New help for greasy works of art

BOSTON — Had Benjamin Franklin spilled some lamp oil on the Declaration of Independence, scientists might be able to identify what made the offending spot today, information crucial to cleaning the document. Researchers have developed a way to assess oily stains on old documents or artworks without creating any new damage.

Revealing what kind of oil made a stain eliminates the trial-and-error method that art conservators often resort to when deciding how to clean unknown spots, scientists reported August 24 at the American Chemical Society’s fall meeting

The first rule of stain removal is identifying what made it, said Eleonora Del Federico of the Pratt Institute in New York City. But that process is especially difficult when it involves works of art. High-tech analyzers tend to be invasive, damaging the artwork.

“There are some techniques that you can’t use in art conservation because you don’t want to take a sample,” Del Federico said.

The new method, using a portable nuclear magnetic resonance spectrometer, doesn’t require a sample to be snipped and inserted into the machine for analysis. Conventional NMR, which analyzes how the nuclei in a substance’s atoms respond to a magnetic field and pulses of radio energy, typically involves an enormous magnet housed in a specialized laboratory. But the bread box–sized, less sensitive version of the standard NMR — called NMR MOUSE — can be brought to the art and held above it. The magnets in the NMR MOUSE don’t produce the broad, uniform magnetic field of regular high-power NMR magnets, so the technique doesn’t yield a detailed analysis of the substance being tested. But it can discern whether an oil stain resulted from a saturated, monounsaturated or polyunsaturated fatty acid. And that information lets conservators know if the stain should get an enzyme treatment or something like petroleum ether, said Del Federico.

Del Federico and her colleagues used the technique to classify 18 oils, including olive, peanut, canola and linseed, according to how they harden over time. Next, the team created a 500-stain library using five kinds of paper, three aging processes, four oils and 10 stain treatments. The library can serve as a reference for identifying stains on fine art and historic documents.

Conservators can also use the approach to monitor the cleaning process, Del Federico said, perhaps switching solvents after some of the oil has been removed.

The technique looks like a good approach for art, said Ali Azarpira of the University of Wisconsin–Madison. NMR MOUSE has already been used to examine the wood structure of delicate violins. Perhaps, he says, its use could be expanded further, for example to probe the structural components of woody plants, information that could guide selection of plants that could be turned into biofuels.