Page 180 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Gettysburg National The leaching of copper components from the patinas of sev­
Military Park bronzes eral outdoor bronzes in the Gettysburg National Military Park,
Pennsylvania, was investigated by Block and colleagues (i987).
These sculptures ranged in age from fifty to ninety years old. Numerous streaks had devel­
oped on the patina surface, disfiguring the bronzes. Patina samples from the statue of General
Oliver O. Howard showed a sulfur-rich layer overlying a chlorine-rich layer in intimate contact
with the metal surface. Pits were filled with a gray material that the researchers inferred was
a combination of various compounds, such as cuprous sulfate. In the absence of X-ray dif­
fraction data for these compounds, however, it is dangerous to assume that scanning electron
microscopy-energy dispersive X-ray analysis is capable of distinguishing or characterizing the
materials formed in outdoor corrosion. In fact, it is very unlikely that any cuprous sulfate would
be able to form as a corrosion product; copper (I) sulfate is very unstable and has never been iso­
lated from corrosion products found on ancient or historic bronze patinas. The study did show,
however, that deep-seated chlorides may be present in outdoor bronze patinas close to the metal
i
surface, and this may have implications for future protection of such bronzes f the chlorides are
reactive. The work of Selwyn and colleagues (i996) showed that most of the chloride species are
probably copper trihydroxychlorides rather than the reactive nantokite.

BrancusVs Infinite Column by Constantin Brancusi (i876-i957) was the
Infinite Column object of an investigative study of the deterioration of a monu­
mental work of art in the outdoor environment (Scott, Kucera,
and Rendahl 1999). The sculpture, erected over the period of 1928-37 in Tîrgu Jiu, Romania, is
part of a sculptural ensemble to honor the children and teenagers who died defending an impor­
tant bridge during World War . Shown in PLATE 38, the sculpture is 29.33 m (96 ft. 3 in.) high.
I
It is shaped with fifteen cast-iron modules over a carbon-steel framework that holds the mod­
ules in position. These cast-iron modules were coated with zinc and then thermally sprayed
with a brass alloy. In some areas, the sprayed modules had developed a green corrosion of basic
14
copper sulfate, too thin to be easily characterized by sampling.
The work has experienced a number of problems over the years, including a darkening of
the exposed brass surface, which would normally be expected to occur. Brancusi was very famil­
iar with hand-finished and hand-polished brass surfaces for his smaller, indoor sculptures,
including many smaller versions of the Infinite Column. He attempted, unwisely, to apply the
same aesthetic to this much larger, outdoor sculpture by using the thermal spray technology to
metallize the cast-iron panels with a golden, polished brass finish. Unfortunately, Brancusi was
apparently not familiar with the technical difficulties of retaining a shiny golden finish on his
outdoor sculpture, and the original finish (approximately 30Zn70Cu) began to darken after
only two years. A new supply of a different brass alloy was ordered from Switzerland, and the

BASI C S U L F A T E S
163