Page 155 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

El Toro, Lower California Antofagasia Gila Co., Arizona

Crystalline varieties of atacamite (Palache, Berman, and
F I G U R E 4 . 4 Frondel 1951) .

also appeared. X-ray diffraction studies showed the green material to be atacamite. The suc­
cessful preparation of viride salsum by Theophilus's method is highly dependent on the reaction
conditions. f the mixture of copper, honey and salt is placed over red wine vinegar in a vessel
I
I
that is then well sealed, a good yield of atacamite can be obtained in a few days. f the reaction
vessel is not sealed, however, verdigris is the principal product. The formation of basic copper
8
chloride does not occur in strongly acidic conditions; therefore, it may be that partial evapora­
tion of acetic acid took place during the work of Naumova and Pisareva, resulting in a pH
increase that favored atacamite formation. This synthetic atacamite included a spherulitic form,
which Naumova and Pisareva state is comparatively rare, although this form is, in fact, common
in laboratory preparations. The spherulitic form of atacamite has been observed in only one
group of paintings: eighteenth-century frescoes from the Avraamovsky Monastery in Russia.

Pigment morphology In archaeological material, atacamite is often present as distinct
crystals; some typical morphologies are shown in FIGURE 4.4.
Photomicrographs of natural mineral atacamite from the collections of the British Museum,
Natural History, are shown in PLATE 29. The particles are a pale green with transmitted illumi­
nation and have a pale blue birefringence when viewed under crossed polars. Natural mineral
botallackite from the Levant mines in Cornwall, England, are illustrated in PLATE 30. Botal­
lackite crystals appear as clear green and have banded features when viewed under transmitted
light. By comparison, particles of botallackite identified by Fitzhugh (i988) in a fifteenth-
century manuscript from Iran (PLATE 31) are spherical with dark spots toward the center of the

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