During the single decade of his short career, from 1880 to 1890, Vincent van Gogh produced more than 800 oil paintings and 1,200 drawings. Yet the prolific Dutch artist sold no more than three pieces of art during his lifetime, a shortfall that left him unable to afford enough new materials to keep up with his frantic work pace. With countless ideas but no fresh canvases to put them on, van Gogh often resorted to painting and drawing on whatever was handy, such as scraps of towel or even completed works that he deemed less valuable or interesting than his new compositions.

Art historians estimate that as many as 30 percent of van Gogh’s paintings could have underlying images. And van Gogh was not the only recycler. For more than a century, x-rays have been revealing hidden compositions under paintings by artists including Pablo Picasso, N.C. Wyeth and Leonardo da Vinci.

The radiation produced by conventional x-ray techniques can produce only black-and-white images of the underlying works. Yet in the past decade, more-focused and powerful beams of radiation have enabled scientists and art historians to see unprecedented details, like the original colors of the buried artwork, in hundreds of reexamined paintings without having to damage the overlying layers.

Masterpiece Forensics

Finding hidden works isn’t the only motivation for subjecting paintings to closer analysis. Curators, dealers and collectors also use imaging techniques to crack down on counterfeiters. It’s relatively easy to spot poorly made fakes just by examining the paints and brushstrokes. But careful counterfeiters, most of whom are artists themselves, use historically correct materials on old canvases and frames. In those cases, analysts look for scannable traits that are known to exist in an original painting, such as hidden layers or previously restored areas, which are harder to fake.

For historians, revealing original sketches can illuminate the artist’s process. “When we look at these hidden details, it’s like we are looking over a painter’s shoulder and following his or her thought process,” says Joris Dik, an art historian and materials chemist at Delft University of Technology in the Netherlands, who helped lead a recent analysis of several van Gogh works. “We can try and understand the choices we see him making, from the pure material choices, paints and such, to his artistic evolution.”

In 2005 historians from the National Gallery in London examined da Vinci’s 1508 painting The Virgin of the Rocks, in which the Virgin Mary, her arm wrapped around the shoul- ders of a child, faces another woman and child in a rocky landscape. To look beneath the masterpiece, the researchers used infrared reflectography, a technique in which infrared light passes into the paint layers and is either absorbed by dark color pigments or reflected by light ones. The reflected light is captured by an infrared camera and digitized into a black-and-white map of the underlying image. The team discovered the outline of a kneeling woman in another landscape of rocks, her face in profile, one arm across her chest and the other outstretched behind her.

The hidden woman’s head was drawn in short, overlapping strokes of paint or ink, which from a distance look like a single, continuous line. This suggests that da Vinci was painting over existing marks. The right arm and landscape, however, were dashed in, and using a fatter brush, an indication that he made these strokes without an outline. Da Vinci, the historians concluded, outlined only parts of a painting, doing the rest freehand.

More-Powerful X-rays

In recent years, researchers at the Art Institute of Chicago used improved infrared reflectography on the previously discovered painting of a young woman underlying Picasso’s The Old Guitarist. The scan revealed the woman’s thoughtful expression and long dark hair but not the colors Picasso imparted to her.

Such black-and-white images were also all that researchers initially had of a portrait of a woman hidden under van Gogh’s Patch of Grass, an 1887 landscape currently at the Kröller- Müller Museum in the Netherlands. Art historians at the museum first discovered the portrait in the 1990s using conventional x-ray technology. Materials made of elements with low atomic weights, such as carbon-based paints, allow x-rays to pass through them easily, so they appear transparent on x-ray film. In contrast, paints that contain heavy elements, such as lead white, block the radiation and appear dark on film. Because ordinary x-rays can record the presence of different color pigments only in black and white, imaging work done on Patch of Grass provided only a vague impression of the hidden portrait.

In 2006 Dik and Koen Janssens, a chemist at the University of Antwerp in Belgium, heard about a newer technique used by Stanford University scientists to decipher unreadable text in the works of the classical Greek mathematician Archimedes. The researchers had used x-ray fluorescence imaging—a technique in which radiation from a ring-shaped particle accelerator called a synchrotron bombards a target—to reveal the hidden text. Dik, Janssens and their colleagues decided to test the technique on van Gogh’s Patch of Grass, with help from scientists at the German Electron-Synchrotron in Hamburg and the European Synchrotron Radiation Facility (ESRF) in France.

Synchrotron radiation is 100 to 1,000 times as strong as conventional x-rays, so it can penetrate layers of paint deeply and with little distortion, quickly scanning underlying properties. The concentrated beam of radiation from the synchrotron knocks electrons off the inner orbits of atoms in the target material. As electrons from the outer orbits move inward to fill the gaps, they emit radiation at characteristic wavelengths called fluorescence radiation. Re- searchers analyze these wavelengths to determine the elemental composition of the material being scanned.

By looking for combinations of elements that correspond to specific colors, the research team unveiled details of the portrait beneath Patch of Grass. Van Gogh used vermilion, a vivid red, and Naples yellow in the portrait of the woman. Vermilion, which is made from mercury sulfide, colored the woman’s hat, lips and parts of her skin. The artist blended Naples yellow, which contains lead antimonate, with zinc-white and lead-white pigments to highlight areas of the painting. A stratigraphic sample of the paint layers—a physical core sample that extends from the surface to the base layers—confirmed the results.

Why would van Gogh cover up his original work? Based on preserved letters, historians know that the artist spent time in Nuenen, a small, rural town in the Netherlands, between October 1884 and May 1885, where he chronicled peasant life in a series of paintings he later sent to his brother, Theo, an art dealer in Paris. Two and a half years after leaving Nuenen, van Gogh joined his brother in Paris. Theo gave the painter money to live, but the artist spent it all on art supplies. Dik believes that van Gogh’s impulsiveness and poverty, combined with his exposure to French avant-garde artists who changed his aesthetic style, most likely led him to paint over the portrait, replacing it with a colorful image from nature that was more in line with the new movement.

More Masters Get Scanned

X-ray fluorescence imaging is also revealing how da Vinci painted such lifelike faces. Researchers from the ESRF and the Center for Research and the Restoration of the Museums of France recently used the technique to study his use of sfumato, a range of subtle optical effects that blur outlines, soften transitions, and blend shadows like smoke.

Last summer, the researchers announced that they had determined the composition and thickness of each layer in da Vinci’s Mona Lisa and eight other faces he painted throughout his career. Their work also uncovered different shadowing “recipes” he used: certain pigments and additives and a technique involving glaze layers or very thin paint.

Dik, Janssens and their colleagues have also applied x-ray fluorescence imaging to works by Rembrandt, Goya and the German romantic painter Philip Otto Runge, as well as several other van Gogh pieces that had been previously scanned with conventional x-rays. Most recently, their analysis of an early 19th-century Runge painting of a demurely dressed, brown-haired woman uncovered an underlying portrait of a different woman, who had loose blond hair and a more revealing dress. “There has been disagreement among scholars as to whether or not this painting even belongs to Runge,” Janssens says. “By revealing the hidden layers—the colors used and style—we have provided art historians with more arguments for or against.”