Page 166 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

accurately identify antlerite; he describes it as an alteration product in the patina formed
on a copper roof that had been exposed for thirty years, representing more "acidic" conditions.
Gettens (1933) mentions antlerite as a predominant species in an exposed bronze patina less
than forty years old, and Marchesini and Baden (1979) reported finding the mineral on the
Horses of San Marco in Venice. Since then, antlerite has begun to make more regular appear­
ances in the literature. It was identified on Lorenzo Ghiberti's (ca. 1378-1455) gilded bronze
Gates of Paradise at the Baptistery of San Giovanni in Florence, on the equestrian monument of
Marcus Aurelius in Rome, and on the Statue of Liberty in New York (Baboian and Cliver 1986).
In Philadelphia, it was found on the statue of William Penn by Alexander Milne Calder (i846-
1923) and on Swann Fountain by Alexander Stirling Calder (1870-1945), according to Lins and
Power (1994), who carried out a detailed study of the formation of these basic copper sulfates on
exposed bronzes in polluted urban sites.
Antlerite is often assumed to be present in the corrosion crust of exposed bronzes because
the pH of rainwater has become more acidic since the mid-twentieth century. This suggests that
antlerite is an indicator of low environmental pH conditions, although this has been disputed.
The major problem with low pH rainwater is the potential dissolution of the patina on an
exposed bronze, leading to streaking and surface disfigurement. Laboratory studies show that
at pH 2.5 both cuprite and copper can react to form chalcanthite, CuS0 4 -5H 2 0, or bonatite,
CuS0 4 -3H 2 0, rather than brochantite or antlerite. At pH levels higher than 4, cuprite dissolu­
tion is notably slower than that of the basic sulfates. The results show, however, that acidified
copper sulfate solutions are readily produced from corrosion crusts of bronze and copper sub­
strates. These solutions may influence cuprite growth according to the following reaction:

Cu + CuS0 4 + 2 H + + Vi0 2 = Cu 2 0 + H 2 S 0 4 5.1

The dissolution of the basic sulfates in the outermost layer of the corrosion crust is initially
accompanied by an increase in pH. For antlerite, this is represented by

= 3Cu 2 + + S0 4 " + 4 0 H " 5.2
2
CuS0 4 -2Cu(OH) 2
or
+ 4 H + = 3Cu 2 + + 2 - + 4 H 2 0 5.3
CuS0 4 -2Cu(OH) 2 S 0 4
For brochantite, this is represented by

= 4Cu 2 + + S0 4 ~ + 60H~ 5.4
2
CuS0 4 -3Cu(OH) 2
or
+ 6 H + = 4Cu 2 + + S0 4 " + 6 H 2 0 5.5
2
CuS0 4 -3Cu(OH) 2

BASI C S U L F A T E S
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