Page 182 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

ions, carbonate ions, copper ions, and sulfate ions, among others. The experimental results
indicate that the solubility product CK SP ) value of brochantite is exceeded, whereas that of mal­
achite is not. Because of these findings, Garrels and Dreyer concluded that most supergene
solutions derived from the weathering of primary copper sulfide deposits have a copper ion
concentration that is less than 0.001 M; f higher than this concentration, brochantite or atac­
i
amite would form.
This finding is of interest because calcium carbonates are often associated with copper
objects in sea burial or in land burial, and the work of Garrels and Dreyer helps to clarify the
circumstances of their formation. Since the copper ion concentration in concretions surround­
ing buried objects is always likely to be higher than 0.001 M during corrosion events, the
absence of malachite as a corrosion product is predicted by the theoretical data. In copper cor­
rosion crusts from marine burial, the basic chloride atacamite is often found, but malachite is
uncommon. In land burial, the basic copper sulfates can occur on copper alloy objects, but these
occurrences will always be rare compared with sulfate formation during outdoor corrosion.
Mattsson and coworkers (i996) and Schweizer (1994) discuss occurrences of the basic sulfates as
corrosion products identified on buried bronzes.

B A S I C S U L F A T E S AS PIGMENTS

Both natural and artificial basic sulfates have been used as pigments. Purinton and Waiters
(1991) found X-ray diffraction evidence for brochantite as the green pigment used in Zahhak
Enthroned with Two Sisters, a Persian painting from the Bukhara region of central Asia (now in
Uzbekistán) that dates to 1615 and is now in the collections of the Los Angeles County Museum
of Art. Naumova and Pisareva (1994) reported that various copper compounds were used as pig­
ments in Russian frescoes from the early sixteenth century. They concluded that most histori­
cal recipes for pigments do, indeed, refer to the production of copper acetates, with a small
number producing atacamite, as might be expected. But they also found that an eighteenth-
century recipe for an artificial blue pigment actually produced posnjakite, Cu 4 S0 4 (OH) 6 -H 2 0,
rather than a carbonate or an acetate. Banik (i989) identified langite, Cu 4 (S0 4 )(OH) 6 -2H 2 0,
the isomorph of posnjakite, as a green pigment in a European illuminated manuscript from the
sixteenth century. The identification method used by Banik, however, was severely restricted
by sample size and type; the pigment needed to be heated briefly to 105 °C to encourage a
crystalline product to form for identification by X-ray diffraction analysis. Naumova, Pisareva,
and Nechiporenko (1990) did identify langite as a pigment in early sixteenth-century frescoes
painted by the gifted Russian master Dionisy (ca. 1440- ca. 1503) in the Cathedral of the Nativ­
ity of the Virgin at the Ferapontov Monastery in Russia. They also found langite in The Holy
Family, a painting by the Italian Mannerist artist Agnolo Bronzino (1503-72) that is now in the
collections of the A. S. Pushkin State Museum of Fine Arts, Moscow.

B A S I C S U L F A T E S
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