Page 292 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

curved. The relief varies on rotation, showing differences in refractive index, the principal RI
determined was μ = 1.548, the other closer to 1.53. The particles tend to have undulóse or no
clear extinction under crossed polars. They show an intense birefringence, the greener particles
retaining a finely variegated light green color under crossed polars, while the lighter blue-green
fibrous particles appear strongly colored with intense blue and light yellow color. Individual
fibers that can be seen show a positive sign of elongation. In preparations at the GCI Museum
Research Laboratory, some of these Β group compounds also occurred as irregular crystalline
fragments of pale blue-green color rather than as fibrous particles. Kuhn (1993) quotes the RI of
basic verdigris as 1.56 and 1.53. These are similar values to those found here for compound B, but
some of the other basic salts have refractive indexes higher than these two values.
Basic verdigris compound c, visually a pale turquoise color, is quite distinctive since it
consists of a mass of well-formed, acicular crystals, some perfectly rectangular and prismatic in
shape. Under plane polarized light, the acicular clumps look light green in color, some of the
crystals curved and fibrous, but the majority appearing as long prismatic needles that have par­
allel extinction under crossed polars. Under plane polarized light, in Cargille liquid of 1.580, the
needles almost disappear on rotation, showing that one RI is very close to 1.580 parallel to the
long axis of the crystal. The RI is lowest at 45 to the long axis and about 1.560. The needles
o
appear blue parallel to the wave plate, indicating that they have a positive sign of elongation.
Basic verdigris compound D macroscopically appears a light green-blue color and micro­
scopically is composed of pale light green to transparent crystalline fragments often tinged with
light blue green. These particles show only low to moderate relief in a melt-mount of RI 1.662
without any noticeable change in relief on rotation under plane polarized light. Under crossed
polars, the particles appear intensely colored with light straw and strong light blue colors. Some
particles appear veined with microcracks or fissures and twinkle on rotation under crossed
polars with no clear extinction. According to Yamanaka (i996), compound D has a layered struc­
ture of the botallackite type. The acetate ions are located between the positively charged copper
hydroxide layers and can be exchanged with various other anions. This may partially explain
why some of these verdigris samples may be difficult to fully characterize, because the presence
of other anions —such as chloride, sulfate, or nitrate —may produce slight variants of the par­
ent compound by substitution of some of the copper hydroxide layers. This substitution may
also help to explain the considerable variation in the analytical data for these verdigris com­
pounds, particularly the powder X-ray diffraction data.
Reexamination of the melt-mount preparations after two years in storage showed that
some of them had already undergone a reaction with the mounting medium; the resin was
slightly discolored and the particles partially dissolved. Preparations in melt-mount should
therefore be examined and photographed when freshly made.

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