Page 302 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Take pure copper plates and hang them over strong vinegar. Place them in the sun with­
out touching them. After two weeks, open the container and take out the plates. Collect the
efflorescence and you will have very pure iarin. (Burnham 1920:13)
There are many variants to preparing verdigris with just copper and vinegar. For example,
Mactaggart prepared basic verdigris using malt vinegar and copper foil, as described in APPEN­
DIX B, RECIPE IS. 1 8 A verdigris sample prepared by the same method at the GCI produced an
X-ray diffraction pattern very different from that of Mactaggart's verdigris. A second attempt
was made using the same ingredients but with burial of the copper foil wrapped in a vinegar-
soaked cloth. This successfully produced crystals of both neutral verdigris and the lighter-blue
basic verdigris salts, which were characterized as being similar to compound B.
I LATE-MEDIEVAL PIGMENT RECIPES Manuscripts from the
fourteenth to the sixteenth century reveal a more detailed knowledge concerning the prepara­
tion of blue copper-based pigments. Most of these manuscripts prescribe verdigris rather than
copper metal as the starting material. A fifteenth-century manuscript, M s 1243 in the Biblioteca
Riccardiana, Padua, Italy (Merrifield 1849), gives this typical recipe for a durable azure:

[M]ix well one part of sal ammoniac and three parts of verdigris with oil of tartar until
it is soft and paste-like, or even softer. Then place it in a glass vessel under hot dung for
a day; afterwards, you will find that the green has turned to best blue. Another way of
making the best blue. R[ecipe] : mix together three parts of sal ammoniac and six parts of
verdigris with oil of tartar until soft and paste-like or even softer. Then place the paste into
a glass ampoule, and when it is well-stoppered and sealed, place it in a hot oven and let it
stand for some days; afterwards, take it out and you will find it turned into the best azure.
(Merrifield 1849:226)

Crosland (1962) identifies "oil of tartar" as a saturated solution of potassium carbonate.
Both methods mentioned in this recipe produce bright blue needles or cylinders mixed with
colorless crystals of varying morphology. Orna (i996) found that these blue crystals gave an
X-ray diffraction pattern that did not match any published data, and the product could not be
characterized. The same synthesis was attempted at the GCI Museum Research Laboratory using
the sal ammoniac, verdigris, and a variant of oil of tartar. APPENDIX B, RECIPE 19, describes
these results.
The synthesis appears to produce a mixture of products. One bright blue clump of crystals
was ground and analyzed by X-ray diffraction, which showed possible matches to potas­
sium copper acetate, 2K(CH 3 COO)-Cu(CH 3 COO) 2 , ammonium chloride, and/or ammo­
nium copper acetate acetic acid, C 1 4 H 5 0 C u N 4 O 2 0 , as shown in APPENDIX D, TABLE IS. Clearly,
this medieval recipe produces a blue product of a somewhat intractable nature. The range of

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