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Pigment Microscopy September 29, 2000
by Chris Hellier

A sophisticated fiber-optic probe connected to a miniature color video camera is aiding researchers in dating and authenticating painted statues and frescoes. This method, known as Raman microscopy, determines the structure and nature of molecules by scattering light and allows researchers to work directly on artifacts without having to remove precious samples. It is "the newest--and arguably the best--single technique to be applied to the area," says Robin Clark of the Christopher Ingold Laboratories, University College London, which pioneered this area of research.

In addition to painted statues and frescoes, Clark has also applied the technique to a range of manuscripts and artifacts including illuminated papyri, 18th Dynasty Egyptian faience, Persian manuscripts, medieval bibles, and Syriac lectionaries. Advances in the sensitivity of Raman microscopes have increased the number of pigments, including indigo, verdigris, and Indian yellow, that can now be identified using the technique. Clark and colleague Peter Gibbs have compiled a list of some 60 natural and synthetic pigments identifiable with Raman microscopy and dated by year of their first known use or synthesis. A larger compilation of post-1850 pigments awaits publication.

In a recent case study of illuminated papyri, supposedly dating back to Ramses II (r. 1279-1212 B.C.), the researchers were able to prove that the illustrations, although skillfully done, were, in fact, modern. Pigments used included Prussian blue, first synthesized in 1704, and synthetic anatase (titanium dioxide), first created as pure white in 1923.

The use of laser-induced breakdown spectroscopy (LIBS), an older method of spectral analysis for studying the composition of materials using lasers, has also recently been used for the first time in conjunction with Raman microscopy to examine the pigments on icons in Heraklion, Greece. "Hardly any laboratories are yet involved in this work," says Clark, "but interest is rapidly developing in both Europe and the U.S."

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© 2000 by the Archaeological Institute of America
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