There's more to a painting than meets the eye. Under the surface of a landscape or a still life lie dozens of meticulously applied layers of paint, forming a complex 3-D structure that is all but invisible to viewers. Now, an imaging technique borrowed from biomedical research promises to let art historians and conservators peer into the depths of paintings without damaging them.
"Right now, if an art conservator wants to understand the three-dimensional layering structure of a painting, they almost certainly take a scalpel to it," removing tiny core samples to study its stratigraphy, said Warren Warren, a Duke University chemist and biomedical engineer. He spends most of his time developing laser systems to image human tissue. But when he visited an exhibit on detecting art forgeries in London's National Gallery a few years ago, he began wondering what art historians and conservators could learn from state-of-the-art imaging technologies.
One method Warren works on is called pump-probe microscopy, which uses carefully timed pulses of laser light to electrically excite the molecules in a sample. As the molecules gain and lose energy in reaction to the pulses, they emit signals that serve as "fingerprints" that reveal their chemical makeup.
Pump-probe microscopy is especially useful for studying biological pigments like melanin in skin. So Warren wondered: Could it work on other kinds of pigments, too? Paint?
"We built a laser system that was designed to do a good job of diagnosing skin cancer and then realized that we could use exactly that same laser system to look at Renaissance artwork," he said.
The low-powered laser pulses travel deep into a painting without scattering, returning a remarkably clear picture of its subsurface structure as well as chemical fingerprints of the pigments in each layer.
The team initially tested the technique on mock-up paintings made with historically accurate Renaissance pigments, proving that pump-probe microscopy can distinguish between the 3-D structures of a purple created by mixing red and blue pigments and a similar shade made by layering red over blue.
Then, the researchers turned their laser eye on an actual Renaissance painting: The Crucifixion, painted by Puccio Capanna around 1330. By imaging small sections of the blue robes of the Virgin Mary and one of the flying angels, they revealed that Capanna used very different pigments to create each one, despite their similar colors, the team reported online in the Proceedings of the National Academy of Sciences.
Pump-probe microscopy could be especially useful for identifying places on aging paintings where the pigments have started to decay. That could help conservators fine-tune their efforts to halt such deterioration.
"Such a boost in technology is what the art conservation and museum fields need to ensure that unique works of art are, and remain, protected in the best possible manner," said Koen Janssens, an analytical chemist at the University of Antwerp in Belgium who was not involved in the research.