The unravelling of a string of shocking old master forgeries began in the winter of 2015, when French police appeared at a gallery in Aix-en-Provence and seized a painting from display. Venus, by the German Renaissance master Lucas Cranach the Elder, to describe the work more fully: oil on oak, 38cm by 25cm, and dated to 1531. Purchased in 2013 by the Prince of Liechtenstein for about £6m, Venus was the inescapable star of the exhibition of works from his collection; she glowed on the cover of the catalogue. But an anonymous tip to the police suggested she was, in fact, a modern fake – so they scooped her up and took her away.

The painting had been placed in the market by Giuliano Ruffini, a French collector, and its seizure hoisted the first flag of concern about a wave of impeccable fakes. Ruffini has sold at least 25 works, their sale values totalling about £179m, and doubts now shadow every one of these paintings. The authenticity of four, in particular, including the Cranach, has been contested; the art historian Bendor Grosvenor said they may turn out to be “the best old master fakes the world has ever seen.” Ruffini, who remains the subject of a French police investigation, has denied presenting these paintings as old masters at all. To the Art Newspaper, he protested: “I am a collector, not an expert.”

The quality of these paintings – their faithful duplicity – jolted the market. The sums of money at stake in art, never paltry to begin with, have grown monstrous. Thirty years ago, the highest auction price for a painting was $10.4m, paid by the J Paul Getty Museum for Andrea Mantegna’s Adoration of the Magi in 1985. In contrast, while the $450m paid for Leonardo da Vinci’s Salvator Mundi in 2017 counts as an outlier, abstract expressionists and impressionists frequently come, in auctions or private deals, with nine-figure price tags.

In lockstep, the incentive to be a proficient forger has soared; a single, expertly executed old master knockoff can finance a long, comfortable retirement. The technologies available to abet the aspiring forger have also improved. Naturally, then, the frauds are getting better, touching off a crisis of authentication for the institutions of the art world: the museums and galleries and auction houses and experts who are expected to know the real thing from its imitation.

What was most unnerving about the alleged fakes sold by Ruffini was how many people they fooled. The National Gallery in London displayed a small oil painting thought to be by the 16th-century artist Orazio Gentileschi – a battle-weary David, painted on an electric-blue slice of lapis lazuli; the work is now suspect. A portrait of a nobleman against a muddy background was sold by Sotheby’s in 2011, to a private collector, as a Frans Hals; the buyer paid £8.5m. Sotheby’s also sold an oil named Saint Jerome, attributed to the 16th-century artist Parmigianino, in a 2012 auction, for $842,500. With care, the catalogue only ventured that the work was from the “circle of” Parmigianino– an idiom to convey that it was painted by an artist influenced by, and perhaps a pupil of, Parmigianino. But the entry also cited several experts who believed it was by Parmigianino himself.

The works were full of striking, scrupulous detail. On Jerome’s arm, for example, dozens of faint horizontal cracks have appeared; every so often, a clean, vertical split intersects them. In French canvases from the 18th century, cracks in paint tend to develop like spider webs; in Flemish panels, like tree bark. In Italian paintings of the Renaissance, the patterns resemble rows of untidy brickwork. On the Saint Jerome, the cracks match perfectly. Prof David Ekserdjian, one of the few art historians who doubted that the painting was a Parmigianino, said he just didn’t feel the prickle of recognition that scholars claim as their gift: the intimacy with an artist that they liken to our ability to spot a friend in a crowd. “But I have to be frank, I didn’t look at it and say: ‘Oh, that’s a forgery.’”

When Sotheby’s sells an artwork, it offers a five-year guarantee of refund if the object proves to be a counterfeit – “a modern forgery intended to deceive”, as its terms specify. In 2016, after uncertainty crackled over the Hals and the Parmigianino, the auction-house sent them to Orion Analytical, a conservation science lab in Williamstown, Massachusetts. Orion was run, and staffed almost solely by, James Martin, who has loaned his forensic skills to the FBI for many art forgery investigations. Within days, Martin had an answer for Sotheby’s: both the Hals and the Parmigianino were fakes.

The “Hals” contained synthetic pigments that the artist, in the 17th century, could not have used. In Saint Jerome, similarly, Martin found phthalocyanine green, a pigment first synthesised four centuries after Parmigianino died. It showed up consistently across 21 paint samples from various parts of the painting – “a bit like taking the pulse of a corpse 21 times,” Martin told the New York Times last year. Sotheby’s refunded both buyers, and filed suits against the sellers, demanding they return their proceeds from the sales.

