How to Heal a Masterpiece

When a painting shows the ravages of time, conservators try a little TLC — tender loving chemistry.

By Curtis Rist|Thursday, April 01, 1999
RELATED TAGS: MATERIALS SCIENCE
On a wall in the Brooklyn Museum of Art's conservation department--the museum equivalent of an intensive care unit--hangs a remarkable oddity, Forest Scene with Brook. A century ago, an early American modernist painter named Ralph Albert Blakelock put his finishing touches on the charming landscape and set it aside. Curiously, the painting never dried. The oils failed to harden, and the entire scene--like the brook itself--has been flowing slowly but inexorably toward the bottom of the canvas ever since. Museum conservators tried hanging the painting upside down to strike equilibrium, but without success. "Today the painting has slid off the canvas and onto the frame," says conservator Carolyn Tomkiewicz. "The image is a complete loss."

Blakelock’s catastrophe, Tomiewicz and her colleagues believe, was probably triggered in part by his use of a synthetic pigment known as Van Dyke brown. The pigment appears to have retarded the process of drying that has kept other oil paintings clinging to their canvases for centuries. The demise of Forest Scene with Brook illustrates a larger issue: coming up with the materials to make a painting permanent may be as big a challenge as painting it in the first place. No single method can guarantee that a painting will last, and even if the chemical composition of an artist’s paints is sound, the resulting work can still be damaged by heat, light, humidity, and the rigors of being stashed in an attic for a generation or two. To help preserve and authenticate paintings, conservators have learned to examine them using techniques more common in a morgue than in a museum. They have learned to remove a tiny core sample from a work of art, analyze it to determine the chemical makeup of pigments, varnishes, and binders, and diagnose any ailments.

Poring over flecks of paint narrower than the width of a human hair, these art doctors have explored the secrets of some of the world’s greatest paintings. When a vandal slashed Rembrandt’s The Night Watch, conservators at the Rijksmuseum in Amsterdam matched pigments and painting strokes to stitch the masterpiece together again. In the Vatican, Michelangelo’s Sistine Chapel ceiling murals took on brilliant new tones once conservators removed centuries of grime by dabbing them with a watery solution of baking soda and other mild ingredients—a technique made possible by careful examination of the layers beneath to determine what wouldn’t dissolve them. “Sometimes the surface you see is as much a result of the hand of the restorer as it is of the artist,” says James Martin, a chemist at the Williamstown Art Conservation Center in Massachusetts. Understanding the chemistry of paints and pigments helps conservators work gently as they preserve original artwork.

Pigments have a long, rich history—almost as old as humanity itself. Prehistoric people first made patterns and images by rubbing chunks of charcoal and iron oxide onto cave walls. The challenge was to make the images permanent. “There’s evidence that very early on, humans began mixing animal fats with the pigments to make them adhere better,” says Melanie Gifford of the National Gallery of Art in Washington, D.C. And as binders evolved over the years, so did artistic styles. Roman artists created striking translucent portraits by painstakingly combining pigments with hot wax, then spreading the mixture onto wooden panels. Medieval scribes frothed up eggs with water and added colors to produce elaborate illuminated manuscripts. The resulting medium, known as egg tempera, was durable because the protein denatured and became insoluble as it dried—which also explains why a splotch of egg left on a breakfast plate is so difficult to wash off.


KIND OF BLUE

James Martin first uses a polarizing light microscope to identify paint pigments from The Virgin and Child with Saints John the Evangelist and Paul.

The color, shape, and size of the magnified pigment particles (middle) from the Virgin’s mantel allow Martin to quickly zero in on a small number of possible matches.

Next, he measures optical properties of the particles such as their refractive index, made visible as the particles rotate between polarizing filters.

Finally, Martin compares the particles with sample slides in his library of pigments (top) and narrows his search to a single match: azurite.

He confirms the sample’s chemical makeup as azurite using an infrared microscope.

The paint sample produces an infrared spectrum (bottom) with peaks that correspond to the chemical fingerprint of azurite.—C. R.


Popular as it was, egg tempera had limitations. The colors were opaque and somewhat pale, and the paint dried the instant the brush touched a surface. A false step was hard to correct. When artists in the early Italian Renaissance began applying egg tempera to large wooden panels instead of manuscript pages, they had to build up their images from tiny brush strokes. “You can’t blend colors together on the painted surface, so gradations have to be made by laying down successive strokes of lighter or darker paint,” says Gifford. “It is an arduous process.”

Meanwhile, a revolution was occurring in northern Europe. Painters, particularly in the Netherlands, were mixing pigments with plant oils such as linseed and walnut. Unlike the olive oils typically used to dress salads, which remains liquid over time, these special drying oils undergo a process known as polymerization. When exposed to air, the molecules of fat within the oils absorb oxygen and link together to form insoluble films (see “The Art of a Molecule,” page 78). Oils allow painters to use techniques unimaginable with egg tempera. The paints can be mixed into delicate glazes, or laid on in thick globs, called impastos, that give an almost sculptural relief to paintings. Working in oils, Renaissance artists could create a vivid illusion of reality by making individual brush strokes disappear. Because the paintings also tended to be varnished with shellac, a purified waxy resin excreted onto twigs by tree-dwelling insects called lacs, surface imperfections could be smoothed out. Light reflected off the pigments so that colors appeared deep and rich, like those of a wet river pebble that grows dull when it dries.

