Page 132 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Synthetic malachite (green verditer) has been identified in several fifteenth-century Italian
panel paintings in the collections of the National Gallery, London (Roy 1993). Technical exam­
ination of a fifteenth-century polychrome terra-cotta relief from Siena, Italy, at the National
Gallery of Art, Washington, D.C. (Edmonds 1997), reveals that synthetic spherical malachite was
used throughout. The major period during which this synthetic pigment was introduced, there­
fore, seems to be the fifteenth century, although where it was being made and by what process
is still unclear. Blue and green verditer were also frequently used as pigments for decorative
facades. In a 1769 portrait by John Zoffany (1733-1810) of Sir Lawrence Dundas sitting in
his library at Arlington Street, London, much of the wallpaper is depicted as green. Assum­
ing that the artist was representing the actual wallpaper color, Bristow (i996) suggests that the
only green pigment available at the time for such decorative purposes was green verditer, as
described in APPENDIX B, RECIPE 4.

Synthesis of blue Blue verditer can be synthesized by adding calcium carbonate
and green verditer to copper sulfate. Mactaggart and Mactaggart (i98o), who pre­
pared both blue and green verditers, showed that the blue basic
carbonate precipitated at low temperatures and under more controlled conditions than those for
the formation of green verditer. The crystal size of the blue precipitate was much finer than that
of natural azurite, with a corresponding paler color, but X-ray diffraction data show it is essen­
tially identical to the natural mineral. The verditers made by the author at the Getty Conserva­
tion Institute Museum Research Laboratory were highly biréfringent, with a dark cross visible
when viewed under crossed polars. Preparations of green and blue verditer viewed with polar­
ized light microscopy are shown in PLATES 24 and 25.
During the nineteenth century, a synthetic azurite known variously as English blue
ashes, copper blue, or mountain blue was made by pouring a solution of commercial potash
1 4
(potassium hydroxide) into a solution of copper (II) sulfate. Riffault, Vergnand, and Toussaint
(1874) report that this formed a precipitate of copper carbonate. 15 The precipitate was then
ground with lime (calcium hydroxide) to which a small amount of sal ammoniac (ammonium
chloride) was added. Lime decomposes sal ammoniac into an ammoniacal copper salt with a
deep blue color.
Naumova, Pisareva, and Nechiporenko (1990) were able to prepare a synthetic malachite by
adding sodium bicarbonate, either as a powder or a solution, to a copper sulfate solution. At a
concentration ratio of HC0 3 ~ to S0 4 ~ greater than 3, an emerald-green precipitate formed,
2
2
consisting of spherulitic malachite with a diameter of 10-20 μιη. The same researchers iden­
tified synthetic malachite from frescoes in the Cathedral of the Nativity of the Virgin at Fera-
pontov Monastery in the Vologda region of Russia. The pigment, which consisted of small
spherulites, was found in background areas painted green. These green areas also contained

BASI C C O P P E R CARBONATE S
115