In December 2016, in a signal of how attribution scandals have spooked the market, Sotheby’s took the unprecedented step of buying Orion Analytical, becoming the first auctioneer to have an in-house conservation and analysis unit. The company had seen enough disputes over attribution to mar its bottom line, its CEO, Tad Smith, said: “If you looked at earnings reports from a year or two ago, you’d see little blips here and there. These were expenses coming from settlements – not a slew, the number was small and statistically insignificant, but they’re expensive.” The cost of insurance that covers such settlements was also rising. With Martin in the building, “the pictures and other objects moving through Sotheby’s now have a much higher chance of being checked”, Smith said. Last year, Martin analysed more than $100m worth of artworks before they went under the hammer or into private sales. Sotheby’s employs him, in part, as a conservator, so he ministers to the health of the paintings and sculptures that pass through. But over the past two decades, Martin has also become the art world’s foremost forensic art detective. He has worked so many forgery cases with such success that he also serves Sotheby’s as a line of fortification against the swells of duff art lapping into the market.

The first major painting sold by Sotheby’s was also a Hals – a real one: Man in Black, a half-length portrait of a hatted gent. Until 1913, Sotheby’s had dealt in books for a century or thereabouts; art made up only a wan side business. In that year, though, a Sotheby’s partner found a Hals consigned to the firm, and rather than forwarding it to Christie’s, as was often the practice, decided to auction it. After a spirited contest of bids, Man in Black sold for £9,000 – a 26% rate of return per annum since Christie’s had last auctioned the work, in 1885, for around £5. It was the first signal, for Sotheby’s, that there was profit to be mined from paintings. Last year, it sold $5.5bn worth of art, jewellery and real estate.

The Frans Hals painting, Portrait of a Gentleman, supplied to Sotheby’s by Mark Weiss. It sold for a reported £8.5m ($10.8m) but was later declared fake. Photograph: Sotheby's

For Sotheby’s, the question of authenticity is not merely, or even primarily, academic. There is more at stake than a satisfying answer to the fundamental conundrum of whether authenticity matters at all – a debate that has been fought and refought in the history of western art. “If a fake is so expert that even after the most thorough and trustworthy examination its authenticity is still open to doubt,” the critic Aline Saarinen once wondered, “is it or is it not as satisfactory a work of art as if it were unequivocally genuine?” Typically, this debate comes to rest at the same place every time. Of course authenticity matters; to study a false Rembrandt as a true one would be to hobble our understanding of Rembrandt as an artist, and of the evolution of art. Now, however, the question’s philosophical whimsy has been replaced by financial urgency. At a time when the art market is synonymous with art itself, a lack of regard for attribution would derail a trade that traffics in the scarcity of authentic Rembrandts.

Leaving straight forgeries aside, any discussion about the “authenticity” of an artwork opens suddenly, like a trapdoor, into the murk of semantics. On the sliding scale of attribution that art historians use – painted by; hand of; studio of; circle of; style of; copy of – each step takes the artist farther from the painting. These variations, often subtle, are compounded by the unease about overpainting; Salvator Mundi had been worked over so many times and so heavily, critics argued, that it was less by Da Vinci than by his restorers. Deliberate fakes, misattributions and poor restorations all encroach into the realm of the authentic. In two decades at the Met in New York, Thomas Hoving, the museum’s director until 1977, must have examined at least 50,000 objects, he wrote in his book False Impressions. “I almost believe that there are as many bogus works as genuine ones.”

Like criminals of every stripe, modern forgers have kept easy pace with the techniques that attempt to trap them. The mismatch between the purported age of a painting and the true age of its ingredients is the workhorse of Martin’s technique. So forgers have grown more rigorous in their harvesting of materials, taking the trouble, for instance, to source wooden panels from furniture they know is dateable to the year of the fake they are creating. (The trick isn’t wholly new; Terenzio da Urbino, a 17th-century conman, scrabbled around for filthy old canvases and frames, cleaned them up, and turned them into “Raphaels”.) Forgers also test their own fakes to ensure they’ll pass. Wolfgang Beltracchi, a German artist who served three years in prison for forging paintings worth $45m, surveyed the chemical elements in his works by running them under X-ray fluorescence guns – the same handheld devices, resembling Star Trek phasers, that many art fairs now train upon their exhibits.

