fresco.jpg SUSI being used on a fresco.  Courtesy of the Institute of Applied Physics, Italy

Physicists in Italy are developing a new device for assessing the condition of some of the country’s most valuable fresco paintings. The tool can reveal information about the condition of a wall painting without ever touching its surface, making the device a potentially valuable aid to restoration and preservation efforts.

Fresco is a style of painting, popular during the Italian Renaissance, in which artists apply pigments to a plaster-covered wall. One of Italy’s most well known frescoes is the ceiling of the Sistine Chapel, painted in 1482 by Michelangelo.

“Frescoes cannot be protected from the environment by putting them in a safe place, because they are on a wall,” Robert Olmi, a researcher at Italy’s Institute of Applied Physics and author of the paper, said via email. “The only solution is early diagnosis and prevention.”

Moisture is the main source of damage to frescoes because the evaporation of water from their surfaces causes paint to detach. The art is also commonly damaged by salt, which can crystallize and form a layer of whitewash over the wall’s surface. 

The handheld scanning tool, called SUSI—the acronym, in English, is “sensor of humidity and integrated salinity”—uses a non-invasive microwave system to detect salt and moisture levels up to two centimeters beneath the surface of a fresco. About the size of a video camera, it works by sensing the material’s resonance frequency, a physical quantity that depends on the number of salt and water molecules in a given area of fresco.

SUSI has already been used to examine several of Italy’s frescoes, including some at the Church of Santa Maria Novella in Florence, which were painted by Florentine artists around 1300.
“In several of the frescoes we were able to monitor the seasonal changes in water content,” said Olmi, who published a paper about SUSI in the August issue of Measurement Science and Technology.

Early detection of moisture, scientists say, is essential for avoiding damage, while information about the type and amount of salt present can help restorers determine the best technique for fixing the artwork.

One of the currently popular methods of non-invasive fresco diagnosis is called thermography, which detects moisture in the painting by sensing infrared radiation. But unlike SUSI’s microwave system, thermography probes only the surface of a painting and does not detect salt levels. Chemical methods are still best for measuring salt content, Olmi said, but are also invasive: They require taking a small physical sample from the painted surface.

Piero Baglioni, a chemist at Italy’s University of Florence who was not involved in developing SUSI, agrees that its ability to detect salts is a boon. But the tool will probably be used in addition to, not as a replacement for, more conventional methods, he said.

“It is probable that a faster screening of water will be performed by SUSI, followed by a more specific analysis using other spectroscopic techniques,” Baglioni said in an email. “But it should be cheaper than conventional methods.”

Originally published September 1, 2006

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