Page 325 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Riffault, Vergnand, and Toussaint (i874) provide the following commentary on making
Scheele's green:

Oxide of copper, combined with various substances, produces quite a number of green
colours, which, unhappily, are highly poisonous, but possess great brightness. The oldest
of these colours is a neutral arsenite of copper, discovered in 1778 by Scheele. Dissolve in
a copper kettle ι kilogramme of pure sulphate of copper in 20 litres of water. In another
vessel prepare an arsenite of potassa by boiling 1 kilogramme of carbonate of potassa and
325 grammes of arsenious acid in 6 litres of water. These two solutions are filtered, and
while they are still hot the arsenite of potassa is slowly poured into the solution of sulphate
of copper, which is stirred all the while. The precipitate of arsenite of copper settles in the
liquor which has become a solution of sulphate of potassa. This is decanted, and the pre­
cipitate is carefully washed with hot water, drained upon a cloth and dried at a low tem­
perature. (Riffault, Vergnand, and Toussaint 1874:234)

This recipe was replicated in the laboratory and produced a dark green precipitate of very fine
particle size. After the precipitate was washed in distilled water and allowed to air-dry for four
days, it was ground and analyzed using the Siemens D5005 X-ray spectrometer with Gobel mir­
rors attached. A reasonable X-ray diffraction pattern was obtained from this preparation. These
X-ray diffraction patterns reveal considerable discrepancies among preparations, confirming
that different pigments were made depending on the route employed to produce them.
Two of the distinct copper arsenites mentioned earlier were prepared in the GCI Museum
Research Laboratory following recipes from Gmelin (i965) : the synthesis of copper orthoarsen-
ite, 3CuO*As 2 0 3 -2H 2 0, is described in APPENDIX B, RECIPE 25, and that of copper diarsen-
ate , 2CuO-As 2 0 3 -2H 2 0, in RECIPE 26. The pigment was also synthesized using Scheele's
I
method: 10.5 g of sodium carbonate was dissolved in 60 ml of water, and the solution was heated
on a hot plate to about 90 °C. Next, 3.25 g of arsenious oxide was slowly stirred into the solution
until completely dissolved. Separately, 10 g of cupric sulfate was dissolved in 200 ml of water,
then heated to the same temperature. The sodium arsenite solution was then added slowly, with
stirring, to the cupric sulfate solution; the resulting precipitate of copper arsenite was washed
in hot water and then dried at 45 °C.
Interestingly, powder X-ray diffraction studies of the product made with Scheele's method
showed that it is essentially amorphous: no pattern whatever was obtained from this solid
precipitate. The recipe from Riffault, Vergnand, and Toussaint, however, did produce a
crystalline solid.
A variety of chemical syntheses can be used for the preparation of this pigment, and
this complicates the diagnostic data for Scheele's green (Fiedler and Bayard 1997). In fact, the
entire subject requires further research to unravel the subtleties of these copper arsenites. For

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