Georgina Adam, who wrote Dark Side of the Boom, a book about the art market’s excesses, told me that many forgers are sensibly choosing to falsify 20th-century painters, who used paints and canvases that can still be obtained, and whose abstractions are easier to imitate. “The technical skill needed to forge a Leonardo is colossal, but with someone like Modigliani, it isn’t,” she said. “Now, scholars will say it’s easy to distinguish, but the fact is that it’s just not that easy at all.” In January, in a celebrated Modigliani exhibition in Genoa, 20 out of 21 paintings were revealed to be counterfeits.

As the tide of money in the market has risen, making decisions about authenticity has turned into a fraught venture. Collectors, realising how much they stand to lose, are now happy to take scholars and connoisseurs – traditionally the final authorities on the authenticity of a work – to court for their mistakes. Realising that their reputations, as well as their bank balances, may wilt under the heat,these experts have begun to subtract themselves from the game entirely.

The estates of several 20th-century artists had once taken on the duty of resolving doubts over attribution, setting up authentication committees, consisting of experts or the artist’s former colleagues or friends – people expected to know the work best. In 2007, a collector named Joe Simon-Whelan sued the Andy Warhol estate’s authentication committee, claiming it had twice rejected a Warhol silkscreen he owned because it wanted to maintain scarcity in the Warhol market. Four years later, after spending $7m in legal fees, the estate dissolved the committee. The authentication boards of other modern artists – Jean-Michel Basquiat, Keith Haring, Roy Lichtenstein, Alexander Calder – have followed. Individual connoisseurs – as the art world calls its experts – won’t always challenge popular identifications, wrote the critic Jerry Saltz in a scorching essay on the vertiginous price of Salvator Mundi. They are reluctant to “rock the already splintering institutional boat. As in the wider world, where people sit by for fear of losing position, it’s no wonder that many old master experts are keeping quiet, not saying much of anything.”

The collapse of these committees feels like a victory of the market over the academy, like a blow to the very cause of trustworthy authentication. (In New York, a small band of lawyers is lobbying for legislation that will protect scholars from being sued merely for expressing their opinion.) In this void of opinion, Martin’s abilities – premised not on the mysterious instincts of connoisseurship, but on the verifiable results of the scientific process – have an even higher valence.

Martin, a tall man with lumber-beam shoulders, has a voice that never surpasses a murmur. He is a consummate nerd; find someone who looks at you the way Martin looks at his Fourier-transform infrared microscope. He trained as a conservator of paintings, but now he assays them: picks out their chemical constituents, inspects pigments and binders, peers under their washes of colour. From a painting’s materials, he can extract the vital detail of when it could, or could not, have been created.

The field of scientific art conservation is not a crowded one; Martin, who set up the first for-profit art lab in the US, has been consulted in nearly every major fraud case in the past 25 years, often working alongside the FBI or other investigators. When he is described as the premier forensic detective working in art today, the accolade comes not only from people such as John Cahill, a New York lawyer who has managed dozens of art transactions, and who called Martin “hands-down the best in the business,” but also from those on the other side of the fence, so to speak. Beltracchi, the German forger, told me that, after his arrest, he had seen an assortment of technical studies collected by the police and the prosecution. He remembered Martin’s well. “His reports contained the most accurate results. His reports were factually neutral and without unrealistic guesses.” By folding Martin into its staff, Sotheby’s has given itself a muscular chance to stamp out problems of attribution before they flare into spectacular, expensive affairs. But it’s hard not to feel, at the same time, that it has cornered a precious resource, at a moment when the art world needs him most.

Martin spent much of last year setting up a new lab in what used to be a photo studio on the fifth floor of the Sotheby’s headquarters in Manhattan. Soon, he will also have a London facility, in the building where the Beatles once recorded A Taste of Honey for the BBC. The New York lab, one large room, is as white and aseptic as a dentist’s clinic. Many of the cabinets are still empty, and the desk surfaces often bear nothing apart from one red pack of Martin’s Dentyne Fire gum. Outside the lab, above the lead-lined double doors, is a warning light; if it’s on, so too is the giant x-ray fluorescence machine, and no one is allowed in.

