Chemistry diagnoses a painting's ills.

A doctor giving a lecture in anatomy points to the dissected arm of a cadaver while his pupils look on intently. The Dutch artist Rembrandt van Rijn painted this scene in 1632. Now, nearly 4 centuries later, the canvas known as "The Anatomy Lesson of Dr. Nicolaes Tulp" has itself gone under the scalpel.

New techniques developed by a team of Dutch scientists have helped art conservators in their recent restoration of this 17th-century masterpiece. By examining tiny paint chips with various spectroscopic methods, the scientists determined the chemical compositions of the paint, glaze, and varnish layers on the canvas. This information provided insight into artistic techniques and guidance to conservators deciding how to clean and repair the artwork.

"There's a big gap in the understanding of the basic processes of aging in painted art," says Ron M.A. Heeren of the FOM Institute for Atomic and Molecular Physics in Amsterdam. To fill that gap, researchers are working with MOLART, a Dutch project to "provide the art conservation community with tools and fundamental research capabilities," Heeren explains. He described the group's techniques this week at the Pittsburgh Conference in Orlando, Fla.

"What they have done is much more fundamental chemistry than what has been done previously" in art conservation, says David Erhardt of the Smithsonian Center for Materials Research and Education in Suitland, Md.

With microscope and scalpel, the Dutch researchers remove flakes of paint about 1 millimeter across, routine practice in restoration projects. The scientists mount them in resin and polish the samples with fine aluminum oxide particles in water. By studying how the surfaces then interact with light, the scientists can explore the chemical composition of the paint chips. "The sensitivity of analytical techniques has improved so much that you don't need large quantities of material," Heeren says.

Before it reached its current home in the Mauritshuis, a museum in The Hague in the Netherlands, "The Anatomy Lesson" hung for many years in the Amsterdam surgeon's guild where Nicolaes Tulp lectured. There the painting experienced mishaps that would make any art lover cringe. It had been rained upon and dirtied by smoke from a fire, for example, says Heeren.

When conservators began examining the painting a few years ago, they noticed many holes. The scientists have now found that the white material in these microscopic craters consists of carboxylates making up a benign soap containing lead. A chemical reaction in or on the painting created this substance, but whether the constituents came from Rembrandt's brush, the environment, or previous restoration efforts remains unknown. The conservators decided that the best strategy was to leave the holes alone.

In another case, the MOLART group detected protein, confirming that Rembrandt used eggs to improve the consistency of paints. With analytical methods, Heeren says, scientists can reveal the secrets of artists from another age.