Chemistry Reference
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titanium white, zinc white, and strontium yellow. The nominal proportion of
1 : 1 : 1 was approximately confirmed by the ratio of 0.94 : 1.04 : 1.03. The
precision was characterized by a relative standard deviation of only 4%.
The repeatability of the method was examined by a multiple sampling at 14
different spots of the modern oil paint cobalt green or Rinman's green (CoO
5
ZnO with key elements cobalt and zinc). A relative standard deviation of only
4% demonstrates a good repeatability. A quite different result was obtained for
14 different spots of a certain color on a real painting. The old Flemish painting
of Figure 5.23 was chosen and the“red”color of the Madonna's dress was
identified as a mixture of vermilion (key element mercury) and white lead (key
element lead). A high scattering of the results indicates that the artist (Van Den
Heuvel in Ghent, about 1646) has applied the“red”color in different mixing
proportions. Obviously, sampling is only representative for a small restricted
area of an oil paint.
The foregoing method is fast and convenient and has already been applied to
a systematic screening of numerous oil paintings under restoration, as well as to
murals, painted sculptures, and book illuminations [204]. The samples came
from 12 museums in the whole of Europe, and many questions of restoration,
conservation, and dating had been answered [203-205]. Some examples will
demonstrate the capabilities of TXRF.
The mentioned painting of Figure 5.23 had to be restored because it suffered
from the so-called“ultramarine disease,”which is feared in all museums
around the whole world. It has discolored the brilliant blue into a pale
greenish-gray in the dark folds of the blue coat. Ultramarine could not be
identified, since the key elements sodium and aluminum could hardly be
detected by TXRF without vacuum. Among blue pigments of the seventeenth
century, ultramarine might be recognized by the absence of copper and cobalt.
Indeed, both elements are absent in all blue parts of the coat, but the key
element cobalt was found in the discolored parts. Cobalt is characteristic for the
blue pigment smalt—a cobalt-bearing glass. Arsenic was determined as an
impurity because cobaltite (CoAsS) and smaltite (CoAs
2
) are sources of
cobalt [203,205]. Consequently, it was inferred that the ultramarine disease
is actually a smalt disease. The less valuable and cheap smalt pigment had been
used in previous restorations. The deterioration of smalt is based on the
migration of cobalt and cannot be reversed. For the future, only the precious
and expensive pigment ultramarine has to be applied for restoration.
Another Flemish oil painting was also examined before restoration. It is
called“Piet Hein goes ashore”and was painted by Reintjens in 1846. A white
cloud on a blue sky was sampled and the white color was identified as a
mixture of permanent white (key element barium), white lead (key element
lead), and titanium white (key element Ti) with a mixing portion of 1 : 3 : 1.
The appearance of TiO
2
on a picture of 1846 arouses suspicion because
titanium white was not discovered until 1908 and was generally not applied
on paintings before World War II. However, the painting had been restored
in the 1950s [197].
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