Forest Scene with Brook R. A. Blakelock, Brooklyn Museum of Art

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.

Sterling and Francine Clark Art Institute, Williamstown, Massachusetts.

The Virgin and Child with Saints John the Evangelist and Paul, in the style of Il Bergognone. Renaissance triptych or nineteenth century fabrication?  Ask a chemist.

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.

The task of grinding toxic pigments fell to dispensable apprentices


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

Conservators dab painting clean by using swabs soaked with saliva.

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. 



James Martin
Clark Art Institute
Brooklyn Museum of Art
National Gallery of Art
Hirshhorn Museum