Before the Biros left the Tate, they say, they walked through a gallery that had several Turner paintings on display. Peter Paul paused in front of Turner’s “Chichester Canal,” peering at the pale-blue sky reflecting off the waterway, which made it seem as if the earth had been turned upside down. In the foliage of several trees, he says, he noticed tiny swirls in the paint. He looked more closely. They were from a partial fingerprint. He felt a jolt: he had noticed partial fingerprints embedded in the potential Turner painting as well. In both pictures, he says, the ridges were deep enough in the original dried paint that they could not have been left by the hands of an owner or a restorer; rather, they were a by-product of Turner’s technique of modelling paint with his fingertips. Indeed, Biro says, he subsequently found fingerprints in hundreds of Turner’s works, and wondered: Why not compare the fingerprint in an undisputed Turner painting like “Chichester Canal” with the one in his own painting, and see if they matched?

The desire to transform the authentication process through science—to supplant a subjective eye with objective tools—was not new. During the late nineteenth century, the Italian art critic Giovanni Morelli, dismissing many traditional connoisseurs as “charlatans,” proposed a new “scientific” method based on “indisputable and practical facts.” Rather than search a painting for its creator’s intangible essence, he argued, connoisseurs should focus on minor details such as fingernails, toes, and earlobes, which an artist tended to render almost unconsciously. “Just as most men, both speakers and writers, make use of habitual modes of expression, favorite words or sayings, that they employ involuntarily, even inappropriately, so too every painter has his own peculiarities that escape him without his being aware,” Morelli wrote. He believed that not only did an Old Master expose his identity with these “material trifles”; forgers and imitators were also less likely to pay sufficient attention to them, and thus betray themselves. Morelli became known as the Sherlock Holmes of the art world.

To many connoisseurs, however, the nature of art was antithetical to cold science. Worse, Morelli made his own share of false attributions, prompting one art historian to dismiss him as a “quack doctor.”

In the early twentieth century, as J. P. Morgan, Henry Clay Frick, and other wealthy Americans bid up prices of Old Masters, the search for a foolproof system of connoisseurship intensified. At the same time, the flood of money into the art market led to widespread corruption, with dealers often paying off connoisseurs to validate paintings. In 1928, the art dealer René Gimpel complained, “The American collector is prey to the greatest swindle the world has ever seen: the certified swindle.”

The public has long been suspicious of connoisseurship. As John Brewer recounts in his recent book “The American Leonardo,” about a Kansas City couple’s battle, in the nineteen-twenties, to authenticate a potential Leonardo, this distrust had to do with more than the system’s reliability; it also had to do with doubts about the authenticity of the art world itself, with its cult of prized artists, its exorbitant trafficking in aesthetic pleasure, and an élite that seemed even more rarefied than most. In 1920, the Kansas couple, Harry and Andrée Hahn, sued the powerful art dealer Joseph Duveen for half a million dollars after he told a reporter that a portrait they owned could not possibly be a Leonardo. The Hahns argued that connoisseurs offered only “air-spun abstractions and nebulous mumbo-jumbos,” and that “smart and tricky art dealers” ran a “racket.” Even the judge in the case warned jurors to be wary of experts who relied on means “too introspective and subjective.” (Though none of the leading connoisseurs considered the painting a Leonardo, and later technical evaluations confirmed their judgment, the trial ended in a hung jury, and Duveen paid the Hahns sixty thousand dollars to settle the case.)

The desire to “scientificize” connoisseurship was therefore as much about the desire to democratize it, to wrest it out of the hands of art experts. Before the Hahn trial, rumors surfaced that there was a thumbprint in the paint. One newspaper asked, “WILL THUMBPRINT MADE 400 YEARS AGO PROVE PAINTING IS LEONARDO DA VINCI’S ORIGINAL?” But identifying the author of a painting through fingerprints still seemed far beyond the reach of science, and the process of authentication remained largely unchanged until Biro came up with his radical idea.

After returning from London, Biro studied books on fingerprinting and conferred with a retired fingerprint examiner. He learned the difference between a latent print—which is transferred with sweat and often needs to be dusted or processed with chemical agents in order to be detected—and a visible print, which is either impressed in a substance or left by touching a surface with something on one’s fingertips, such as ink. He learned fingerprint patterns, including loops, whorls, and tented arches. And he learned how to tell whether two fingerprints had enough overlapping characteristics to be deemed a match. “He basically trained himself,” Laszlo recalls. “He read and studied everything.”

