Medical imaging technique helps virtually dissect artwork

Researchers have developed a 3D imaging technique that uses lasers to create virtual cross-sections of historic paintings -- without causing any damage. And they tried it out on centuries-old artwork.

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For those of us who've tried to sneak a picture in the Sistine Chapel or have been scolded by docents for standing too close to the van Gogh... it seems like just breathing on art could damage it. So how do scientists and conservators conduct analyses without destroying works of art?

Studying the layering structure of a painting could help us understand the artist’s materials and methods -- which is really useful information for art conservators and historians. But that involves collecting a small sample with a scalpel. “The current practice is to take as few samples as you can get away with,” study co-author William Brown of the North Carolina Museum of Art tells Nature. “The ability to take multiple non-destructive ‘samples’ anywhere on the painting is a tremendous advantage.”

Borrowing from biomedical imaging, researchers are using lasers to create 3D analyses of historic artwork. For a non-destructive look at the pigments used and where they lie, a team led by Warren Warren of Duke University turned to “femtosecond pump-probe microscopy,” a technique used to image cross-sections of tissue. It’s particularly useful for studying biological pigments like melanin in skin.

“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,” Warren tells Science

Using 3D imaging, they can create a chemical profile of the paint layers -- a virtual cross-section of the painting. Science explains
  • Carefully timed pulses of laser light are used to electrically excite the molecules in a painting.
  • As the molecules gain and lose energy in reaction to the pulses, they emit signals -- or fingerprints that reveal their chemical makeup.
  • The low-powered laser pulses travel deep into a painting without scattering, returning a picture of the subsurface structure along with the chemical fingerprints of the pigments in each layer.

First, the team tested the method with mockup paintings, discriminating between two different methods used to create purple: red mixed with blue compared with red layered over blue. And then they tested the tech on the real deal: The Crucifixion (pictured), a 14th-century painting by Puccio Capanna. Science reports

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. Mary’s robe is composed of a thick layer of ground-up lapis lazuli, a deep blue stone that at the time was “more expensive than gold,” Warren says. The blue of the angel’s robe, on the other hand, was created through a complex layering of several less precious pigments, with just a hint of lapis lazuli.

In addition to documenting and preserving history, the technique could also rediscover the original appearances of paintings where colors have already faded. Additionally, it could help identify forgeries by examining differences in the 3D structure of brushstrokes.

Image: T.E. Villafana et al. / National Academy of Sciences

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