Yellows evolved as well. Around 1300 painters began using lead-tin yellow, a pigment formed by heating lead and tin oxides in a crucible to tremendously high temperatures; chemists could control the hue by varying the temperature. The pigment disappeared from use around 1750, when the recipe was lost, according to some accounts. Another shade, called Indian yellow, was made on the subcontinent by feeding mangoes to cows and concentrating the urine to retrieve the calcium and magnesium salts that create the color. (The British government outlawed the process in 1908 on the grounds of cruelty.) Since many of these pigments are toxic, “the task of grinding them and mixing them with oils during the Renaissance fell to some lowly—and dispensable—apprentice,” says Sandra Webber, a conservator from the Williamstown lab.
Chemists can use these distinct chapters in the technological history of paint to help determine the authenticity of a work of art. In the 1980s, during an inspection of a painting known as The Virgin and Child with Saints John the Evangelist and Paul, believed to date from the 1400s, a chemical analysis of a tiny chip of paint found a nasty surprise: zinc. Ambrogio da Fossano, called Il Bergognone, the purported painter, could not possibly have had a zinc-based pigment on his palette some four centuries before the element’s discovery. “The painting was deemed a forgery and moved downstairs into the basement storage room,” says Martin.
Suspicious that the zinc pigment was applied during a modern restoration, Martin took a closer look at the painting. in 1994 with a group of undergraduates. Lifting a scalpel to the surface of the rich yellow tunic worn by the Christ child, he gently pressed in and removed an all but invisible core sample. He encased this fleck of paint in epoxy and polished it smooth to expose a cross section of the chip. Then, working with a microscope and a computer monitor, he examined the geologic-looking strata of varnish, paint, and gilding. He focused on the pigment particles in the layer of yellow paint, and next slid them into a scanning electron microscope. In addition to producing a picture, this microscope produces a line graph with peaks corresponding to the elements present. Martin found lead and tin, an indication that the pigment was lead-tin yellow—which dated the painting to before 1750. “We can’t authenticate paintings using scientific techniques alone, but we can present evidence that art historians can then interpret,” says Martin. The painting was attributed to the school of Il Bergognone and returned to its wall in the neighboring Clark Art Institute.
Jackson Pollock’s works with cigarette butts, added
for texture, have been stable so far
Chemists can also help with conservation—and ease the damage inflicted by past barbarisms. In many old paintings, the original work has been lost beneath countless layers of touch-ups and revisions by later artists. “Art used to be done in secret, so that no one had any idea what the condition of the original painting was,” says Martin. Using a variety of techniques, from X-rays to infrared imaging, modern conservators can find out. In the case of Il Bergognone’s Virgin and Child the results are not encouraging: a later, inferior hand painted a floral motif across the bottom of the painting and gave the Virgin a wardrobe change by outfitting her in an entirely new dress of dark cloth with a fleur-de-lis pattern. Often such changes are irreversible. “If oil paint is used on top of oils, it binds to the surface and can’t be removed,” says Martin. This practice was common during the Victorian era. Damaged areas were filled in, nude figures were covered modestly in clothes, and sometimes entire characters were transformed. One Spanish portrait at the Williams College Art Museum features St. Lucy bearing her trademark symbol—a plate holding a pair of eyeballs. In the nineteenth century she was recast as the primly appealing St. Cecilia, carrying a book and wearing a veil. Fortunately, it was possible to reverse the makeover because it was applied on top of varnish rather than directly onto the oil paint.
Sometimes, after decades or centuries, an oil painting sprouts a ghost. A hidden image gradually appears in a corner of the canvas. In Pierre-Auguste Renoir’s canvas At the Concert, painted in 1880, the figure of a man in evening dress now floats faintly above the girl. Here, an infrared video camera and monitor show the image more clearly. Infrared energy from lights penetrates surface paint and is absorbed by black pigments made of carbon; the camera catches this energy.
Deciding what to fix and what to leave alone has become a touchy issue for modern conservators. But in the past, restorers had no such qualms. One of the many treasures now at the Clark, by the Italian painter Perugino, was originally painted on a wood panel. During the 1940s, before the museum acquired it, a restorer peeled the paint off like a fruit roll-up and reapplied it to a flat piece of masonite. “There’s a photograph of a painting being held up in the air, and you can see the light shining through from behind,” says Webber. “It’s really scary. I mean, it would be fun to try—but not with a Perugino!”
Using microscopic analysis to determine the chemical composition of the original pigments, conservators now apply paint only to areas where the damage is so distracting it cannot be left alone. Cotton swabs soaked with saliva are used to dab paintings clean; the enzymes in saliva dissolve surface proteins and grime safely, without harming the picture. Moreover, the work of the conservators is reversible because they use materials that are stable but don’t form permanent bonds with the painting. “We do everything in our power not to alter the intent of the artist,” says Martin. “Even if we think a shiny surface would look better than a matte finish, we can’t change it.” Still, he admits that no system is perfect, and each cleaning and analysis removes something of the original: “Every time an object comes in this door, it gives something of itself up in order to continue to live.”
Works of modern art present a whole new set of problems for conservators. Twentieth-century painters have experimented not only with abstract forms but with abstract substances as well. Jackson Pollock, for instance, used drums of World War II surplus paints for his splashiest effects, and he often added texture by mixing in things such as cigarette butts. These appear to be stable, at least so far. Another abstract painter, Franz Kline, used house paints for his trademark black-on-white forms; already the whites are yellowing. Willem de Kooning, an abstract expressionist, often used nonhardening safflower oils that remain tacky even today, some 30 years later. “There is some dust on the surface that can be brushed out carefully, but the canvases basically can’t be cleaned, ever, without removing the paint,” says de Kooning expert Susan Lake, a painting conservator with the Smithsonian Institution’s Hirshhorn Museum and Sculpture Garden in Washington, D.C. As for the persistent rumor that de Kooning also mixed pigments with mayonnaise—an ersatz cross between egg tempera and oil—Lake is still searching for the evidence. “I haven’t found cholesterol yet in any of the paintings I’ve looked at,” she says. Despite such tricky substances, conservators of modern paintings have an easier time than conservators of modern sculpture, where a favorite medium just now is chocolate.
Deciding whether it’s worth the effort to conserve what an artist obviously planned as an ephemeral form has given rise to a debate among critics and historians: Should art ever be allowed to die? Some say yes: even the act of preserving these works, they believe, would alter the artist’s original intent. But the chemist would like to have a crack at conserving everything else. “It’s a challenge, because you never know what you’re going to find when you look closely at a canvas,” says Martin. And therein lies the art.