One Friday in mid-February, the room held only two items of art. A carved wooden chair sat on a counter; on a stand was a painting that, for reasons of confidentiality, may be described here only as “a late-19th century American work”. When a painting checks into the lab, it is first submitted to a visual examination in bright, white light; then the lamp is moved to one side, so that the light rakes over the surface at an angle, showing up restored or altered areas. The canvas in Martin’s lab was at the next stage; it had been photographed under ultraviolet and infrared, and then under x-rays to discover some of the painting’s chemical elements.

On a computer, one of Martin’s two colleagues cycled through the images. Under infrared, the painting’s browns and yellows and greens turned into shades of grey, but no spectral underdrawings peered back out. (Not that underdrawings would have suggested anything about authenticity one way or another; they’d merely have been a further nugget of information to consider.) Mapped for lead by the x-ray fluorescence unit, the painting looked faded and streaked with dark rust; the streaks betrayed where restorers had perhaps applied touchups with modern, lead-free paint. Mapped for calcium, the painting showed yellow-green splashes where conservators had made repairs with a calcium carbonate filler.

Not every object needs to move beyond these non-invasive phases. (At Orion, Martin was once able to unmask a fake Modigliani after seeing, under infrared, a faint grid, which had been drawn by a forger who wanted to guide his work.) If Martin has to disturb the painting, he will place it under a stereo microscope and, squinting through the two eyepieces, pick out a grain of paint with a scalpel. He demonstrated with a sample of phthalocyanine blue, a synthetic pigment he picked out of a box that held paint cakes of different colours. Working with the same steady, cautious manner in which he speaks, he teased out a particle smaller than the width of a human hair, flattened it gently, then nudged it on to a slim, small rectangle of metal, where it was held in place between two tiny diamonds.

“You don’t drink a lot of coffee before you do this,” he said, grimacing.

Cracks in the surface of a 16th-century painting as seen through Martin’s microscope. Photograph: Joshua Bright/The Guardian

The metal plate then goes into the Fourier-transform infrared microscope, like a slide. The spectrometer pumps infrared light through the flecks of pigment; a computer analyses the light’s behavior and returns a tidy spectrum graph. Martin has looked at so many of these spectra that he recognises on sight the patterns thrown up by different pigments, but even if he didn’t, the computer could rifle through databases of the spectrum patterns of other known chemicals, find the nearest match, and tell Martin what, in this case, he already knew: that his sample was phthalocyanine blue.

By a system of triage – sorting, for instance, for artists with a high incidence of being faked in the past, or for works accompanied by scientific analysis reports that are suspiciously long – only a small percentage of the tens of thousands of objects passing through Sotheby’s is diverted to the lab. Martin thinks of them as patients showing symptoms. Sometimes, like a doctor doing general checkups, he will tour the galleries at Sotheby’s just before a sale, reading every work with a handheld infrared camera. In the past year, his lab has stopped several lots from going to market, preventing possible disputes after the sale. In one case, a painting valued at $7m was removed from sale after the lab found that it had been completely and irretrievably overpainted by a restorer. “An appraiser would’ve said it’s worthless,” Martin said. “So it wasn’t sold.”

The arduous process of Martin’s work divorces art from its aesthetic. It reduces compositions of great prestige or high beauty to their very particles; it frees Martin up to think of art as pure matter. In this way, he comes closer to the artist than anyone has before, often becoming only the second person to think as intensely about the materiality of the object, about the chemical nature of its pigments or the physical properties of its canvas. The art he analyses derives its worth from unique, flashing inspiration. His own talent, if anything, has more in common with the forger. It lies in his capacity to be unflashy but diligent – to perform a step time after time without a slackening of attention, to never leave a molecule unturned, to never conclude more about a work than what it tells him about itself.

When Martin turned 13, his father gifted him a microscope, a chemistry kit, and art lessons – a splendid piece of foreshadowing. He used them all, but he was particularly attracted to art. The family lived in Baltimore, and whenever they visited Washington DC, Martin spent his time at the National Museum of Natural History, drawing the dioramas, while the others wandered the capital. His father worked in army intelligence. “As a child, I’m not sure I understood what he did. I do remember being in airports and trying to guess who was a spy,” Martin said. He devoured detective stories and loves them still, particularly Patricia Cornwell’s novels about Kay Scarpetta, the forensic pathologist. “We both examine patients that cannot speak their past,” he said.

