The Dirt on Art: Chemists test laser cleanup of paintings

To keep the world’s treasury of paintings looking up to snuff, museum conservators fight the beauty-stealing tides of dirt, age, and more acute forces, such as floods and fire. Now, it seems, they might consider adding lasers to their centuries-old closet of cleaning gear.

Conservators usually freshen up paintings with the equivalent of gentle scrubbing, often with chemical solvents. Because they have much higher precision, lasers can potentially improve on these techniques for removing unwanted soot, paint, or varnish, says Marta Castillejo of the Institute of Physical Chemistry Rocasolano in Madrid. Although restorers sometimes use lasers to bust dirt off museum pieces, they haven’t embraced them widely, especially not for paintings.

“We all recognize that it’s a valuable tool and has great potential,” says conservator Craig Deller of the Deller Conservation Group in Geneva, Ill., “but I think there may be hesitance.”

In the Sept. 15 Analytical Chemistry, Castillejo and her coworkers report how pulses from an ultraviolet laser affect the condition of tempera paintings similar to ones hanging on many museum walls. They also used spectroscopy to monitor the material removed by the laser as it comes off the painting.

The research team prepared model paintings for the experiments using a variety of organic and inorganic pigments and artificially aged them with fluorescent light. Over some of their samples, the researchers also applied layers of varnish, which often cover real paintings and can yellow with time.

The studies show that the laser system can safely remove some varnish from a painting as long as a thin layer of it remains behind to protect the underlying paint, says Castillejo. On unvarnished samples, some types of paint fared better than others under laser cleaning. For instance, some pigments–mostly inorganic paints–showed signs of discoloration.

“The results of this research demonstrate the viability of the laser cleaning . . . of paintings,” says Castillejo.

“They’re doing the right thing,” says Charles Tumosa of the Smithsonian Center for Materials Research and Education in Suitland, Md. “You have to show the technique is safe before you use it.”

Even so, Tumosa says, the data don’t merit an enthusiastic acceptance of laser cleaning. For example, he says that he’s unconvinced that a painting remains unharmed when a laser removes some of its varnish. Moreover, lead white–the pigment most susceptible to color change–is one of the most widely used in art.

“It’s not the magic solution for every problem,” Castillejo admits.

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