Page 328 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

In the acetate reactions, the following sequence may occur:

+ 4Cu(CH 3 COO) 2 = + 6CH 3 COONa 9.3
6NaAs0 2 3Cu(As0 2 ) 2 -Cu(CH 3 COO) 2
With the sulfate method, the reactions are:
+ 6NaAs0 2 + 2CH 3 COOH + = 9.4
4CuS0 4 Na 2 C0 3
+ H 2 0 +
3Cu(As0 2 ) 2 -Cu(CH 3 COO) 2 + 4Na 2 S0 4 C 0 2
The recipe given by Beam (1923) uses the acetate method and calls for 100 parts of soda ash
2
(sodium carbonate), 00 parts of white arsenic (arsenious oxide, As 2 0 3 ), 150 parts of soda ace­
tate (sodium acetate), and 50 parts of copper sulfate. The sodium carbonate is dissolved in
2
water, and half the required amount of white arsenic is added. Steam is then passed through
the solution until all the arsenic oxide is dissolved. The rest of the arsenic oxide is then added,
and the mixture is boiled until the arsenic oxide is completely dissolved, which can take more
than six hours. The resulting solution of sodium arsenite has a syrupy consistency and must be
diluted and allowed to settle. The copper sulfate is then dissolved to make a fairly concentrated
solution (1:25), aided by heating and constant stirring. When all the copper sulfate has dis­
9
solved and the temperature of the solution is about 0 °C, then the clear liquor of the prepared
sodium arsenite solution, after being filtered to remove any undissolved particles, is added at its
boiling point, with continuous stirring. The requisite amount of a dilute solution of acetic acid
or sodium acetate is then stirred into this mixture until the characteristic green color develops.
Care must be taken at this stage not to stir too much: to be properly formed, the pigment
must have a coarsely developed crystal structure to produce the brilliant green color. Likewise,
Beam urges great care in regulating the temperature of the reacting solutions, otherwise the
shade of the pigment will vary enormously; he discovered empirically that the best shades are
produced only at about 0 °C. The product is filtered and washed well to remove all soluble salts
9
and then dried at a low temperature. PPENDIX B, RECIPE 27, describes the author's laboratory
A
synthesis of emerald green using this recipe.
Two other, completely different synthetic preparations of emerald green were made us­
ing recipes derived from Gmelin (i965). The syntheses are described in APPENDIX B, RECI­
PES 28 and 29.
In addition to synthesizing emerald green using various recipes, two samples labeled emer­
ald green from the Forbes historical pigment collection were also examined during this study.
Powder X-ray diffraction patterns were taken for the these two historical pigments and for the
two different synthetic preparations made in the laboratory. The results are shown in APPEN­
DIX D, TABLE 26. Schweinfurt green has a characteristic array of d-spacings with an unusually
large number of strong reflections that span a large range of angstroms. The 1938 ICDD files con­
tain one minimal entry for emerald green (ICDD 1-51), which is shown with the data derived
3 3

T H E ORGANI C SALT S O F C O P P E R
311