Page 131 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

bluish green verdigris, a copper acetate and one of the earliest artificial pigments, for use in
manuscripts (Laurie 1914).
The synthetic version of malachite is green verditer. It has the same powder X-ray diffrac-
tion pattern as the natural form, and it usually forms as a precipitate in the shape of spherulites.
Some of these globular particles may exhibit a dark cross when viewed under crossed polars.
Gettens and Fitzhugh (1974) identified this globular form in a cross section of a painting of
St. Vincent Ferrer by Francesco del Cossa (ca. 1435- ca. 1477), which dated to about 1472. This
confirmed that green verditer was already in use at that time. Because the synthetic product is
rather pale, however, its use became less common in Europe starting around the eighteenth cen-
tury (Harley 1970). Fitzhugh (1979) studied the Utagowa School of Japanese paintings dating
from the sixteenth to the nineteenth century in the collections of the Freer Gallery of Art, Wash-
ington, D.C. A green, copper-containing pigment was identified as malachite by X-ray diffrac-
tion, but it did not have the typical microscopic appearance of the natural mineral. The particles
were rounder than usual but not the typically rounded spherulitic particles found in the artifi-
cial version, so the origin of this pigment is still in question.
There is ample evidence that recipes for making artificial copper pigments go back to medi-
eval times. By around 1910, the recognized way of making synthetic azurite, known as blue
verditer, involved adding lime and potassium carbonate to copper sulfate, then treating the
precipitate for a number of days with sal ammoniac (ammonium chloride) and copper sulfate.
This process was described in the manuscript of Jean Lebègue (1368-1457), who compiled much
information concerning pigment manufacture and use during the Elizabethan era (Laurie 1914).
Sometimes the resulting product contained impurities of calcium sulfate and copper sulfate.
Laurie found that the artificial pigment could be distinguished from azurite because it was not
biréfringent, and when immersed in oil of cassia, its refractive index was lower than that of azu-
rite. 1 3 Some recipes, such as the one described here, will produce complex products. Most of
these recipes are discussed in the section dealing with verdigris compounds (see CHAPTER 9).

B L U E AND G R E E N V E R D I T E R
Use of blue and green Because azurite and ultramarine were so expensive, artificial
verditer in art blue copper pigments were used extensively in medieval paint-
ing. They were probably more significant at that time than all
other blue pigments together (Thompson 1932).
Laurie (1914), who notes that blue verditer was once synthesized in England in large quan-
tities, identified this artificial azurite in various English illuminated manuscripts—for example,
in folio 1826 of the Coram Rege Rolls, which dates from 1672. Laurie (1935) also mentions that blue
verditer was used throughout the eighteenth century in oil paintings; he identified it in Madame
de Pompadour by François Boucher (1703-70) in the National Gallery, Edinburgh.

C H A P T E R T H R E E
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