Every few weeks, photographs of old paintings arrive at Martin Kemp’s eighteenth-century house, outside Oxford, England. Many of the art works are so decayed that their once luminous colors have become washed out, their shiny coats of varnish darkened by grime and riddled with spidery cracks. Kemp scrutinizes each image with a magnifying glass, attempting to determine whether the owners have discovered what they claim to have found: a lost masterpiece by Leonardo da Vinci.

Kemp, a leading scholar of Leonardo, also authenticates works of art—a rare, mysterious, and often bitterly contested skill. His opinions carry the weight of history; they can help a painting become part of the world’s cultural heritage and be exhibited in museums for centuries, or cause it to be tossed into the trash. His judgment can also transform a previously worthless object into something worth tens of millions of dollars. (His imprimatur is so valuable that he must guard against con men forging not only a work of art but also his signature.) To maintain independence, Kemp refuses to accept payment for his services. “As soon as you get entangled with any financial interest or advantage, there is a taint, like a tobacco company paying an expert to say cigarettes are not dangerous,” he says.

Kemp, who is in his sixties, is an emeritus professor of art history at Oxford University, and has spent more than four decades immersed in what he calls “the Leonardo business,” publishing articles on nearly every aspect of the artist’s life. (He even helped a daredevil design a working parachute, from linen and wooden poles, based on a Leonardo drawing.) Like many connoisseurs, Kemp has a formidable visual memory, and can summon into consciousness any of Leonardo’s known works. When vetting a painting, he proceeds methodically, analyzing brushstrokes, composition, iconography, and pigments—those elements which may reveal an artist’s hidden identity. But he also relies on a more primal force. “The initial thing is just that immediate reaction, as when we’re recognizing the face of a friend in a crowd,” he explains. “You can go on later and say, ‘I recognize her face because the eyebrows are like this, and that is the right color of her hair,’ but, in effect, we don’t do that. It’s the totality of the thing. It feels instantaneous.”

Other authenticators have also struggled to explain their evaluative process, their “eye.” Thomas Hoving, the former director of the Metropolitan Museum of Art, who died in December, liked to speak of the “ineffable sense of connoisseurship.” The art historian Bernard Berenson described his talent as a “sixth sense.” “It is very largely a question of accumulated experience upon which your spirit sets unconsciously,” he said. “When I see a picture, in most cases, I recognize it at once as being or not being by the master it is ascribed to; the rest is merely a question of how to fish out the evidence that will make the conviction as plain to others as it is to me.” Berenson recalled that once, upon seeing a fake, he had felt an immediate discomfort in his stomach.

In March, 2008, Kemp checked his e-mail and saw another submission—a digital image of a drawing on vellum, or fine parchment. Ever since Dan Brown published “The Da Vinci Code,” five years earlier, Kemp had been flooded with works, many of them purportedly embedded with cryptic symbols, and, after a lifetime of dismissing forgeries and copies and junk, he was instinctively wary. About thirteen inches long and nine inches wide, the picture showed the profile of a girl, on the cusp of womanhood, with pale skin and glowing brown hair pulled back in a long ponytail. Her left eye, the only one visible in the profile, had a lifelike translucency. Her upper lip pressed secretively against her lower one, and a red bodice peeked out from underneath a green dress. The artist had meticulously rendered the girl’s features with pen and colored chalks (“Her face is subtle to an inexpressible degree,” Kemp later wrote), and Kemp felt a shiver of recognition. He enlarged the image on his computer screen until it became a mosaic of pixels. He looked closely at the shading—it seemed to have been drawn with a left hand, just as Leonardo had done.

Kemp tried to contain his excitement. A major work by Leonardo had not been discovered for more than a century. This drawing had no clear provenance—a trail of invoices, catalogue listings, or other records that can allow a work to be traced back to an artist. Rather, the drawing seemed to have come, as Kemp later put it, “from nowhere.” In 1998, Kate Ganz, a prominent dealer, had paid a little less than twenty-two thousand dollars for the drawing, at an auction at Christie’s. (The auction house did not disclose the previous owner’s identity, saying only that the picture had been the “property of a lady.”) At the time, the drawing was thought to have been executed in the nineteenth century, by a member of a German school of artists known for imitating Italian Renaissance painters. If the drawing was by Leonardo, it had slipped past some of the world’s most respected connoisseurs and collectors—people whose eyes are honed to look for fortune in addition to beauty. As Hugh Chapman, an assistant keeper in the Department of Prints and Drawings at the British Museum, later told the Times, “The market is a fairly efficient place. This would be an amazing miss.”

A purported Pollock fingerprint on a stretcher. *Photograph by Georgianna Lane / Courtesy Fine Art Registry * Photograph by Georgianna Lane / Courtesy Fine Art Registry

In January, 2007, Ganz sold the drawing at her gallery in Manhattan for roughly what she had paid for it. As is common in the art world, the identity of the new owner was a secret. Officially, the purchasing agent was listed as Downey Holdings, a Panamanian business with an address in Jersey, in the Channel Islands, which is popular as a tax haven. The purchase was made under the guidance of Peter Silverman, a Canadian collector who has a reputation in the business (though he dislikes the term) as a “picker”—someone who scours auction houses for undervalued works. Silverman told me that he had bought the drawing for a collector in Switzerland who is one of “the richest men in Europe.” Many people in the art world have speculated that Silverman himself is the owner. He denied this, but added, “Even if it were true, I wouldn’t say.”

Upon seeing the drawing, Silverman thought that it had to be from the Renaissance, and before long, he said, he began to consider “the ‘L’-word”— Leonardo. He submitted the drawing to tests that have become a standard part of the authentication process. Many of the drawing’s pigments were analyzed, and it was determined that none of them had been invented after Leonardo’s time period. A sample of the parchment was sent to the Swiss Federal Institute of Technology, in Zurich, for radiocarbon dating. The parchment was dated between 1440 and 1650, making it conceivable that the drawing was by Leonardo, who was born in 1452 and died in 1519. After receiving these results, Silverman contacted Kemp and sent him the image.

As Kemp well understood, countless artists could have made the drawing in that two-hundred-and-ten-year span. And many modern forgers come out of the field of restoration, where they learn not only how to copy an artist’s style but also how to exploit historically appropriate materials: organic pigments, antique wooden frames infested with beetles, canvases blackened by centuries of smoke. In the nineteen-thirties, the notorious Dutch forger Han van Meegeren, who produced at least nine fake Vermeers, used a canvas from the seventeenth century that still had its original stretcher. (Like many forgers, Van Meegeren insisted that he was “driven by the psychological effect of disappointment in not being acknowledged by my fellow artists and critics.”)