Issue Date: December 17, 2012
Many analytical chemists find themselves drawn to cultural heritage science and the opportunity to use their expertise to study beautiful pieces of art or historically important artifacts.
For Robin J. H. Clark, the opportunity first came in 1992, when the University College London chemist was asked to identify blue pigments in a 16th-century German choir book illustration. The following year, Sotheby’s asked Clark to study a rare 13th-century illuminated Paris Bible “as a test case to see whether or not we could, in a few hours, identify the pigments present,” he says.
In each case, Clark knew he’d need to use Raman microscopy, an analytical technique that permits scientists to identify the molecular makeup of artwork without harming the precious objects.
But without a comprehensive guide to pigments used in antiquity and what their Raman spectra look like, Clark was stuck. So he went to the library. In art history and archaeology libraries, he explored what pigments were used throughout the ages. In science libraries, he tracked down the chemical properties and spectra of those pigments.
After doing this groundwork to identify pigments on the choir book and Bible, Clark figured he’d summarize the background material in a review paper. That paper, “Raman Microscopy: Application to the Identification of Pigments on Medieval Manuscripts,” was published in Chemical Society Reviews in 1995 (DOI: 10.1039/cs9952400187). It, along with two subsequent papers describing the pigment databases Clark assembled, has been cited more than 700 times in Thomson Reuters’ Web of Science Citation Index (Spectrochim. Acta, Part A1997, 53, 2159; 2001, 57, 1491). According to the American Chemical Society’s Chemical Abstracts Service, Clark’s papers are among the most cited papers in art conservation science.
After these early projects, Clark went on to use Raman microscopy to study eight Gutenberg Bibles. He has helped show that a contested Vermeer painting was likely not a forgery and that 19th-century A.D. Mauritian stamps were. He’s studied both authentic and fake 13th-century B.C. Egyptian papyri; 9th-century A.D. Iraqi stuccoes; and a wide variety of Russian, Albanian, and Greek religious icons.
“We’ve had many millions of pounds’ worth of artwork underneath our Raman microscope,” Clark says, “so
Clark says that one of the most interesting projects he’s had a hand in is that of the Vinland Map, which purports to be a Viking representation of Iceland, Greenland, and the northeastern seaboard of North America from 1440, some 50 years before Christopher Columbus landed in America. The map’s authenticity is one of the “longest-running controversies in the art world,” Clark says.
The map surfaced in 1957 and was later bought by the American philanthropist Paul Mellon for about $300,000 and donated to Yale University. But despite heated debates about the authenticity of the map, by 1972, Clark says, no scientist—only historians and conservators—had been given access to analyze it.
That’s when Illinois-based analytical chemistry consultant Walter C. McCrone was given an opportunity to study the map. “When his team analyzed the ink, they found titanium dioxide in the form of anatase. There was nothing wrong in principle, because that’s a mineral,” Clark says. “But the anatase found had a particular size and shape, which does matter. The diameter of the particles was about 0.15 μm, and they were white and rounded. All of these features are characteristic of synthetic titanium dioxide that was produced from the 1920s onward, rather than of mineral anatase, which is found in a wide variety of grain sizes and colors,” he adds.
McCrone’s 1972 conclusion was damning, but his team’s scientific work has not stopped counterclaims or conferences—as recent as 2009—purporting otherwise.
A bounty of scientific research has been done in the interim decades. For example, in 2002, carbon dating of the map’s parchment set the age to 1434. But Clark argues that the age of the ink is what’s important for dating the map itself. Most scientific research, including Raman analysis by Clark, supports McCrone’s initial conclusions that the map is a forgery, he says.
One of the biggest challenges with authentication requests, Clark says, is that the people making them “want a black-and-white answer.”
“If you go through and find one modern pigment that’s synthetic and the date of its first synthesis is after the date of the work of art, then the artwork is probably a forgery,” Clark says. In cases where modern restoration work has introduced the pigment, he says, the modern pigments lie on top of the ancient ones.
“But if you don’t find a particular modern pigment, you can’t say it therefore is authentic,” Clark adds. “At the end of the day all you can say is that the thing is consistent with being authentic because there’s nothing you’ve found on it that is modern.”
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