Page 287 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

crystal structure was determined by Tamura and coworkers (i98i). The structure consists of infi­
nite zigzag chains with HCOO and OH groups bridging neighboring copper ions. The structure
is of interest since many other formate salts adopt a dimeric structure rather than an infinite
chain structure.
In one of the first studies of the basic formates, Fowles (1915) found that when solutions
of the neutral copper(II) formate were boiled, 2Cu(HCOO) 2 -Cu(OH) 2 -2H 2 0, an insoluble
basic salt, was formed. Fowles observed that copper (II) hydroxide readily combined when
shaken with a cold solution of copper formate to form the salt Cu(HCOO) 2 -3Cu(OH) 2 that
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tended to revert to Cu(HCOO) 2 -2Cu(OH) 2 . f the salt, Cu(HCOO) 2 -3Cu(OH) 2 , freshly pre­
pared, is added to a dilute solution of the neutral formate, it will change into a heavy, green crys­
talline powder after ten weeks. f an excess of a saturated solution of the neutral salt is used,
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complete conversion requires seventeen days. The compound obtained, Cu(HCOO) 2 * Cu(OH) 2 ,
is a fine, emerald green color.
If copper strips are kept in an atmosphere of 80% RH with 200 ppm of formic acid, dark
green corrosion spots form on the copper surface. Powder X-ray diffraction analysis shows data
very similar to that for the laboratory synthesis of 2Cu(HCOO) 2 -Cu(OH) 2 -2H 2 0. This illus­
i
trates the potential importance of the new data; f only the ICDD ( JCPDS) files were relied on
for reference information, the corrosion formed on the copper strips would remain unidenti­
fied. Two refractive indices could be determined from the mounted crystals in Cargille liquids,
one at μ = i.620 and the other at about 1.652.
Pey (1998) makes an unusual mention of the formate anion in reference to the characteri­
zation of some green pigments found in the Hafkenscheid collection, a historical collection of
Dutch pigments that dates from the early decades of the nineteenth century. Pey describes one
pigment, papegaaigroen (parrot green), as "possibly a formo-arsenite," which would make par­
rot green an analog of Schweinfurt green; 3 no further analytical data are provided, however.

THE COPPER A C E T A T E S

The chemistry of verdigris The preeminent organic salt of copper is verdigris, essentially a
copper acetate, of which there are a bewildering number of pos­
sible varieties or mixtures. The name, however, is not always specific to copper acetate. Some
compounds called verdigris may actually be a mixture of carboxylate salts, or they could be basic
copper chlorides, carbonates, or other more exotic compounds.
Verdigris, which ranges in color from pale blue through turquoise to green, was made for
use as a synthetic pigment or as a medicinal preparation. It is also an undesirable and disfigur­
ing corrosion product on copper or bronze alloys. Verdigris has been known from at least Greek
and Roman times and is mentioned by Theophrastus and Dioscorides. The origin of the word
aeruginous, often used to describe "copper rust" (verdigris) goes back to Pliny.

C H A P T E R NIN E
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