
so eloquently describes in the Mor alia:

Then since we urged him on, he continued and said that the air in Delphi being thick and
dense and having vigour because of the repulsion and resistance it encounters from the
mountains, it is also thin and sharp...therefore penetrating and cutting the bronze because
of its thinness, it scrapes a great quantity of earthy patina from it, but holds it and tight­
ens it again, the density not allowing the diffusion, the deposit blooms and takes sheen and
splendour on the surface. 12

In fact, outdoor corrosion events are much less complex than those that may occur during
burial; consequently, it is possible to assess to some extent exactly how corrosive certain aspects
of the outdoor environment may be to exposed copper alloys. During the past two hundred
years or so, the emergence of atmospheric pollutants has changed the environment in the West.
Before then, exposed bronzes may have slowly acquired a cuprite or light green patina with a
sulfate and, possibly, a carbonate component. With the increase in sulfur-containing contami­
nants, patinas in urban areas began to turn different shades of green. Atmospheric soot and
smoke darkened the surfaces of bronzes, creating subfusc (brownish) or blackish crusts, which,
when attacked by low pH fog or mist, might produce continued corrosion and a surface streaked
with light green rivulets that disfigured the patina.
I ATMOSPHERIC POLLUTION Complaints about the insidious
effects of smoke were already being voiced in imperial Rome during the early centuries C.E. By
the 1800S, it was recognized in England that air pollution was injurious to health and also to
materials, particularly architectural stone. Voelcker (i864) investigated the deterioration of
limestone buildings and found the crust to be of calcium sulfate, which formed from the origi­
nal calcium carbonate by reaction with the sulfurous polluted air. This was recently confirmed
by numerous studies into the cause of building-stone decay in the outdoor environment.
The development of patina on exposed copper alloys depends on atmospheric gases, pol­
lutant gases, particulate matter, rain, wind, sun, soot, and, in some cases, proximity to the sea
or to industrial activity. These factors are fundamentally more significant in corrosion than
the nature of the alloy substrate, although this, too, can play an important role. Because of
these diverse factors, the surface appearance of exposed bronzes may vary markedly. A typical,
streaked copper-alloy patina on a version of Auguste Rodin's The Thinker, I888, at the Musée
Rodin, Paris, is shown in PLATE 5. This bronze, like many others, would originally have been
patinated by the foundry before being placed outside. Such artificial patinations, however, do
not necessarily provide long-term protection and are themselves subject to chemical change
with time, gradually becoming converted to the basic copper sulfates, especially in urban areas.

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