Page 293 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I DECOMPOSITION STUDIES Several recent studies have
derived thermal decomposition data for neutral copper acetate. Mansour (i996) found that
dehydration occurred at 190 °C and that there was only partial decomposition to cuprite and
tenorite at 220 °C. Maiti and Ghosh (i983) state that the decomposition products at 300 °C com­
prise cuprite, tenorite, and copper, with the final product of the thermal decomposition being
the copper oxide tenorite.

The history of verdigris An interesting historical account of the preparation of verdigris
in France from the twelfth through the nineteenth century was
given by Benhamou (i984). During this time, verdigris was used as an ingredient in oil-based
paints to embellish the exteriors of French country houses and Dutch homes; it also gave an
attractive green to sedan chairs. Verdigris was mixed with oil or distemper to illustrate postage
stamps and to tint paper, as well as to color woods for marquetry. In metallurgical processes it
was used in the soldering and coloring of gold jewelry. The tints and colors based on verdigris
were usually green, but the compound could also be used to produce black on both wood and
"
cloth, perhaps from tenorite formation within the fibers. Montet writes, I am told that another
common use of verdigris is to dye hats black; and a famous dyer of this city told me that he used
only verdigris to dye woolen cloth black" (Montet 1757:27).
Benhamou's historical account notes that the production and trade of this important pig­
ment was, interestingly, controlled by a monopoly of women for eight hundred years. Any man
who attempted to turn his hand to the task was ridiculed, a social technique that was apparently
effective in protecting this occupation from masculine encroachment. During this period, the
synthesis of verdigris started with 53.34 cm (21 in.) copper disks that were specially imported
from Sweden. These were cut into twenty-eight strips and hammered smooth before being
buried for three to four days in old verdigris to season them. The copper strips, which could be
used through a number of production cycles, were then placed in partially fired earthenware
pots, each holding about one hundred strips.
New jars were moisture proofed by soaking for about ten days in vinasse (wine vinegar),
distilled from a strong red wine. The jars were then scrubbed in the same liquid to cleanse them
of tartarous deposits. Grapes were used for chemical transformation of the copper. The grapes
were squeezed from their skins, sun dried, and pressed before being soaked in vinasse until they
doubled in size. The plumped grapes were then formed into large balls, sealed in the earthen­
ware jars with three pots of wine, and left to ferment for up to twenty days. At just the right
moment during fermentation, the liquid was drained from the jars, the grapes were layered with
heated copper strips, then left for three to four days until the strips developed white crystals.
The crystal-covered strips were then placed on a rack and wetted occasionally with wine or
vinasse, a process that continued for about a month, or until the strips developed the desired
green and spongy growth. 7

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