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Analytical Chemistry

Pinpointing Pigments In 3-D

Art: Researchers can now identify the exact position of pigment particles in artwork noninvasively

by Sarah Everts
January 23, 2014 | A version of this story appeared in Volume 92, Issue 4

Pigment grains in the cross-section of a 14th century masterpiece.
Credit: Proc. Natl. Acad. Sci. USA
A new microscopy technique can pinpoint the exact position of lapis lazuli particles in a cross section (right) of the Virgin Mary’s blue robe in the 14th-century painting “The Crucifixion” by Puccio Capanna (left).

A technique for visualizing the three-dimensional location of pigments in human tissue is set to help cultural heritage researchers do the same for pigments used in valuable masterpieces. The technology’s many potential art applications include finding forgeries or informing conservation strategies.

The new technique builds upon femtosecond pump-probe microscopy, a noninvasive method used to measure pigments such as hemoglobin and melanin in living tissues. A team of researchers led by Warren S. Warren at Duke University figured out how to tweak this method so that it can be used to image pigments in each of the many layers of a painting without harming it. They did proof-of-principle experiments on a 14th-century painting called “The Crucifixion” by Puccio Capanna (Proc. Nat. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1317230111).

Knowing the exact depth and location of different pigments in the many layers of a painting helps researchers trying to authenticate a masterpiece or those planning a smart approach for restoring it, explains Costanza Miliani, a conservation scientist at Italy’s National Research Council Institute of Molecular Science & Technologies, in Perugia. Currently, researchers can identify the kind of pigments found on a painting noninvasively. But they can’t map the layering of those pigments from surface to canvas noninvasively, she says. Restoration projects can require removing varnish layers from a painting’s surface without harming the pigment layers underneath. So it would be very useful to be able to see exactly where pigment layers begin and end, Miliani adds.

The technique may also allow conservation scientists to map distinct brushstrokes in a painting, Warren says. Brushstrokes often can be correlated to the hands of individual painters and thus be useful for authentication. “This may also help us distinguish between apprentice and master,” he explains. “So even if a famous artist signed the work, we may be able to see what fraction they actually painted.”

Until now, the only way to figure out the 3-D location of a pigment particle was to physically remove a cross-sectional sample from the artwork. However, conservation scientists are increasingly trying to avoid invasive sampling. When sampling does occur, it is often in relatively benign locations of the paintings: at the artwork’s edge or near an already existing crack, Miliani says. Because the new technique is noninvasive, it would allow conservation scientists to analyze the depth of pigments in any area of interest in a painting.

Modifying the laser-based femtosecond pump-probe microscopy technique so that it could detect artist pigments anywhere on a painting was not a trivial endeavor, Warren says. The chromophores in tissue pigments fall within a relatively small window of absorption. But artist pigments range from organic molecules to minerals, which have a wider range of absorption. So his team had to modify the equipment to detect such pigments. Furthermore, the lasers used in the machinery have to operate at extremely low power to protect valuable works of art. Warren’s team had to boost the system’s sensitivity so that it would work at such low laser powers.

The work is “a significant step forward toward achieving the ability to noninvasively probe the molecular properties of artworks,” says Northwestern University’s Richard P. Van Duyne. But he worries that the laser power may still be too high for precious artwork. The half-million-dollar cost of purchasing the instrumentation also could be a barrier for many museums, he adds.

Next up, Warren’s team aims to make a cheaper, more portable version of the laser instrument that can be used to study art that cannot be moved for security reasons or paintings that don’t fit on a lab bench.


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