Page 301 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The sets of three experiments given in APPENDIX B, RECIPE 16, are broadly similar to this one,
taking strips of copper and exposing them over warm vinegar. The experiments were carried
out in the laboratory using strips of pure copper, bronze, and brass exposed to a variety of mod­
ern vinegar types. In virtually all of this experimental work, X-ray diffraction analysis showed
that the green product made was neutral verdigris; in one case, a mixture of two basic acetates
was produced.
Another Mappae clavicula recipe exposes copper to urine:

[recipe 96] Coat copper, beaten out into sheets with honey and put beneath it in a pot broad
laths of wood and pour over it a man's urine. Let it stand, covered for fourteen days. (Smith
and Hawthorne 1974 : 4i)
This recipe was replicated using oak wood chunks and a pure copper sheet covered with urine
in ajar for fourteen days. A darkish green product formed only in small patches over the cop­
per sheet. It was poorly crystalline, giving no powder X-ray diffraction pattern. Examination of
the same product with Fourier transform infrared (FTIR) spectroscopy showed significant peaks
in the spectrum, associated with nitrogen bonds, indicating that one of the major products must
be a compound between copper (II) and one of the nitrogenous compounds of urine, such as
urea, uric acid, or creatinine. This is not unexpected, given the reactivity of copper ions with
both oxygen and nitrogen groups. The identity of the compound is currently being investigated.
The following Mappae clavicula recipe uses copper filings:
[recipe so] Changing copper—take six minas [unit of weight] of salt and four minas of
filings or scrapings of copper. Mix the filings in a pot with ground salt, sprinkling vine­
gar over them and leave for three days and you will find it has turned green. (Smith and
Hawthorne 1974:38)

The replication experiment found that the copper filings did indeed turn green, with the prin­
cipal product identified as atacamite, as described in APPENDIX B, RECIPE 17.
Many historic recipes call for the preparation of verdigris using copper sheets or strips that
are smeared with honey to which salt is added; the sheets or strips are then hung over vinegar.
These recipes invariably produced atacamite. The resulting product was a mixture of light green
and darker green mottled crystals over the honey-smeared copper surface. Copper compounds
enveloped the sodium chloride, and concentric or globular crystals formed over the copper.
X-ray diffraction analysis of the resulting products showed them to be atacamite.
Orna, Low, and Baer (i98o) provide some important information on manuscripts from the
ninth to the sixteenth century. In addition, an eighth-century Latin manuscript from Lucca,
Italy, known as Codex Lucensis 490 or Compositiones variae 17 (Burnham 1920), contains a recipe
that tells how to make a product called "flowers of copper," identified as "iarin," which appears
to be verdigris:

C H A P T E R NIN E
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