Page 289 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

A number of subtleties in the assemblage of basic copper (II) acetates are listed in TABLE 9.2.
Rahn-Koltermann and colleagues could not produce [Cu(CH 3 COO) 2 ]Cu(OH) 2 -5H 2 0 (com­
pound B), for example, during their research and therefore concluded that it does not exist.
Gauthier found that manufactured verdigris could be any one of the compounds on the list or
mixtures of A and B, or Β and D. The basic salt Cu(CH 3 COO) 2 [Cu(OH 2 )] 2 (compound c) can
occur only as a single species on the basis of the equilibrium diagram, according to Gauthier.
Rahn-Koltermann's group utilized both powder X-ray diffraction and infrared spectros­
copy in determining data for compounds A, C, and D. They also found evidence for the existence
of a new phase not previously reported in the literature that has been assigned the formula
Cu(CH 3 COO) 2 [Cu(OH) 2 ] 4 -3H 2 0, which is analogous to compound D. This new substance is
labeled compound H. The existence of a new basic salt complicates the discussion of these com­
pounds. In fact, there may be a continuum of these basic copper (II) acetates comprising a num­
ber of salts with the following stoichiometry:

[
[Cu(CH 3 COO) 2 ]* Cu(OH) 2 ]y2;H 2 0 9.1
where typical values are χ = 1,y = 3, and ζ — . This would equate to compound D, as found
2
by Gauthier, although compounds A, B, C, and H can all be considered possible variants on this
general formula.
The only relevant neutral salt that has been well characterized, based on the most
recent ICDD files, is the neutral copper(II) acetate monohydrate (compound F). This salt,
Cu(CH 3 COO) 2 -H 2 0, has a distinctive blue-green color. As shown in TABLE 9.3, there are two
other neutral salts in the ICDD files with more limited relevance: the anhydrous copper(II)
acetate (compound E) and the anhydrous copper (I) acetate (compound G).
Compound F is the product most easily made with modern, chemically pure ingredients. In
the past, this neutral acetate often had to be prepared by dissolving crude verdigris in vinegar
and collecting the neutral verdigris crystals as grapelike clusters grown from the evaporation

TABLE 9.3 CHARACTERISTICS OF SOME COPPER ACETATES

CHEMICAL NAME FORMULA X-RAY LISTING CRYSTAL SYSTEM

copper (II) acetate (E) Cu(CH 3COO) 2 I C D D 2 7 - 1 1 2 6 tetragonal
copper (II) acetate hydrate (F) Cu(CH 3 COO) 2 -H 2 0 I C D D 2 7 - 1 4 5 monoclinic
copper (I) acetate (G) Cu(CH 3COO) I C D D 2 8 - 0 3 9 2 not known

a
Letters in parentheses refer to in-text citations of individual compounds.

C H A P T E R N I N E
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