The efficiency of micro-Raman spectroscopy in the analysis ofcomplicated mixtures in modern paints: Munch's and Kupka'spaintings under study
2016
Twenty one mock-up samples containing inorganic pigments
primarily used at the turn of the 19th and 20th century were
selected for comparative study and measured by micro-Raman and
portable Raman spectrometers. They included pure grounds
(chalk-based, earth-based and lithopone-based), grounds covered
by resin-based varnish, and different paint layers containing
mixtures of white, yellow, orange, red, green, blue and black
pigments, usually in combination with white pigments (titanium,
zinc and barium whites or chalk). In addition, ten
micro-samples obtained from seven paintings of two world-famous
modern painters Edvard Munch and Frantisek Kupka have been
investigated. Infrared reflection spectroscopy (FTIR), portable
X-ray fluorescence (XRF) and scanning electron microscopy with
energy dispersive spectroscopy (SEM–EDS) were used as
supplementary methods. The measurements showed that blue
pigments (ultramarine, Prussian blue and azurite), vermilion
and ivory black in mixture with whites provided characteristic
Raman spectra, while Co-, Cd- and Cr- pigments' bands were
suppressed by fluorescence. The best success rate of
micro-Raman spectroscopy has been achieved using the 780 nm
excitation, however, the sensitivity of this excitation laser
in a portable Raman instrument significantly decreased. The
analyses of micro-samples of paintings by E. Munch and F. Kupka
showed that micro-Raman spectroscopy identified pigments which
would remain unidentified if analyzed only by SEM–EDS (zinc
yellow, Prussian blue). On the other hand, chromium oxide green
and ultramarine were not detected together in a sample due to
overlap of their main bands. In those cases, it is always
necessary to complement Raman analysis with other analytical
methods.
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