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Laser treatment restores art

September 25, 2002

FIRING laser beams at priceless paintings might seem the height of folly, putting the painting`s delicate pigments at risk. So while conservators have for decades used ultraviolet lasers to clean up grubby sculptures, they have always fought shy of using them on soiled paintings.

But now a study has shown you can fire away with impunity. Lasers are not only safe, but also more effective than conventional chemical techniques, says Marta Castillejo, a researcher with the Spanish Council for Scientific Research in Madrid.

Oil paintings are protected by a thin layer of varnish. But over time, this becomes oxidised and coated in dirt, hiding the picture`s true colours beneath an ever darkening veil.

Traditionally, restorers have removed this layer of varnish with scalpels and solvents. But progress is painstakingly slow, and the solvents sometimes harm the paint.

Laser cleaning, on the other hand, is a non-contact process. "The effect of the laser is restricted to the surface being treated," says Castillejo.
She has found the a UV laser can safely remove the outer surface of varnish without affecting the chemical composition of the underlying pigments. "Provided you leave a thin layer of varnish you will not damage the paint underneath," she says.

In their study, published in the current issue of Analytical Chemistry (vol 74, p 4662), Castillejo and her team used a krypton fluoride gas laser that produces a fixed UV wavelength, to see if it could evaporate the dirty varnish from the surface. Rather than risk a masterpiece, they tried the laser on an artificially aged painting containing a variety of pigments, both organic and inorganic.

As the laser light hits the paint, a separate tool "sniffs" the plume of gas created by the laser. As it burrows deeper, the system begins to detect traces of paint molecules. "Then you know you`re close to the surface," says Castillejo.

Analysis of the painting afterwards revealed that the underlying pigment had remained stable. A prototype has since been developed and some trials have been carried out on real paintings, with startling success, says Castillejo.
Duncan Graham-Rowe

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