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Posted: April 3, 2007

Regaining a historic sunset with surface-enhanced Raman spectroscopy

(Nanowerk News) US chemists are racing against time to recreate sunsets which have disappeared from the watercolors of American painter Winslow Homer (1836-1910).
The Art Institute of Chicago is planning a major exhibition of Homer's work in Spring 2008. But art conservators working behind the scenes have discovered that the striking white skies in watercolors like 'For to be a farmer's boy' (pictured) were originally painted in unstable red and orange dyes which have faded almost entirely - leaving behind only a few particles of pigment, invisible to the naked eye.
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Striking white sky was originally a red and orange sunset (Winslow Homer, 1836-1910) (Image: Art Institute of Chicago)
Now researchers at Northwestern University, Illinois, are hoping to work out the skies' original colors, in time for the exhibition.
But no conventional analytical technique can identify Homer's red 'lake' pigments (made by precipitating an organic dyestuff, like madder, brazilwood, lac or cochineal, onto an inert inorganic binder such as alumina or chalk). The problem is that many are fluorescent, crowding out the Raman spectroscopy normally used to fingerprint artists' palettes - while not enough dyestuff is left on the artworks for high performance liquid chromatography.
Instead, Francesca Casadio, a conservation scientist working at the Art Institute, is hoping to apply surface-enhanced Raman spectroscopy (Sers), a technique pioneered in 1977 by Richard Van Duyne at Northwestern University. Sers dramatically increases the number of photons scattered by the Raman effect, making it a more sensitive fingerprinting tool, and quenches fluorescence. This is due to surface electromagnetic waves (plasmons), excited by laser light, which enhance the Raman signals of molecules adsorbed on a thin hundred-nanometre silver layer coating a substrate of silica spheres.
'Without Sers it would be impossible to work out what pigment was where,' said Casadio. The idea, she said, is to take tiny particles of red lake pigment from the painting and prepare them on a Sers substrate. Casadio, Van Duyne and graduate researcher Alison Whitney have together built up a Sers lake pigment fingerprint library for comparison, but they will now have to move from pure compounds to real, potentially contaminated, artworks.
To crack Homer's sunsets, said Casadio, the team will need to work out a reliable way of preparing microscopic watercolor samples for Sers analysis. The group is using 19th century red lake dye recipes to perfect the intricacies of extracting dyestuffs and preparing samples, with the Netherland Institute for Cultural Heritage, Amsterdam.
'We're hoping we can work this out before the exhibition,' said Casadio. Conservators would never repaint altered skies onto Homer's original watercolors, she stressed, but a digital recreation is possible. Even this would be subjective in the nuances of pigment shade and tone. Without modern chemical analysis, though, Homer's faded sunsets would remain forever lost to view.
Source: Chemistry World (Richard Van Noorden)
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