In a universe a twist away from ours, Martin might have become a forger himself. Late in his teens, he joined an art school where students were taught how to grind their own pigments and stretch their own canvases. For practice, he set up an easel in the Baltimore Museum of Art and copied the works he liked; he grew so accomplished that once, as he was leaving with his copy of William Merritt Chase’s Broken Jug, the museum director spotted him and asked if he was returning the painting to storage.

“I was very good technically,” Martin said, “but like most art forgers, I didn’t have my own creative way of doing things.” He thought he’d become an illustrator of medical textbooks, but then heard about a conservation programme at the Winterthur Museum in Delaware. The portfolio he submitted included his copy of the Chase, as well as of other painters – all at such a high level of craft, said Richard Wolbers, who taught him at Winterthur, “that we were blown away”. He was such a good copyist, in fact, that he was almost rejected. “Later, I heard that the committee worried that if they trained me to be a conservator and taught me all the science, I’d be a natural forger.”

After Winterthur, Martin was hired by the Clark Art Institute, a museum in Williamstown, Massachusetts, to conserve paintings. A couple of years later, he set up the museum’s first conservation lab, filled with equipment that he bought or begged from chemistry departments in nearby universities. At the time, in 1990, the apparatus of analysis – the microscopes, the spectroscopes, the infrared cameras – was bulky, expensive and difficult to operate. Few museums had their own labs, Martin said. “The Guggenheim, the Brooklyn Museum, MoMA [Museum of Modern Art], the museums in San Francisco – none of them had the facilities.”

In getting to know a painting, conservators in these museums relied first on the tactility of their craft – “listening to the sound of the swab on the canvas”, Martin said, or “feeling the pull of the swab in the varnish”. Most conservation departments owned microscopes, some perhaps even x-ray machines. But if they needed some serious technology – Fourier-transform infrared microscopes, say, or scanning electron microscopes – they could turn only to the lab in the Metropolitan Museum of Art, or to those in universities. Even then, an expert was still needed to interpret the data. “Small museums really didn’t have any place to go. Some people took paintings to the vet to get them x-rayed.”

Martin’s lab began by assisting conservators who had no equipment of their own. “If someone was trying to get a varnish off a painting and didn’t want to damage it by using a solvent that was too strong, they’d send me a sample,” he said. “I’d tell them: ‘It’s polyurethane. You’re not going to get it off.’ Or: ‘It’s shellac. You need to use alcohol.’” A conservator wondering if the strange sky in a landscape was overpaint – paint applied by later restorers – could mail Martin a tiny cross-section tweezed out of the work, so that he could examine it under a microscope. “We’d see the layers in the cross-section: varnish, varnish, varnish, then blue sky, then more varnish, then more sky. So we’d establish that the topmost layer of blue was overpaint.”

In its materials, an artwork holds its biography, so inevitably, Martin became an arbiter of authenticity. Nearly all of the privately owned art labs in Europe and the US have been founded in the past decade – not coincidentally, around the time that the world’s multi-millionaires realised how hollow their lives had been without art. But in the 1990s, at Clark, and then again at Orion, which he founded in 2000, Martin was often the sole resource for collectors and merchants.

James Martin using his stereo fluorescence microscope in his New York lab. Photograph: Joshua Bright/The Guardian

Some of his stories from these years have the baroque pulpiness of Elmore Leonard plots. Martin narrates these with care; he is alive to the sensational aspects of his work, but by default, he wears an air of studious detachment. There were the two questionable gentlemen from Tel Aviv, who slipped a pair of paintings out of architects’ tubes, shook them open as if they were rugs, and asked him to confirm that they were Modiglianis. (They weren’t.) There was the client who sent Martin to test a painting at an auction house, claiming he wished to bid on it, but then also had Martin stop by a warehouse to assess “a horrible copy” of the same painting. (Martin now thinks the client wanted to know how close the fake was to the genuine work.) There were the two ferocious dogs chained near the front door of a house in Los Angeles, guarding the stolen Chinese sculptures held within. There was the collector who offered to fly Martin to an undisclosed location, have him picked up by a security detail, and bring him in to examine an old Mexican stele, a stone carving supposedly worth $50m. The night before his flight, Martin was unable to sleep, so he Googled the collector and found that he had recently been released from federal prison after serving time on weapons charges.

Next morning, Martin called the collector and turned down the case.

“Oh,” the collector said. “Did you read about the murders?”

“No,” Martin said. “What murders?” The collector, it turned out, had once been implicated in the killings of two people over a matter of Mexican steles. Martin never got on that plane.

