Page 326 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

example, the following syntheses can produce different products, all of which might go by the
name of Scheele's green: In a method used by Payen (i835), arsenious oxide was dissolved in
boiling water, and copper (II) sulfate solution was added until an aliquot gave a good color when
precipitated with potassium carbonate. In another process, Gentele (i906) used a solution of
potassium carbonate and potassium hydroxide that was added to a solution of copper sulfate. A
solution of arsenic trioxide and potassium carbonate was then added to this alkaline mix after
it had cooled to produce a copper diarsenite. (This process calls for the alkali to be dissolved
with the arsenic trioxide rather than with the copper sulfate.) A lighter shade of pigment was
produced by stirring boiling copper sulfate into the basic arsenite solution, which may, with
some difficulty, form copper metaarsenite. In a subsequent process, Church (i9is) recounts that
hot solutions of arsenious oxide and copper sulfate were mixed together and precipitated with
small additions of potassium carbonate until the color of the desired shade was obtained.
PLATE 60 shows two photomicrographs of a sample of Scheele's green from the Forbes pig­
ment collection. The sample was mounted in two ways: in melt-mount (RI 1.662) for polarized-
light microscopy, and for X-ray diffraction studies. The pigment was found to be composed of
clusters of small rounded particles with an anomalous blue-white birefringence. The particles
appear only faintly green or colorless under plane-polarized light as seen in PLATE OCA. The
same sample is shown viewed under crossed polars in PLATE 60B.
A photomicrograph of copper diarsenite is shown in PLATE 6i, and two views of copper
orthoarsenite are shown in PLATE 62. A number of copper arsenite mineral species are listed in
the ICDD files. Some of them may be germane to a discussion of Scheele's green, given that many
different routes were used historically to prepare the pigment. This group includes trippkeite,
CuAs 2 0 4 ; lammerite, Cu 3 (As0 4 ) 2 ; mixité, Cu 3 (As0 4 ) 2 - H 2 0 ; lindackerite, Cu 5 As 4 0 15 - 9H 2 0;
6
cornubite, Cu 5 (As0 4 ) 2 (OH) 4 ; clinoclase, Cu 3 (As0 4 )(OH) 3 ; and olivenite, Cu 2 (As0 4 )(OH),
although, insofar as it is known, the only possible comparison is with trippkeite.
A comparison of the X-ray diffraction data for trippkeite with the data for two samples of
Scheele's green from the Forbes collection reveals significant differences between the data sets,
as shown in APPENDIX D, TABLE 25. Both of the Forbes samples have peaks at 3.11, while tripp­
keite has a major peak at 3.16. However, the relative intensities for the d-spacings are not at all
consistent, and further research into the chemical relationships of the copper arsenites in gen­
eral is required to clarify the situation.
A sample of trippkeite from Copiapo, Atacama, Chile, from the collections of the Smith­
sonian Institution, appears very pale green under plane-polarized light and bright yellow green
under crossed polars with almost parallel extinction; some particles are rather undulóse. The
most impressive characteristic is that the crystals are composed of numerous fiberlike particles
that flake easily in melt-mount to fine acicular crystals. Examination with the standard quartz
wave plate showed that the individual crystals have second-order blue color parallel to the slow

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