Page 183 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Natural posnjakite, as well as a mixture of synthetic posnjakite and malachite, was found
in the paint layers of the fifteenth-century iconostasis from the Kirillo-Belozersky Monas­
tery, near Vologda, Russia. Naumova, Pisavera, and Nechiporenko (1990) found posnjakite in
the background paint on some pillars in the monastery. The microscopic pigment particles, or
"plates," were blue with a gray interference color and were anisotropic. The plates sometimes
reached ι mm in size, and, when viewed under crossed polars, they consisted of several mono-
crystals. These were grown together with different orientations and with the appearance of sec­
tors having low interference colors. The researchers note that the optical properties and crystal
structure of synthetic and natural posnjakite are identical, although the crystals of each have dif­
ferent morphological features. This was further investigated with laboratory synthesis trials,
which showed that a mixture of synthetic malachite and posnjakite can be coprecipitated as a
pigment. Details of this synthesis are given in APPENDIX B, RECIPE 6.
Van Eikema Hommes (i998) mentions the use of a mixture of ivory black and "couperose,"
an old term for a copper compound that would have been added to the ivory black, since
the ivory-black oil paint would have dried much more slowly without the addition of this
copper-based pigment. It is suggested that the term "couperose" refers to copper sulfate. Per­
haps a basic sulfate, such as brochantite, is meant, but the use of copper (II) sulfate pentahydrate
would be unlikely.
The copper-based pigment Peligot blue, named after its inventor, the French chemist
Eugène-Melchior Péligot (18I1-92), began to appear around 1858. Peligot blue may be prepared
with any soluble copper salt, but the sulfate is preferred (Riffault, Vergnand, and Toussaint
1874). A very dilute solution of the copper sulfate is treated with an excess of ammonia and
then precipitated by potassa (potassium carbonate) or soda (sodium carbonate). Exactly what
this recipe should produce is not entirely clear. In a laboratory replication experiment, begin-
ning with copper (II) sulfate and using potassium carbonate as the final addition, a light blue
powder precipitated from the solution and was identified by powder X-ray diffraction as a mix-
ture of brochantite and a synthetic copper sulfate hydroxide hydrate (of very similar formula to
posnjakite). Further details of this synthesis are also given in APPENDIX B, RECIPE 7.
It is interesting that the recipe for Peligot blue may produce brochantite, and this might
help explain why the basic sulfate is sometimes found as a pigment in stock from nineteenth-
century art suppliers. It was not uncommon at that time for paints to be fraudulently formulated
with copper minerals instead of the genuine pigment that was stated on the label, and this prac-
tice may still continue on a smaller scale today. Gettens and Fitzhugh (1974), for example, iden-
tified brochantite in a sample of powdered artificial green pigment supplied from old stock by
an English "colourman."

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