The FBI first came to Martin in 1994. A suspicious number of works ascribed to the 19th-century artist William Aiken Walker, who often painted black sharecroppers in the American south, were emerging in the market. “They’d sell at really small country auctions for $5,000 or $10,000 – so low that nobody would pay for analysis,” Martin said. From the paintings, Martin sampled a yellow pigment called PY3, which had been manufactured in Germany and was not available to American artists until the late 1940s, decades after Walker died. Walker also used lead white paint, Martin found; the forger used zinc white. A former vitamin salesman named Charles Heller was eventually indicted for a spree of counterfeiting, but he pleaded guilty to lesser charges and served one year in prison.

With even a little study, a con artist would know not to use zinc white; some forgers go on to become diligent researchers, accessing technical journals and case studies to learn what experts search for. Martin recalled a painting once referred to him, around 3.5 sq metres in size and dated to 1932. In a first round of study, he discovered nothing amiss. But the work’s provenance – its documented history of ownership – was shaky, so he ran a second pass under a microscope. For most of a day, he scanned the painting in dime-sized increments, until his eyes dried up. Was anything embedded in the paint: dust, or hair, or an insect wing? Did the dirt look as if it had been smeared on deliberately? Finally, embedded in a speckle of blue, he found a slim fibre; with a scalpel, he snipped it off and subjected it to infrared spectroscopy. The fibre turned out to be polypropylene. Perhaps someone had worn a polar fleece while painting the forgery?

For a while, Martin cited this example in a two-day course he taught. Last year, though, he read a translation of Faussaire (or Forger), a French novel written in 2015 and containing a wealth of sound wisdom for forgers. “If you want to get hold of antique lead,” one character advises another, for instance, “then you can just pick up bits of it from the old buildings in Rome.” The same character warns of the dangers from “microparticles from your clothes … You must always work in an old smock. Never nylon or a modern apron.” Martin is convinced the detail came from his anecdote; it was one reason he decided to stop teaching his course altogether.

As a crime, art forgery can seem trifling – less a sinister outrage than a half-complete Robin Hood jape that merely robs the rich. After Beltracchi’s arrest in 2010, the Frankfurter Allgemeine called art forgery “the most moral way to embezzle €16m”; Der Spiegel noted that, unlike crooked bankers, Beltracchi hadn’t swindled the common man. But the crime can have real victims, and Martin has met so many of them that he has developed a gentle bedside manner to break bad news. He has seen people who used the money set aside for their children’s education to buy a painting, only to find it to be fake. “So we aren’t just talking rich people. In some situations, it’s a person’s whole life.”

The inflation of the art market, and its attendant litigiousness, imposes fierce pressures upon anyone called to judge the authenticity of an artwork. Martin’s harshest experience of this came during the bitter legal battle over the fate of the Knoedler gallery. The Knoedler, once New York’s oldest gallery, closed in 2011, days after Martin issued a report concluding that a Jackson Pollock it had sold for $17m was fake.

The bogus Pollock was only the inauguration of a scandal. Over 15 years, Knoedler had sourced and sold 40 paintings ascribed to a range of leading modern artists: Willem de Kooning, Mark Rothko, Richard Diebenkorn and Robert Motherwell, among others, earning roughly $80m in the process. When the ambiguity of the works’ provenance raised needles of suspicion, 10 buyers sued Knoedler and its director, Ann Freedman; all but one of these lawsuits have been settled out of court. In 2013, investigators learned that the forgeries had been painted by a Chinese immigrant, who was by then 73 years old, in his garage in Queens, and placed with Knoedler by an art dealer who pleaded guilty. Knoedler’s executives claimed they had no knowledge of the fraud, and argued that scholars had verified the works before sale.

In at least four of the lawsuits, which carried on for years, the plaintiffs hired Martin to test the paintings they had purchased. He found them all to be forgeries. A purported Rothko from 1956, which sold for $8.3m, used a ground layer of white paint between the canvas and the oils; through that decade, though, Rothko had used a transparent ground layer. In an apparent Pollock, the artist seemed to have misspelled his own signature as “Pollok”. Further, in 16 Knoedler paintings he analysed, Martin found the same ground layer of white paint and other anachronistic pigments repeating themselves across the works of several artists, as if Motherwell, De Kooning and Rothko had all travelled forward in time, met in a bar, and swapped tubes of paint.

