Page 291 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Josaphat, a German illuminated manuscript in the collections of the J. Paul Getty Museum
(83.MR.i79). The manuscript, which dates to 1469, is by Rudolf von Ems (ca. 1200-1254), with
illustrations by the Diebold Lauber atelier. No previously published X-ray diffraction data pro­
vide a match with this German verdigris sample.
Another example of the application of this data is shown in APPENDIX D, TABLE 16. Data
for a verdigris synthesis from a recipe in the Mappae clavicula (Smith and Hawthorne 1974 :
sec. 6) that uses copper foil, soap, and vinegar are compared with data from the Schweizer
and Mühlethaler version of compound Β and from the author's synthesis of compound A. Also
shown in this table are data from the synthesis of a basic verdigris by Mactaggart. 6 The com­
parison shows that, apart from matching a few lines of some compound A and Β data, the Mac­
taggart sample is principally the neutral verdigris salt, compound F.
I OPTICAL PROPERTIES OF VERDIGRIS Verdigris produced
from the exposure of copper foil to vinegar is in the form of clear, light blue-green tabular crys­
tals with a refractive index of less than 1.66. When the crystals are mounted in a melt-mount
of index 1.539, they have a slightly higher index than the medium, suggesting that they have
an index of about 1.55. The crystals reveal a second-order blue parallel to the slow direction of
the quartz wave plate, indicative of a positive sign of elongation. Not all verdigris samples have
clear extinction; some, such as the sample of distilled verdigris from Mactaggart illustrated in
PLATE 55, showed undulóse extinction and mixed blue-straw colors with the quartz wave plate.
Some basic verdigris crystals show a long fibrous aggregation of curved crystals.
Optically, the copper acetates are usually blue green and of variable crystalline size and
shape. The crystals are, in general, anisotropic with pale blue birefringence and refractive
indices of less than 1.66. Some typical polarized-light microscope preparations are shown
in PLATES 55-57.
The most common salt, neutral verdigris (compound F), is a deep blue green; under the
microscope, it is characterized by crystalline fragments that often show conchoidal fracture and
clear relief when mounted in a melt-mount of RI 1.662. The particles tend to show a uniform
light blue color without visible defects within the crystalline fragments, which have a refractive
index of about 1.55. When viewed under crossed polars, the neutral salt reveals muted brown,
yellow, and dark blue colors, none of them very intense, and often with clear extinction.
Basic verdigris compound A is a pale blue or blue-green color and under the microscope
appears as turquoise-colored fibrous crystals. The brushlike crystal aggregates show intense
green, yellow, and blue birefringence under crossed polars with some crystals appearing almost
white. The size of the individual fibrous crystals is very small.
Basic verdigris compound Β is a deep blue-green color that appears dark green to deep blue
green under the microscope and may be composed of at least two different crystal forms. Most
of the particles appear to be composed of bundles of fine fibers of varying orientation, some

C H A P T E R N I N E
274