Theft, imitation and outright deception can make a painting's history even murkier than centuries of accumulated grime. But getting to the bottom of a piece of art's origins can be crucial for restoration—and forensics.
In recent decades, art scholars, restorers and forensic specialists have relied increasingly on scientific techniques to determine the chemical composition of a work's pigments to try to ascertain when, where and by whom it was likely made. One ostensibly ancient Virgin with Child painting was revealed to be a 1920s fake after testing revealed that it contained Prussian Blue, a pigment that was invented in the 1700s—long after the painting would have been made if it were original.
Chemical processing of paint samples can provide useful molecular profiles, but it also means physically damaging a chip. Other methods using x-ray fluorescence, scanning electron microscopy and infrared spectroscopy have helped scholars and technicians peer into ancient paint, but they can be time- and labor-intensive.
A new study shows how sound waves can detect a dozen different inorganic pigments using Fourier-transform photoacoustic infrared (PAIR) spectroscopy (which makes use of signal processing functions developed by French physicist Joseph Fourier). The process is based in part on an 1880 discovery by Alexander Graham Bell, who demonstrated that shining a modulated beam of light onto an object could create a subtle acoustic wave.
The researchers were able to use PAIR's argon-ion laser to detect a range of common inorganic hues, including: four blues (cobalt, ultramarine, Prussian and azurite), three greens (malachite, chromium oxide and viridian), two yellows (cadmium and chrome) and three browns (iron oxide, ochre and Mars). A description of the work will be published in the October issue of Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.
Tiny samples, which could later be restored to their paintings, were heated with the laser beam. This heat produced a change in pressure, making small acoustic waves, which were picked up by a super sensitive microphone. Each compound had a different sound profile that distinguished it from the rest. And because the samples are not damaged during the process, the researchers noted, they can be tested multiple times—a bonus not every analysis method can boast.
"The behavior of paints, pigments, glazes, etc. depends critically on the conditions associated with their production, storage and long-term display," the researchers noted in their paper. "Without a full comprehension of the reactivity of the chemicals involved, the attempted preservation of artworks can sometimes lead to more damage than would occur by just simply leaving the works untreated."
The researchers proposed that these simple readings could be included in a database for quick reference in the future. "Once such a database has been established, the technique may become routine in the arsenal of art forensic laboratories," Ian Butler, a chemistry professor at McGill University and coauthor of the new study, said in a prepared statement.
Image of Virgin with Child, painted by an unknown Italian forger in the 1920s who used Prussian Blue, which was not invented until the 1700s, courtesy of Wikimedia Commons