Page 304 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
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gently by dipping the brush in the vinegar, and afterwards grind up the colour with a little
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roch alum and a little gum arabic, so as to make it into a cake. (Merrifield 1849:658, 6o).
Since corrosive sublimate is mercuric chloride, this recipe may have been corrupted by
incorrect copying of the original source. Consequently, the synthesis was not attempted in the
laboratory. It is interesting, however, to see how alum and ammonium chloride commonly
appear in recipes for these synthetic greens and blues.
This recipe in M s 992 calls for "tartar of Bologna," which is approximated by the modern
cooking aid "cream of tartar" (sodium potassium tartrate):
[recipe 17] To make another brilliant green—Take oz. vi of the best verdigris, oz. ij of tar
tar of Bologna, and dr. j ss. [weight units] of roch alum.; pulverize the whole, and grind each
article separately, then grind them together rather stiffly with distilled vinegar, put the pow
der into a glass vase with a little saffron, and expose it to the sun. Then pour on it a bocale
[jug] of distilled vinegar, and the longer it is exposed to the sun the more beautiful will be
the colour. (Merrifield 1849:660).
"Roch alum" is potassium aluminium sulfate. A recent laboratory synthesis of this recipe is
given in APPENDIX B, RECIPE 20.
The same manuscript contains a recipe for refining verdigris:
[recipe 32] How to refine verdigris—Take the verdigris, grind it well, steep it in the best
vinegar for 3 or 4 days, strain it, then pour the strained liquid on other well-ground verdi
gris; let it settle for 2 days more, strain it again gently, leaving the lees of the verdigris at the
bottom of the vase; put the liquid which has been strained in a glass vessel with a little saf
fron, and keep it well covered. (Merrifield 1849:664).
This recipe is fairly straightforward: crude basic verdigris is dissolved in good quality vinegar.
The residue is decanted from the solution, which is then left to slowly evaporate in a covered
vessel. This will produce a crystalline deposit of refined, neutral verdigris.
Another recipe from M s 992 for green verdigris is as follows:
[recipe 33] A most beautiful green colour—Take the powdered verdigris, dissolve it with
lemon juice, and let it settle for 24 hours; then strain the most fluid portion very carefully,
leaving the lees at the bottom of the vase. Put the strained liquid into a glass vase, and add
to it a little of the above-mentioned pasta verde, let it dry, and when you use it, add to it
some more lemon juice, and the more you add the more beautiful the colour will be, so that
it will be like an emerald; take care, however, that you do not permit the brush to touch
water. (Merrifield 1849:664).
T H E ORGANI C SALT S O F C O P P E R
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