Just as the binding mediums for paints evolved, so did the pigments. The earliest artists simply picked up chunks of rock or charred bone and began to sketch. But as people discovered new ceramic glazes and fabric dyes, the pigments available for painting increased in number and complexity. Even a color as fundamental as white has gone through an elaborate evolution. The Romans developed a pigment called lead white, which they created by processing lead with vinegar. Although lead white is highly toxic, artists love how the paint made by mixing the pigment with oil covers surfaces without letting anything show through. When light passes through any uniform substance, it bends; scientists can measure this bending, which they call the material’s refractive index. On their own, oil and lead white pigment are both transparent, but they become beautifully opaque when mixed. That’s because the tiny pigment particles have a higher refractive index than the oil. Light entering the paint bends back and forth as it moves between particles of pigment and oil. Instead of passing through the paint, it’s scattered and reflected back.

FINGERPRINTING A PAINTING

By training an infrared microscope (top) on loose flakes from The Virgin and Child with Saints John the Evangelist and Paul, Martin can analyze the materials in the painted surface without destroying samples.

A more detailed analysis of deeper layers requires the removal of a tiny chip of paint—in this case from the tunic of the baby Jesus.

Martin encases the chip in a faceted block of hard epoxy (middle), then polishes it to reveal a smooth cross section (bottom).

Under a microscope illuminated with ultraviolet light, the layers come into focus. They include the gesso, charcoal from the artist’s first sketch, red clay, gold leaf, lead-tin yellow, and many layers of varnish and materials used in modern restoration.

Painters stopped using lead-tin yellow after 1750, so the painting must have been made before then. And the order and composition of the layers is in keeping with practices of Il Bergognone and his contemporaries.—C. R.


A rival to the whiteness of lead appeared after the discovery of zinc in the eighteenth century. Chemists introduced zinc oxide, which became popular in the 1830s. Although it had poorer hiding power than lead white because its refractive index came closer to that of oil, the zinc-based paint had a bluer hue, which some painters found desirable.

During the Renaissance, the most prized color was ultramarine blue, which was made from precious crushed lapis lazuli and usually reserved for the blue in the Virgin Mary’s mantel. “It was so valuable that artists would scrape it off paintings and reuse it,” says Martin. In 1824 the French government held an international competition to find a less costly replacement. They awarded a patent to a compatriot who developed a synthetic version. 

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.


THE ART OF A MOLECULE

When oil paints are applied to a canvas, they do more than just dry. In fact, since oils contain no water, technically they don’t dry at all. Instead they undergo a series of chemical changes in which molecules link together through a process called polymerization. If all goes well, the painting turns into a cross-linked network that’s bonded to the canvas. “It’s a complex process that scientists are still trying to understand,” says James Martin, a chemist from the Williamstown Art Conservation Center.

The key molecules in oils are known as triglycerides. They’re composed of four linked pieces, a glycerol molecule with three fatty acids attached. Drying oils, such as linseed and walnut, are unsaturated; their fatty acids contain plenty of double bonds between pairs of carbon atoms. These double bonds are inherently unstable, which makes the triglycerides highly reactive with other chemicals. When paint is first spread on with a brush, the carbon atoms jump into action by bonding with oxygen in the atmosphere. This process, known as autoxidation, gives an oil painting its dry-to-the-touch feel within a few days or weeks. But the painting would not be able to resist the incessant tug of gravity without the next step: polymerization. The triglycerides in each oil molecule form carbon-to-carbon bonds with adjacent molecules, forming a vast chemical network.

Martin likens the reaction to cooked pasta: unpolymerized triglyceride molecules move about freely, like single strands of spaghetti. “But if you toss a whole bunch against a wall, it will cross-link and stick together; the bonds will be quite strong,” he says. With oil paints, certain pigments—such as carbon black and Van Dyke brown—often inhibit the process of polymerization by preventing the triglycerides from hitching up. Paints containing these pigments can dissolve or rub off when a painting is cleaned—making it important for art conservators to find out the exact chemistry of a painting before they begin working on it. —C. R.


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.

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.

ART PHANTOMS

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.

Renior himself obliterated the image, which historians believe to be a portrait of one of his patrons, possibly after the man refused to pay for the painting. So how did it reappear?

Ordinarily, paints are opaque: the refractive index of the pigments and the oils are different, which means that light travels through them at different speeds. As it bends and reflects, the light scatters and cannot pass through to the layer below. But as the top surface of paint polymerizes—a process that continues for centuries—the refractive index of the oil increases. In the case of the Renoir, the refractive index of the oil has drawn close to that of the pigment. As a result, less light scatters; instead it passes through what has become a more transparent glaze and reveals what lies beneath it.

—C. R.


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.

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