Stressed-out art: a preservation paradox.

Stressed-out art: A preservation paradox

Without intervention by art conservators, the process of decay would slowly transform museums into warehouses filled with hopelessly deteriorated objects. The healthy appearance of thousands of venerable artworks suggests the custodians of cultural heritage have successfully kept decay at bay.

But appearances can mislead, warns Marion F. Mecklenburg of the Smithsonian Institution's Conservation Analytical Laboratory in Suitland, Md. Drawing from his quarter-century of experience as an art conservator, he suggests that even routine "cleanings" of oil paintings may hasten deterioration rather than delay it. Mecklenburg recalls unnerving occasions when paintings felt "mushy" during such procedures. Although mild soap solution may safely remove superficial dirt, conservators often use stronger organic solvents to remove top layers so old or discolored that they diminish a painting's aesthetic value.

"Things are happening [to paintings] that we didn't even dream of thinking about," Mecklenburg reported last week at a meeting of the Materials Research Society in San Francisco. Most investigations by scientific conservators have focused on the chemical effects of acetone, alcohols and other organic solvents. "But a lot of what goes wrong with museum objects is not chemical, it's mechanical," Mecklenburg contends.

For example, he found that even a 5-second acetone swab on model paintings that mimic old oil paintings can produce potentially damaging stresses. As solvent films evaporate, they draw heat from their surroundings and rapidly cool underlying regions to near-freezing temperatures. The paintings' different layers -- such as the fabric and the sealing glue that prevents paint layers from penetrating the fabric -- contract and expand differently in response to such temperature changes. Mecklenburg says he has measured "extraordinarily high stresses" in acetone-swabbed paintings.

Solvents pose other mechanical dangers. Each time a conservator applies a solvent to a painting, some of the oil base of the paint layers may leach out. "The paint becomes less flexible and more prone to mechanical damage," Mecklenburg told SCIENCE NEWS. He says researchers are only beginning to learn how these subtle effects work. Museum workers could use such information to implement safer procedures for handling, shipping and storage.

Collaborating with scientists at the Canadian Conservation Institute in Ottawa, Ontario, Mecklenburg has compared computer predictions of surface stress distribution with tests on artificial surfaces designed to behave like old paintings. Both the experiments and the computer models indicate that vibrations from impacts may not threaten oil paintings as much as curators suspect, he says. When the scientists dropped framed test paintings from carrying heights, the surfaces didn't shatter unless the impact occurred at a corner of the frame. This finding, though potentially reassuring to nervous handlers, hints that other "hidden" mechanical stresses -- such as those from solvents -- may underlie much of the deterioration observed in museum pieces.