Page 298 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Variants of verdigris One of the more accurate descriptions for verdigris preparation
from the Middle Ages comes from Theophilus, whose twelfth-
century work, De diver sis artibus, records recipes for two variants of verdigris: viride salsum
and viride hispanicum. His viride salsum was made as follows:

If you wish to make a green colour take a piece of oak of whatever length and width you
like, and hollow it out in the form of a box. Then take some copper and have it beaten into
thin sheets, as wide as you like but long enough to go over the width of the hollow box.
After this, take a dish full of salt and, firmly compressing it, put it in the fire and cover it
with coal overnight. The next day, very carefully grind it on a dry stone. Next, gather some
small twigs, place then in the above-mentioned hollow box so that two parts of the cavity
are below and a third above, coat the copper sheets on each side with pure honey over
which you sprinkle pounded salt, place them together over the twigs and carefully cover
them with another piece of wood, prepared for the purpose, so that no vapour can escape.
Next, have an opening bored in a corner of this piece of wood through which you can pour
warm vinegar or hot urine until a third part of it is filled, and then stop up the opening. You
should put this wooden container in a place where you can cover it on every side with dung.
After four weeks take off the cover and whatever you find on the copper scrape off and keep.
Replace it again and cover it as above. 15

Banik (i989) researched this recipe and concluded that the resulting products were often
mixtures of copper salts, including basic and neutral copper acetates, with some malachite and
I
additional phases that could not be characterized. f the salt predominates in this kind of mix­
ture, then the principal product will be atacamite, as found in the GCI Museum Research Labo­
ratory's work. However, the interaction of the honey with the other ingredients in this recipe is
of interest. Some nantokite, CuCl, was found in areas associated with the honey, which suggests
that the reducing sugars in the honey had a significant effect on the corrosion of the copper in
this case. How the nantokite would eventually transform to other products is difficult to resolve
at present, but further interaction between the nantokite and copper under humid conditions
could be expected to produce atacamite, an assumption confirmed by X-ray diffraction studies
at the GCI Museum Research Laboratory. Other phases were present in the material produced
by this recipe, as well, including a dark green substance that may be a copper chelate complex.
Maekawa and Mori (i965) suggest that this dark green complex may form in high humidity and
in the absence of light, which the recipe requires. The discovery that many so-called verdigris
pigments in illuminated manuscripts are actually basic chlorides that could easily have been
made by Theophilus's viride salsum technique attests to the historical veracity of these recipes.
The replication experiment for this recipe is given in APPENDIX B, RECIPE 13.

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