Biro asked the conservation department at the Tate for images of “Chichester Canal” that were sufficiently high in resolution to show the fingerprint. For days, Biro says, he compared enhanced images of the fingerprint with the one on the rainbow painting; he felt certain that they came from the same person.

Yet the art establishment refused to recognize the painting based on his approach. (As Laszlo puts it, the art world is “very jealous and sinister.”) In 1994, after years of frustration, the Biros took the painting to a Turner scholar, David Hill, at the University of Leeds. He thought that the composition and coloring strongly pointed to the hand of Turner, and he enlisted John Manners, a fingerprint examiner with the West Yorkshire Police, to verify Biro’s conclusions. “Not my cup of tea, really,” Manners said of the painting at the time. “Of course, some Turner canvases are magnificent. Not this one, in my opinion.” Still, he said, the fingerprints definitely matched: “It is a Turner.” Hill called the fingerprints the “clinching piece of evidence.”

The story of the fingerprints circulated around the world—“BURIED TREASURE VERIFIED BY SCIENCE,” the Toronto Globe and Mail declared—and many Turner scholars relented on the question of attribution. “It was the pressure of the media,” Biro said. “They were beginning to look foolish.” In 1995, the painting, called “Landscape with a Rainbow,” was sold as a Turner at the Phillips auction house in London. An undisclosed bidder bought it for more than a hundred and fifty thousand dollars—a sum that would have been even higher had the painting been in better condition. It was the first art work officially authenticated based on fingerprint identification. Biro asserted that he had uncovered the painting’s “forensic provenance,” telling a reporter, “The science of fingerprint identification is a true science. There are no gray areas.” Having developed what he advertised as a “rigorous methodology” that followed “accepted police standards,” he began to devote part of the family business to authenticating works of art with fingerprints—or, as he liked to say, to “placing an artist at the scene of the creation of a work.”

In 2000, Biro took on an even more spectacular case. A retired truck driver named Teri Horton hired Biro to examine a large drip canvas, painted in the kinetic style of Jackson Pollock, that she had bought for five dollars at a thrift shop in San Bernardino, California. After inspecting the work, Biro announced that he had found a partial fingerprint on the back of the canvas, and had matched it to a fingerprint on a paint can that is displayed in Pollock’s old studio, in East Hampton. André Turcotte, a retired fingerprint examiner with the Royal Canadian Mounted Police, supported the results. But the International Foundation for Art Research, a nonprofit organization that is the primary authenticator of Pollock’s works, balked, saying that Biro’s method was not yet “universally” accepted. Biro, in a report on Horton’s painting, wrote that he had been warned that “science prying into the closed world of connoisseurship is likely to make me many enemies.” Horton, meanwhile, became a modern-day Harry and Andrée Hahn, dismissing the method of traditional connoisseurs as “bullshit,” and the whole art world as a “fraud.”

Biro told me that he maintains a firewall between his research and the sale of a painting, and that he receives the same fee—two thousand dollars a day—regardless of the outcome of his investigation. “If I stopped being disinterested, my credibility will be gone,” he said. But he felt “morally obliged” to stand behind his findings.

The effort to authenticate the painting became a crusade. Horton went on “The Tonight Show with Jay Leno,” and her struggle was valorized in a 2006 documentary called “Who the #$&% Is Jackson Pollock?” In the film, Biro is depicted as a champion of science and of a woman with an eighth-grade education battling an autocratic establishment. The main antagonist—“the effete, nose-in-the-air art expert,” as he later quipped of his role—is Thomas Hoving. He is shown, in a suit and tie, sitting before Horton’s picture and declaring, “Dead on arrival.” Later, offering a rationale for his response, he noted that Horton’s picture featured acrylic paint, which had not previously been documented in Pollock’s drip paintings.

