Page 69 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

ι. Black smooth surfaces, occasionally with a green or brown hue, best represented the
"original surface" of the bronze, with a cuprite crust beneath. Patina samples from
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these black areas contained cuprite and atacamite.
2. Black crus tal zones, often with a light green layer beneath and cuprite below that,
contained soot particles that may exfoliate as hard crusts. Analysis showed that these
black crusts always contain quartz, atacamite or paratacamite, feldspars, and occa­
sionally brochantite and antlerite.
3. Brown and orange patinas are cuprite, which is observed on recent sculptures. Some
sculptures from the 1950s had orange vertical streaks, perhaps due to active leaching
of the patina and surface dissolution of the outer layers.
4. Black islands on the patina were observed, each surrounded by a light green area that
appeared to be corroded. These isolated islands of black crust on the surface were
similar in composition to the black crusts described in point 2.
5. Light green zones on the surface occurred on horizontal and inclined areas where
rain impacted the sculpture. These areas appear etched and rough, and the patina
consisted of brochantite and some cuprite. Vertical areas exposed to strong wind were
also light green.

Many of these observations agree with those of Rathgen made seventy years earlier, partic­
ularly those relating to the formation of rough, blackish crusts on the bronze surface. These
crusts have been observed on bronze surfaces for many years, and most of the affected outdoor
bronzes examined by Strandberg in 1998 had been exposed outdoors for more than fifty years. 17
I
I RATES OF CORROSION N THE OUTDOOR ENVIRONMENT After
World War I, corrosion scientists became interested in rates of corrosion, and numerous expo­
I
sure trials of copper alloys were begun. Tracy (1951) examined the corrosion of English-made
copper sheets from the roof of Christ Church, Philadelphia. The roof had been exposed for 213
years. A corrosion rate of 0.20 mg/dm per day was calculated; this is equivalent to 0.8I6 μιη
2
per year. This low rate of corrosion is attributable to relatively clean air during the early years
of the copper roof's exposure. Rajagopalan, Sundaram, and Annamalai (1959) compiled corro­
sion rate data from different geographical regions, and selected data are given in TABLE 1.5.
Most of these corrosion rates are similar to those found by other studies, although the rate for
marine atmospheres in Panama appears rather high compared with other results.
Observations based on modern laboratory testing simulating outdoor corrosion conditions
are useful in providing further evidence for the events that occur in patina formation and for the
variations that may occur in different environments; such tests make it possible to measure
and record the parameters of corrosivity. Holm and Mattsson (1982) summarized some of the
available data for corrosion rates of exposed copper in different environments. The following

C H A P T E R O N E
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