Page 74 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

acetone-soluble components in exposed copper patinas on the Statue of Liberty and at AT&T
Bell Laboratories in Murray Hill, New Jersey. Copper sulfates, nitrate, chloride, acetate, for­
mate, and oxalate were found in a water extract from the Statue of Liberty patina. The acetone
extract revealed the presence of several monocarboxylic acids, alkanes, and polynuclear aro­
matic species. A sample from the Murray Hill location showed the presence of acetate and for­
mate ions distributed throughout the patina layer; that is, they were not confined to the
outermost surface. Although present at very low concentrations, usually much less than 1% by
weight, these organic fractions may act as binders, helping the patina to form a cohesive layer.
Even at these very low concentrations, more than fifty monolayers of these copper organometal-
lic compounds may be present, and this could create a significant binding capacity to hold parts
of the patina together.
The complexity of these natural patinas formed in the atmosphere makes it difficult to sim­
ulate corrosion reactions in the laboratory. For example, many exposure trials in laboratory-
controlled atmospheres containing known amounts of S0 2 and N 0 2 have been carried out with
interesting and relevant results. The correspondence with natural corrosion processes in the
atmosphere, however, has been mitigated by the discovery that small amounts of ozone greatly
accelerate the corrosion of copper. This casts doubt on the validity of predictions based on expo­
sure to SO 2 and N 0 2 alone, regardless of the relative humidity values used.
Although many patinas on outdoor bronze sculpture comprise a thin layer of cuprite over­
laid with a green corrosion crust of basic copper sulfates, many individual studies show more
complex patterns. For example, Mach, Reinhardt, and Snethlage (i987- 88) examined the dis­
tribution of copper, iron, phosphorus, lead, and sulfur in the corrosion layers of some bronzes
in Austria. The distribution patterns showed that the copper content decreased toward the sur­
face while the iron content increased, which is what would be expected. With increased thick­
ness of the corrosion layers, the migration of copper ions is severely reduced; consequently,
elemental species from airborne particulates are more prevalent in the outer corrosion layers.
The deposition of these pollutants not only prevents a further homogeneous accretion of the
protective patina, but it may even weaken the patina layer through chemical and physical pro­
cesses taking place in the outer corrosion layers. In the opinion of Vendl (1999), this may impair
the protective effects of the patina. This is a difficult issue to evaluate, and the situation will vary
greatly from one bronze to another. There is no doubt that in some patinas, typical spherical car­
bonaceous soot particles and ash become incorporated into the corrosion crust. The differential
galvanic activity of these particles and the disruption of subsequent patina growth must result
in the surface acting as a potentially active corrosion agent—if mist or fog of very low pH, for
example, is absorbed by this crust. Consequently, even f the patina of a bronze is retained dur­
i
ing conservation, some surface cleaning is always undertaken because soot and particulate dep­
osition is very damaging to the surface of the object.

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