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Tuesday, January 28, 2014

A new way to analyze precious works of art

This is what happens when elderly parishioners take it upon themselves to restore artwork. This was a 19th century fresco by Spanish painter Elias Garcia Martinez in the Santuario de Misericodia church in Borja, northeastern Spain.
In order to preserve and restore art work, you need to understand exactly what materials and techniques the original artist used. For example, you can create nearly the same shade of purple by either applying a thin red glaze on top of blue pigment or by mixing blue and red pigments together first. If you don’t know which pigments and methods the artist the used, you could easily botch up your restoration efforts.
Unfortunately, conventional imaging can’t penetrate very deeply into paint layers and don’t offer 3D molecular information, whereas methods that can give that kind of information require the removal of physical samples. But no longer.  Tana Villafana of Duke University and her colleagues have used pump-probe microscopy to create 3D molecular maps of paintings.

The technique was originally used to visualize organic pigments in biological samples as a way to screen for skin cancer. Briefly, you use a series of short pulses of light to electrically excite (pump) the sample and then another series of pulses to detect (probe) that change. 
The tricky part is finding the proper wavelengths to pump through the apparatus to give you the information you need. Skin pigments are much less variable than the range of materials used to color paintings, and perhaps more importantly, they aren’t combined in myriad ways. However, the scientists were successful in making a nondestructive 3D image of an intact 14th century painting (The Crucifixion by Puccio Capanna).
Left: a femtosecond pump-probe microscope positioned over a section of The Crucifixion by Puccio Capanna.
Top right: a false-color face-on view of the section, showing blue lapis lazuli fragments.
Bottom right: a 60-┬Ám-thick cross-section of the painting, revealing the thickness of the pigment.
PNAS PMID: 24449855.

The authors believe this technique can be used not only to completely deconstruct paintings so that they can be properly restored, but to more fully understand the degradation of pigments so that artwork can be better protected and preserved.

Villafana TE, Brown WP, Delaney JK, Palmer M, Warren WS, & Fischer MC (2014). Femtosecond pump-probe microscopy generates virtual cross-sections in historic artwork. Proceedings of the National Academy of Sciences of the United States of America PMID: 24449855