Decoding DaVinci

The layers in old-master paintings can now be mapped non-invasively.

The beauty of a painting — its design, subject matter and colour — is the first thing that strikes a museum visitor. But art historians want to know more than what can be seen on the surface. They want to know the stratigraphy and hence the succession of layers composing the total painted work — including a preparatory layer right on the raw wood or canvas medium, an under drawing, the actual layers of paint above and possibly a layer of varnish. A viewing method, borrowed from medical resonance imaging technology, will now allow this detailed study of deep layers in paintings to be made non-invasively.

Other techniques, such as shining X-rays on the surface and looking at the fluorescent light coming back out generally allow only a shallow mapping of layers into the body of the painting.

The new profiling technique, pioneered by a group of Italian and German scientists, uses nuclear magnetic resonance (NMR). The NMR process works on a microscopic level. First a magnet helps to get all the hydrogen atoms in a sample to point in the same direction — sort of like getting a thousand soldiers to all look at a flag and then salute at the same moment. Then the atoms are exposed to a bath of radio waves. These waves oblige the hydrogen atoms to swivel about in place, as if they were doing exercise. Eventually the atoms (or more particularly the protons at the heart of the atoms) re-emit a radio wave.

It is these emitted waves that are detected by a nearby sensor.

With the help of a computer, the wave information can be turned into an internal map of the whereabouts of the hydrogen atoms. In the case of medical imaging, the hydrogen atoms are usually found in water molecules throughout the body. By subtle analysis of the resultant map a trained doctor can spot the location of a tumour since its water content will be slightly different from that of the surrounding healthy tissue.

In a study of a painting, by comparison, 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.

One of the researchers, Federica Presciutti, a chemist at the University of Perugia, says that the thicknesses of the layers in the painting, even at the deepest level, can be determined. And although the age of the layers cannot be determined absolutely, it is possible to tell which layers are older than others. The whole process is non-invasive. Furthermore, the magnet used in the new approach is single-sided with the scanner being brought right up to — but not touch — the painting.

The operation of the new painting scanner was reported in the 21 July issue of the journal Applied Physics Letters.