Page 113 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Tenorite may form by alteration of other compounds, as shown by Gutscher and cowork­
ers (i989), who investigated the transformation of azurite (copper carbonate) into tenorite in
wall paintings. X-ray diffraction studies of the paint film revealed an agglomeration of tenorite
particles growing on the azurite, which had been used as a pigment. This mineral complex
was in close proximity to calcite, CaC0 3 , which was also present in the paint film along with
two different calcium silicates, Ca 2 Si0 4 - 0.3H 2 O and Ca 2 Si0 4 - 0.35H 2 O, and traces of calcium
oxide, CaO. The presence of the calcium compounds implies the existence of a high local pH,
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which is one of the conditions favoring tenorite formation. Replication experiments showed
that the mineral azurite could be altered into tenorite f mixed with calcium oxide in the pres­
i
ence of moisture. The conversion of azurite to tenorite has also been observed on a polychrome
sculpture excavated from an alkaline environment.
FIGURE 2.1 shows Pourbaix diagrams for the system C u - C 0 2 - H 2 0 at 25 °C with C 0 2 at
levels of 44, 440, 4400, and 44,000 ppm. Based on these diagrams, alteration of azurite to
tenorite is expected at levels of 4400 ppm and 44,000 ppm, as shown in FIGURES 2.1c and 2.ID,
respectively. These carbon dioxide levels are greater than ambient concentrations, which are
approximated by the Pourbaix diagram for 44 ppm C0 2 , as shown in FIGURE 2.1A. Azurite pig­
ment particles were found near carbonates in the paint layer on the polychrome sculpture, again
suggesting that even at ambient C 0 2 levels, localized areas of higher carbon dioxide concentra­
tion can occur and promote tenorite formation. Where environments are in contact with the
atmosphere—for example, at about 0.8 Eh and at a pH between 8 and 14—tenorite, rather than
malachite or azurite, is the expected stable phase. The alteration of malachite to tenorite in wall
paintings under these conditions has not been reported, as far as the author is aware, but could
be expected to occur under the highly alkaline conditions reported by Gutscher and coworkers.
One of the earliest reports describing the blackening of copper pigments in wall paintings
that could be related to tenorite formation was by Lucas (1934), who studied the color change of
trefoil marks on a painting of a couch in the form of a cow from the tomb of Tutankhamun. He
described the markings as being "now of a very dark brown, almost black colour which mani­
festly were blue originally and still show a little blue underneath the black" (Lucas 1934:286).
Orna, Low, and Baer (1980) proposed that the original pigment in the painting examined by
Lucas may have been covellite, CuS, which is a light-to-dark indigo blue and can be found as a
natural mineral. There is no evidence, however, that covellite was ever used as a pigment in
Egyptian wall paintings; the traditional colorant was Egyptian blue. Another example of pig­
I
ment alteration given by Lucas is from wall paintings in the tomb of Amenophis I , 1 2 where the
blue coloration had darkened in places and become almost black. Lucas noted that this black­
ening was not due to smoke or carbon, but he did not supply an analysis of the altered mate­
rials. It is quite possible that the color change in some of these wall paintings is due to the
formation of tenorite rather than discoloration of the Egyptian blue pigment.

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