Science Allows In-Depth Look at Old Masters

A non-invasive imaging method allows old-master paintings to be seen as never before

Inside Science News Service
July 30, 2008

The beauty of a painting—a blend of its design, subject matter, and color—is the first thing that strikes the eye of a museum visitor. But the surface of a painting is just that, the surface, and the true nature of a painting is contained in the many layers below the surface. Thus, art historians want to know about a painting's stratigraphy—the succession of layers that include a preparatory layer on the canvas or wood medium, an under drawing, the layers of paint above, and possibly a finishing layer of varnish. A new viewing method, borrowed from the kind of medical resonance imaging (MRI) used in hospitals, now allows this detailed study of deep layers in paintings to be made non-invasively, researchers recently reported in the journal Applied Physics Letters.

The new profiling technique, pioneered by a group of Italian and German scientists, uses nuclear magnetic resonance (NMR), the basis for MRI medical scanning. The NMR process works on a microscopic level. First a powerful magnet helps to get the hydrogen atoms in a painting to point in the same direction, then the atoms are exposed to a bath of radio waves. These waves oblige the hydrogen atoms to swivel around in place. Eventually the atoms (or more particularly the protons at the heart of the atoms) re-emit a radio wave.

These waves are detected by a sensor and, with the help of a computer, the wave information is turned into an map that shows the locations of the hydrogen atoms. In medical imaging, the hydrogen atoms are usually found in water molecules throughout the body. By analysis of the NMR map a trained doctor can spot the location of a tumor because its water content will be different than that of surrounding healthy tissue.

In a study of a painting these same techniques can provide information about the binding agents used in the painted layers. Historically, these binders consisted of such things as egg yolk or oil. Learning the nature of the binding agent is often enough to distinguish a naturally aged master-work from an artificially aged fake. By using the NMR process on adjacent areas in a painting, Federica Presciutti, a chemist at the University of Perugia in Italy, said a three-dimensional map can be created that allows a detailed look at the layers. Presciutti, a lead researcher in developing the new technique, said the map can also be used to identify parts of the paintings that have been restored, or altered, in the past.

NMR device being used to study the painting “Madonna with Child” by Gentile da Fabriano, 1411. Credit: Federica Presciutti

The map also allows the thicknesses of the layers in the painting, even at the deepest level, to be determined, Presciutti said. And although the age of the layers cannot be determined absolutely, it is possible to tell which layers are older than others. Furthermore, the magnet used in the new approach is single-sided, which allows the scanner to be brought right up to, but not touch, a painting. The massive MRI magnets used in hospitals are too unwieldy to be used in the art world.

Delving deep into a painting and decoding the properties of the layered material in an old painting can establish the age, origin, and authenticity of the work. Like geologists studying past eras in Earth’s history by looking at strata, curators typically probe the layers of a painting by removing a tiny sample in order to inspect its layers with microscopes employing electron beams or visible light. The problem with this "core sample" approach is that, although the sample is small, removing it does damage to a painting.

Art historians and curators do use noninvasive techniques to study some properties of old paintings, such as shining x rays on the surface and looking at the fluorescent light coming back out. This and other noninvasive techniques are limited, however, in that they only allow a shallow mapping of layers into the body of the painting. That problem is solved with the new NMR technique, according to Presciutti's research team.

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