Page 263 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The possibility that the excavated pigment cake could have been an alteration product from
the burial environment was not considered by the authors. Since sampleite has been found in a
number of different contexts as a corrosion product on copper alloy objects, however, the pos­
sibility must be considered that the formation of this compound could well represent the trans­
formation of a more common pigment, such as malachite, to sampleite.
Bridge and colleagues (i978) examined the occurrence of sampleite from Jingemia cave in
Western Australia and concluded that the mineral had formed from naturally occurring copper
sulfides in contact with guano. In their discussion of the nature and origin of these copper phos­
phates, Bridge and coworkers suggest that the similarity in the X-ray diffraction data between
sampleite and the synthetic copper phosphate Ο ι 3 ( Ρ 0 4 ) 2 · 3 Η 2 0 (ICDD file numbers 1-54 and
22-548), which was determined to be present as a corrosion product on ancient bronzes by Otto
(1959, 1963), may be a misattribution because the ICDD file data for sampleite were not avail­
able at the time of Otto's writing. This suggestion is not necessarily correct because additional
examples of the copper (II) phosphates were subsequently found, for example, from the Swed­
ish corrosion work of Mattsson and coworkers (i996).

Pseudomalachite: A copper The basic copper phosphate known rather oddly as pseudo-
phosphate pigment malachite (apparently, it does resemble malachite) is only rarely
found, despite occupying a large region of the stability diagram
(see FIGURE 7.1) and occurring in a more reasonable range of pH. In fact, pseudomalachite has
been found thus far only as a painting pigment and not as a corrosion product. Banik (1990)
identified the pigment in illuminated manuscripts of the sixteenth century. The only other
report of pseudomalachite used as a pigment is from the frescoes in the Cathedral of the Nativ­
ity of the Virgin at the Ferapontov Monastery in Russia (Naumova, Pisareva, and Nechiporenko
1990). The pseudomalachite, identified in paint layers of the saints' clothing in the wall paint­
ings and on the pillars of the monastery interior, was found as large dark-green masses with a
fibrous and concentrically zoned structure similar to malachite. The pigment was finely ground,
and the crystals had diverse shapes with sharp edges that are characteristic of the natural min­
eral. The extent to which this particular copper phosphate mineral was available for use as a
pigment is still not known.

T U R Q U O I S E

Turquoise is the only commonly occurring mineral phosphate of copper. It is a semiprecious
stone, relatively easy to abrade and polish, and often of a blue-green or gray-green color, although
the best quality stones are sky blue. The mineral does not occur as a corrosion product, but it
3
was commonly used as an ornamental stone for beads, inlays, and small carved stone amulets
and other objects throughout the ancient world. Reference is often made to two versions of this

C H A P T E R S E V E N
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