Page 66 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

bronzes with oil twice a year as a practical conservation treatment. They cautioned, however,
that any excess oil should be rubbed from the surface to avoid the accumulation of dust and dirt.
In 1913 a new committee to study patina was established in Germany. One of its members was
Rathgen, who placed the bronzes he examined into five empirical groups (Rathgen 1924):
1. bronzes with a blackish, rough-surfaced patina
2. bronzes with a blackish, mostly matte surface with gray to yellow-green patchy areas
3. bronzes covered one-third to one-half with a green patina
4. bronzes that were extensively, but not completely, covered with a green patina
5. bronzes covered with a shiny patina, primarily in brownish tones

Rathgen made some interesting observations on the corrosion of four bronzes on the Rhine
bridge in Cologne. Differences in patina were so notable that one might assume the sculptures
were cast in different alloys, but, in fact, they were all cast in an alloy of 86% copper, 4% tin, and
10% zinc. Rathgen noted that there was, however, a salient difference: the two bronzes by Louis
Tuaillon (1862-1919) on the west bank were rough sand castings without further working, and
they had developed a rough, dark patina. In contrast, the bronzes by Johann Friedrich Drake
(i805- 82) and Gustav Bläser (1813-74) on the east side had been meticulously chiseled and were
covered with a smooth green patina. Rathgen writes:
[I]t is understandable that a rough surface offers a larger surface open for attack by atmo­
spheric components. It especially offers the soot the possibility to attach itself, whereas it is
washed off by the rain from smooth surfaces. Soot holds a danger, as it contains in its fine
pores large amounts of sulphuric acid [W]ith the exception of a statement by Magnus,
the influence of sulphuric acid is hardly addressed in earlier publications. From the stone
base of the sculptures it can be seen, however, that by the combined action of sulphuric acid
and humidity, the copper of the bronzes is transformed into the water-soluble copper sul­
phate. Thus the five metre high base of the sculpture on the Cologne Rhine bridge has a
green colour where the copper solution runs down during rainy weather. This happens as
the soluble copper salts convert, with the carbonic acid of the stone into insoluble copper
carbonates, which are retained in the pores of the stone and into soluble calcium salts
which are washed away. (Rathgen 1924:46-47)

The conclusions reached by Rathgen were quite accurate, since the basic copper sulfates are
soluble in acidic rainwater and may be converted to hydrated copper (II) sulfates by a variety of
routes. For example, brochantite may react with sodium chloride to become atacamite, and this
in turn may react with sulfur dioxide to form soluble copper salts, such as the hydrated sulfates.
Rathgen's observations of the dissolution of the previously formed patina by later acidic depo­
sition have been confirmed by recent research. Rathgen also drew attention to the importance

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