Page 176 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

implicate fungal or lichen activity in the corrosion from nearby deteriorating stonework. Par­
ticulate matter is represented by numerous identifications of quartz, albite, and gypsum. Bird
droppings may also influence the prevalence of Tutton's salt, although there are other possible
sources of ammonia in the exterior environment, including ammonium sulfate particulates and
atmospheric ammonia.
Antlerite was more abundant in samples from the rear of the bronzes, suggesting once again
that the local microenvironment inside the niche is more aggressive than the partially exposed
front, due to condensation and stagnant conditions in these types of sheltered areas. This agrees
with the findings of Burmester and Koller (i985), who studied the bronze doors of the Augsburg
cathedral in Augsburg, Germany, and reported that gypsum and antlerite are more prevalent
in protected areas. This finding, however, is disputed and may simply be related to local condi­
tions for gypsum formation rather than to the formation of antlerite or to a more aggressive
microenvironment.
The condition of an exposed sculpture's surface may also be strongly influenced by historic
coatings. Marabelli, Mazzeo, and Morigi (1991) record that the bronze group of the Neptune
fountain in Bologna had been coated with a protective black varnish, commonly applied to
bronzes in Italy since the mid-nineteenth century. The constant spray of water had formed a
hard layer of salt encrustation, which contained gypsum, over the varnish layer. Areas free of
spray had different colored crusts. In the Venice area, where the industrial and marine atmo­
spheres merge, Leoni and Panseri (1972) found that the common patina components on objects
coated with black varnish were still copper and lead sulfates rather than chlorides.

C A S E S T U D I E S OF E X P O S E D BRONZES

The Statue of Liberty The Statue of Liberty, shown in PLATE 36, was inaugurated on
28 October 1886 and represents one of the most carefully stud­
ied outdoor bronzes. 10 The monument's surface consists of copper sheets hammered to shape
and then riveted together. The sheets had an original cuprite patina added during manufacture,
but a mostly brochantite patina eventually formed over the cuprite. (The chemical background
of this monument is discussed in CHAPTER 1.) By 1900, obvious signs of corrosion had appeared:
the copper surface had turned various shades of brown to black as well as pale green. Engineers
from the U.S. War Department carried out a detailed condition study in 1905 and concluded that
the monument was perfectly safe and could be allowed to form a natural patina without inter­
vention. After a series of minor repairs, the statue was left to weather into the twenty-first cen­
tury. Thickness data on the maturing patina were obtained by ultrasonic measurements of both
11
protected and unprotected areas of the copper surface, and the rate of corrosion was measured
at 0.04-0.06 mils (0.0010 - 0.0015 mm per year). This was consistent with the data derived from

BASI C S U L F A T E S
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