Photonics Enables Conservators to Get a Clearer Picture of Art
Art Institute of Chicago, Fourier transform infrared microscopy, Raman microscope equipped with an Andor CCD detector, a holographic notch filter and various dispersion gratings.
She also uses multiple lasers operating at different wavelengths, such as an air-cooled, frequency-doubled Nd:YAG laser at 532.1 nm, a HeNe laser at 632.8 nm and a diode laser at 785.7 nm. She focuses these on the sample, being careful to keep the power low so that the artwork is not damaged. The beams produce Raman scattering that reveals details about molecular bonds.
Conservation scientists are employing photonic techniques such as Fourier transform infrared microscopy, show here being used to study the coatings on a design for a fan by Paul Gauguin. These nondestructive approaches enable conservators to establish the authenticity, date of production and material makeup of a work of art. Courtesy of Francesca Casadio, Art Institute of Chicago.
That information, combined with x-ray fluorescence spectra and other data, enables Casadio to extract the chemical composition of objects and to categorize artworks by their time or place of production. For instance, differences in the location of the peaks within Raman spectra allowed her to date the porcelain from Meissen, the site of the first European factory for such objects, by the pigments in the glazes used to decorate the pieces in the 18th versus the 19th century.
In her work, she uses commercial products, although she does opt for some unusual options. “The open-architecture microscope is a ... nonstandard feature that the company offers,” she said. “It comes in very handy for nondestructive, in situ work on large objects.”
She also makes use of FTIR spectroscopy, which requires compressing a small sample of the piece of art into a thin film and placing it in an FTIR microscope. The infrared absorption spectrum is recorded, and Casadio looks for peaks that act as chemical fingerprints. Many materials have distinctive characteristic absorption spectra from 2.5 to 25 µm. Sulfides, oxides and some other materials are the exception, and they are better probed with Raman spectroscopy.
As a result, she would like to have an instrument capable of Fourier transform Raman spectroscopy to handle fluorescing organic components. Ideally, it would feature many sources of excitation wavelengths or perhaps a tunable laser, because artwork materials vary and because the best wavelength to use varies with the material. The ability to extend the microscope over long horizontal distances also would be desirable, she said, because that would enable oversize objects to be handled and perhaps allow the mapping of a surface. Such instruments, unfortunately, are beyond the budget of a fine-arts museum.
However, even with advanced technology, it is unlikely that this type of artwork sleuthing will be something a scientist does alone. Spectral measurements, after all, do not tell a painting’s entire story. “The synergy between scientist, conservator and art historian is the most important point in my profession,” Casadio said. “Scientific data alone disclose only part of the picture.”
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