Issue Date: June 2, 2008
The Louvre Laboratory
WANDERING AROUND the impressive grounds of Paris' Louvre Museum, one finds it hard to believe that three floors beneath the grandiose art and architecture is an operational 3-MeV particle accelerator.
While thousands of visitors wander through the museum's myriad wings every day, trying to view as many of the Louvre's 35,000 works as possible, some 60 scientists spend their days analyzing these same artifacts in subterranean laboratories.
"We work on about 500 cultural artifacts a year, from Paleolithic times all the way to the 20th century," says Anne Bouquillon, who heads the Stone, Ceramic & Metal Research Division of the Center for Research & Restoration of the Museums of France (C2RMF).
Scientists enter C2RMF through one of the Louvre Museum's inconspicuous side entrances bordering the Seine River. To reach the underground research center, they must pass through a limestone archway, then slip down a discreet stone staircase and along a short passageway before entering the center through handsome glass sliding doors bordered by bronze.
Throughout the three floors of underground office and research space, C2RMF executes its multimission agenda: authentication and origin studies, conservation, and development of new techniques to study cultural artifacts. Because of the center's in-house location, any art in the Louvre that's under investigation can be transported to the labs through tunnels without risk of damaging it from exposure to the outside environment. But the center's reach extends further than the Louvre.
With a multi-million-dollar operating budget coming from France's Ministry of Culture and the French National Research Labs (CNRS), as well as grants from the European Union and the French science funding agency, C2RMF provides scientific support for all of France's museums. The Louvre's art pieces take up about half of the workload, though, says Philippe Walter, codirector of the CNRS lab of C2RMF.
The research center was established in 1995 in the then newly renovated facilities below the Louvre, with the merging of two separate French museum research and conservation facilities that had been operating since the 1930s.
In many ways the research center looks like any other scientific facility. There are benches and glassware in the wet labs. And in the cavernous instrument room, bundles of cables link electronic equipment to computers, optical instruments sit on motion-stabilizing tables, and LEDs flash to a soundtrack of electronic humming.
But amid this equipment sit valuable pieces of cultural heritage. On the day C&EN visited, an ancient Greek clay pot from the 3rd century B.C. and decorated with swirling pigmented designs was resting on top of a lab bench, waiting to undergo X-ray diffraction analysis. Bouquillon is working with a curator who wants to know whether a collection of these pots was painted by the same artist, in the same place. She'll compare the elemental fingerprint of the pot with those of other similar receptacles to try and answer the question.
Curators come to C2RMF scientists for clues about the origin and life story—or "provenance" in museum-speak—of cultural heritage objects, Walter says. "We have more interesting requests than is possible to answer."
ENTER THE 3-MeV particle accelerator. Recently, the machine helped solve a mystery within the red eyes of a Mesopotamian figurine from the 3rd century.
The figure's eyes were thought to be red glass, but when the researchers sent high-energy protons into them and obtained X-ray spectra from the proton-induced electronic transitions, they discovered the eyes were actually rubies. Then, by comparing the relative abundance of trace elements in the two rubies through the same technique, the scientists figured out that the gems most likely originated in Myanmar (formerly Burma). Not only did the data establish the statue as the oldest archaeological sample to include rubies as decoration, but they also revealed previously unknown trade routes between ancient Babylon and Myanmar.
Besides geographical, artistic, or historical provenance, C2RMF researchers have also been using physical techniques to give art historians hints about editorial decisions made in the minds of artists, hundreds of years after the fact. For example, the investigators used a technique called infrared (IR) reflectography to image preliminary charcoal sketches buried underneath subsequent layers of paint on canvases. The technique permits a comparison of the artist's plan with the final product, revealing when a painter altered the subject's expression or edited people and objects out of the completed work.
But C2RMF does more than the detective work instigated by curators. It also carries out science-driven projects. For example, Walter has been reproducing cosmetic and perfume recipes found in ancient Egyptian texts to better understand how these items were formulated thousands of years ago. "The ancient Egyptians may not have known chemistry like we do now, but their recipes were developed from years of testing," Walter says. "For example, we want to know why they cooked many ingredients, including incense, with honey, during certain precise periods to make perfume. Was the mixture an emulsion or were the components soluble, and how did this affect the final product?"
Walter has also reformulated hair dye recipes from 2,000 years ago and found that a concoction of lead oxide and slaked lime, Ca(OH)2, causes the formation of lead sulfide nanoparticles inside strands of hair (C&EN, Sept. 11, 2006, page 12). All this explains the bottle in Walter's office: it's Grecian Formula destined for a comparative study of ancient and modern hair dyes, he explains.
NEW TECHNIQUES are additionally being developed by C2RMF for studying cultural heritage artifacts. One major challenge to using standard IR, X-ray, and spectroscopic techniques to study museum pieces is simple geometry.
"Unfortunately, artwork doesn't often conform to the standard sample sizes of commercial equipment," says Jacques Castaing, a staff scientist at C2RMF. "If you want to do X-ray diffraction, it's hard to put a big vase on the beam line." It can also be difficult to do experiments at archaeological field sites or at different museums.
The center has consequently built custom versions of several pieces of instrumentation, such as portable X-ray diffraction and fluorescence spectrometers that weigh only 40 kg and were recently field-tested in Italy.
Walter has also been working to develop new techniques to study the crystalline nature of individual layers of pigment on paintings, not just their elemental makeup. "With X-ray [fluorescence] you get an idea of the chemical composition but not the ordering of the atoms in that pigment layer," says Walter. "The problem is that many paint pigments have the same elements but not the same crystalline structure." And to solve the crystal structure of pigment layers using X-ray diffraction, one needs far more sample than the 30-µm-thick strokes made by artists.
In a recent Nature Materials paper (2008, 7, 468), Walter and colleagues reported a first step toward determining crystalline structures of pigments in paint strokes on a masterpiece by combining powder X-ray diffraction with computer tomography.
Essentially, they took X-ray diffraction spectra at many different angles with respect to the artwork and used computer tomography to reconstruct the atomic organization of materials with similar atomic densities. Currently, they've done the proof-of-principle work on different types of crystalline carbon, but the idea is to precisely determine the crystal structure of pigment layers on paintings.
Walter hopes this technique will bolster the emerging field of "molecular archaeology," he says. "We don't just want to study the elemental composition. Now we want to go to molecular, supramolecular, and crystalline levels, too."
With tens of thousands of artifacts to study, from Renaissance painting to ancient pottery, the challenge for C2RMF researchers is to have a broad knowledge of everything from materials science to analytical chemistry but also be good on specifics, Walter says.
Yet being scientifically flexible has its perks, Castaing says. He points to the Rembrandt self-portrait that recently made a cameo appearance in the underground labs to have some pigment analysis done on its way from the Louvre's collection to an exhibit in Cincinnati. "When most people go on vacation, they go to museums. But I get to go there for work."
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