Page 251 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

F I G U R E 6. 3 Gilded brass
1
clock, French, ca. 781, with
a type of corrosion known
as "brown fuzzies": , full
A
view, and , B detail, revealing
the disfiguring brown-black
spots. The clock, based on
a design by the architect
François-Joseph Bélanger
(174 4 -1818) , has a movement
designed by Jean-Bap tiste
The
Lepaute (1727-1801) .
corrosion excrescences were
identified as nauwamite, a
zinc-substituted basic copper
sulfate hydrate. Wallace Col­
lection, London (clock F 2 6 9 ) .
Bequest of Lady Wallace.

A

with a low Mohs hardness of . It was first reported in 1982 from the type locality of Aberllyn
2
mine, Llanrwst, Bettws-y-coed, Gwynedd, Wales (Nickel and Nichols 1 9 9 1 ) .
Seeley suggested a possible mechanism for the formation of this product on brass objects. 7
Atmospheric pollutants such as sulfur dioxide can cause local dezincification, leaving traces of
zinc sulfite on the surface of an object, and these traces are rapidly oxidized to zinc sulfate. Zinc
sulfate is highly deliquescent, therefore even slightly elevated relative humidity can cause local­
ized spots of condensation. The zinc sulfate solution thus formed will dissolve more sulfur diox­
ide, which will be oxidized to sulfur trioxide, facilitated by the copper (II) ions also present on
the surface. In the presence of the increasingly strong concentration of sulfuric acid in the drop­
lets, together with atmospheric oxygen, more copper-zinc alloy will dissolve, resulting in the
production of a mixed copper-zinc basic sulfate. This effect could be even greater in two-phase
brasses or brasses with lead or other anodic inclusions present.
Seeley's explanation is perfectly reasonable, since one possibility for the presence of these
corrosion spots on objects such as those in the Wallace Collection is that the corrosion origi­
nated in the 1 9 5 0 s when "pea-soup" smogs afflicted major cities such as London. The pollutants,
such as sulfur dioxide, were capable of reaching levels of thousands of parts per billion and also
significantly affected indoor air quality.
The general equation for copper sulfide formation is

Cu 2 + + xU 2S = CuS x + 2xH + 6.3

C H A P T E R S I X
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