What Lies Beneath

Scientist J. Bianca Jackson is an art sleuth. Using a technology found in airport security body scanners, she searches for lost artwork hidden beneath layers of paint and plaster. Her technique, called terahertz imaging, has enabled Jackson to unveil an old mural behind the white wall of a Latvian cathedral, as well as ritualistic decorations, obscured by clay, from the dwellings of a Neolithic settlement in Turkey. Most recently, she helped curators at the Louvre Museum in Paris investigate the complicated history of a Roman fresco, which had been restored—and likely forged—by a 19th-century Italian art collector.

Over time, artwork can disappear or transform due to a number of causes, such as building renovations, natural disasters, or forgeries. While terahertz imaging isn’t a new concept—it’s been used in physics research labs for nearly two decades—it has only recently been applied to recover forsaken art, and Jackson is one of its foremost proponents. “She’s absolutely on the top,” says Gérard Mourou, a French physicist and terahertz expert who has been an adviser to Jackson throughout much of her training. “She does the best job in this area.”

Jackson, a spunky woman with a crescent-moon smile, rectangular, tortoise-shell glasses, and dark hair that escapes her ponytail in helical wisps around her face, first used terahertz imaging to detect corrosion beneath the protective covering on airplane turbine blades when she was a graduate student in applied physics at the University of Michigan. At Mourou’s suggestion, she eventually shifted her focus from aviation to art—fitting for the daughter of an art teacher—and became a postdoctoral fellow at the Louvre, where she set up the terahertz laboratory, the first lab of its kind to be based at a museum (it has since moved to another location).

A terahertz imaging device works by aiming terahertz waves, a type of electromagnetic radiation, at an object of interest and measuring the intensity of the radiation that bounces back as a function of time. Waves that hit an object’s surface come back first, while waves that penetrate deeper layers take longer to return; Jackson calls the latter “trailing signals.” A computer then transforms data from the reflected waves into information about the object’s material properties. As the data is being acquired, an image is produced on the screen that can be analyzed further once the scan is complete. Information about the trailing signals interests Jackson the most: It tells a story about whatever might lie beneath an object’s façade.

Unlike similar techniques used to peer beneath a surface, such as X-rays and ultraviolet rays, terahertz radiation (which lies between microwaves and infrared light on the electromagnetic spectrum) is non-ionizing, meaning that it doesn’t displace electrons in the atoms that comprise the item under investigation. In other words, the subject remains intact. “One of the reasons we are excited about it as an art history institution is that this type of energy is not damaging in any way to the artifacts,” says Laura D’Alessandro, head of the Conservation Laboratory at the University of Chicago’s Oriental Institute, where Jackson applied the technique to examine bird mummies.

Most recently, Jackson used terahertz imaging to shed light on the history of a fresco housed at the Louvre called Trois Hommes Armés de Lances, or Three Men Armed With Spears. The piece had come from the troves of Giampietro Campana, a 19th-century Italian collector known for restoring, and sometimes forging, Greek and Roman works.

Recent restoration work by a graduate student named Pauline Lascourrèges found that the fresco consisted of about 80 fragments, which Campana and his workshop had probably excavated from catacombs near Rome and plastered together. The amalgam featured a trio of men dressed in a type of ancient Greek clothing called himation.

Curators were wary of the fresco’s authenticity, however. As she was cleaning the artwork, Lascourrèges had uncovered another layer of paint beneath the surface, which revealed three incomplete visages of Roman origin. The discovery suggested that when Campana’s crew had reassembled the fresco, they had also painted over it—“rather awkwardly,” according to Daniel Roger, a curator of Roman art at the Louvre. While Lascourrèges had used various imaging tests to learn about the painting’s history, she turned to Jackson and her novel technique to see what else could be uncovered.

One day this past June, Jackson went to work scanning a portion of the fresco under particular scrutiny: the body of one of the himation-clad men. She set up her device (relatively portable at about 50 pounds) on a motorized stage next to the painting, which moved the instrument as it collected data at a rate of five square millimeters—corresponding to five pixels—per second. She spent all day examining just one-third of one man—an area smaller than 13 square feet.

Back in the lab, Jackson examined her data. At first, “I wasn’t sure what I was looking at,” she says. Then she recognized “something that sort of looked like a mouth,” as well as other facial features in an area corresponding to, roughly, the man’s hips. After showing the image to others and asking them what they saw—“like a Rorschach test,” she says—Jackson was convinced that she had uncovered another ancient Roman face.

In order to confirm Jackson’s observation, curators would need to remove the 19th-century paint layer, says Roger. Pending further discussion, the fresco—now in storage—is, in a way, resting in the catacombs of the museum.

Meanwhile, Jackson continues honing her technique as a postdoctoral fellow at the University of Rochester. When she finishes, she hopes to start a consulting firm to help others investigate more mysteries using terahertz imaging.

An earlier version of this article stated a publication date of March 15, 2013. The publication date was May 15, 2013.