Biro, undaunted, visited Pollock’s old studio and extracted pigment samples from the floor, where the artist had once spread his canvases and applied paint. In a report, Biro wrote that he had used a “microchemistry test”—a method of mixing a paint sample with other chemicals to analyze its characteristics. “The very first sample of paint I tested,” he said, “turned out to be acrylic.” He also revealed that gold paint from a matchstick embedded in the floor was the same as gold paint found in Horton’s picture. Hoving remained unmoved. He dismissed the fingerprints, and said of Horton, “She knows nothing. . . . I’m an expert, she’s not.” In reviews of the film, Hoving was denounced as a “pompous fool” and a “villain”; Biro was called a “hero.”

Based on Biro’s findings, Horton was offered two million dollars for her painting, but she held out for more. Biro assured her that the art world could not continue to resist a forensic method that had been used to convict criminals for more than a century. And though many connoisseurs and collectors opposed his technique, more and more accepted it. He told me that he had authenticated two Picassos, half a dozen Turners, a Thomas Hart Benton, and close to a dozen other Pollocks. Several of the world’s top connoisseurs sought Biro’s expertise. Three years ago, two leading Pollock scholars, Claude Cernuschi and Ellen G. Landau, cited Biro’s evaluation of a suspected Pollock, saying, “Artists’ fingerprints do not show up just anywhere. Their presence cannot be dismissed or simply explained away.” Around this time, Biro helped Martin Kemp attribute a painting, partly on the basis of fingerprints, to one of Leonardo’s assistants. In an earlier e-mail to a client, Biro wrote, “The world is changing. Not as fast as one would hope but it is changing nevertheless.”

In 2009, Biro and Nicholas Eastaugh, a scientist known for his expertise on pigments, formed a company, Art Access and Research, which analyzes and authenticates paintings. Biro is its director of forensic studies. Clients include museums, private galleries, corporations, dealers, and major auction houses such as Sotheby’s. Biro was also enlisted by the Pigmentum Project, which is affiliated with Oxford University. His work is published in museum catalogues and in scientific publications, including Antiquity and the official journal of the Royal Microscopical Society. In the media, he has become one of the most prominent art experts, featured in documentaries and news reports. (He was once mentioned in this magazine, in The Talk of the Town.) He even has a cameo—as the man who “pioneered the whole technique” of fingerprint authentications—in Peter Robinson’s popular detective novel “Playing with Fire”; the story is about a charming, “chameleonlike” con man who runs an art-forgery ring. On his Web site, Biro notes that law enforcement has adopted his approach: “My analytical techniques were presented internally at a training course at the F.B.I. I am not permitted to go beyond that.”

Biro told me that the divide between connoisseurs and scientists was finally eroding. The best demonstration of this change, he added, was the fact that he had been commissioned to examine “La Bella Principessa” and, possibly, help make one of the greatest discoveries in the history of art.

During one of the visits I made to Biro’s home, he offered to share with me what he had learned about “La Bella Principessa.” We were in the living room, and the sweet scent of his wife’s French cooking kept wafting in from the kitchen. “You’ve never tasted anything so good,” Biro said. He went over to a varnished desk, where there was a computer, and clicked on an icon. An image of the drawing appeared on the screen. He zoomed in on the upper-left edge of the parchment, and pointed to a small mark on the surface: a fingerprint. It looked like little more than a smudge, and I squinted at the blurry lines.

Even in a high-resolution photograph, the fingerprint was unreadable; Biro called it “complete visual confusion.” Many fingerprint examiners, he said, would have been stymied. Then, as if he were lining up a row of mug shots, he called up a series of photographs from a multispectral-imaging camera. Because the images had been made with different wavelengths of light, none of them looked exactly the same. In some photographs, the texture of the parchment—the background “noise,” as Biro put it—was pronounced. In others, the ridge patterns in the fingerprint were accentuated and the parchment all but faded away. From one photograph to the next, Biro said, “the smudge becomes clearer.” Still, it was not clear enough. His next step, he said, was “proprietary.” Using advanced image-processing software, he subtracted the background noise from each image, until only the clearest parts of the fingerprint remained. Finally, he said, clicking on another icon, “You get this.”

The smudge had been transformed into a more legible print: now, at least, there were the outlines of ridges and bumps. When I asked Biro if he worried that his method might be flawed, he said that during nearly two decades of fingerprint examinations he had “not made one mistake.” He added, “I take a long time and I don’t allow myself to be rushed.”