A fake Mark Rothko painting is shown to the court during a trial in New York in 2016. Photograph: AP

Eventually, Martin was proved right; when the FBI raided the Queens garage, it even found the tubs of white that had coated the canvas in the fake Rothko. But, until then, the trials were a torrid experience. Knoedler recruited experts to attack Martin in court. “They went after him with a vengeance, saying he’d soiled the evidence, accidentally or on purpose,” said the lawyer John Cahill, who represented some of Martin’s clients. Knoedler’s attorneys served six subpoenas on Martin, to extract more than 8,000 documents and emails related to the case. Instead of being an expert witness, he was forced to defend himself – the care and soundness of his methods, his very character – in court.

When Martin talks about the Knoedler trials, even the memory of the ordeal draws a look of horror on his face. “He’s a real boy scout, and his integrity means a lot to him, so he suffered,” Cahill said. It was an attempted impeachment of Martin’s whole career. “His entire power relies on being objective, on not being part of the party,” said Narayan Khandekar, who runs Harvard’s Straus Center for Conservation and Technical Studies. “He comes under a lot of pressure, because people have a lot of money at stake on the outcome of his analyses. But he’s been very, very brave to stand up and stay stolidly on track with what he does.”

Martin had always loved science for its ability to guide him in pursuit of truth, and he felt a deep distress when his objective facts were countered with dirty tricks and personal vilification. In 2016, after his clients settled with Knoedler, Martin found it difficult to return to work. He wanted to never have to provide expert testimony again, and to go away to paint for a while; he’d already primed a set of boards.

“It was surreal, what happened to me,” he said. “No scientist should have to go to through this.” When, later that year, negotiations began for Sotheby’s to buy Orion, Martin was ready to be cocooned within a larger institution. He’d rather probe works before they hit the market, he decided, than go through the acrimonious aftermath of a sale even once more. Above his desk in Sotheby’s, Martin keeps pinned a pair of sketches of himself from his time in the Knoedler courtroom, as if to remind himself of what he has gratefully left behind.

In conversation, Martin uses many homespun metaphors, but his favourite is that of the three-legged stool. Deciding the authorship of artworks, he says, relies on connoisseurship, technical analysis and provenance. He values the opinions of connoisseurs, considers them complementary to his own skills; his tests can definitively reveal if a painting is not by Da Vinci or Modigliani, but they are unable to affirm authorship, except in rare cases.

Science has a habit, though, of showing up the sagacity of scholars. In a 1932 trial in Berlin – the first in which a forensic exam was used to scrutinise art – two connoisseurs squabbled about the authenticity of a set of 33 canvases, all purportedly by Vincent van Gogh, all sold by an art dealer named Otto Wacker. It took a chemist, Martin de Wild, to trace resins in the paint that Van Gogh had never used, and to prove the paintings fake. Since then, the science has improved, even as human judgment has remained the same, vulnerable to the potential thrill of discovering new work, and to market pressures. During the Knoedler trial, Cahill remembered, one expert admitted that he couldn’t tell one Rothko canvas from another, or indeed whether a Rothko had been hung upside-down or right side up.

In any case, however fond he is of the three-legged stool, Martin may have to think soon of a different item of furniture. The humanities are in decline everywhere; in England, the art history A-level course came close to being abolished altogether in 2016. The populace of connoisseurs is thinning out. “In British art now, for a major artist like George Stubbs, there’s no recognised figure that we can all go to and say: ‘Is this by George Stubbs or not?’ Because various specialists have died recently, and there’s no one to replace them,” Bendor Grosvenor, the art historian, said. Meanwhile, researchers at Rutgers University have developed an AI system that, in tests, detected forged paintings with 100% accuracy by scanning and comparing individual brushstrokes. One leg is growing longer, another growing shorter, the stool becoming decidedly imbalanced. And so, if the art market wants to beat back the threats posed by sophisticated forgeries – if it wants to preserve its financial vigour, rooted as it is so absolutely in the notion of authenticity – it will have to turn more and more to the resources of science.

As a thought experiment, it is possible to envision the immaculate forgery – the one that defeats scientist and connoisseur alike. Our villain is a talented copyist, well practised in the style and the themes of his chosen artist. He is also a resourceful procurer of materials, able to rustle up every kind of age-appropriate canvas and frame, pigment and binder. He fits his forgery neatly into a chain of provenance – giving it the title of a now-lost work, or providing false documents to claim that it had been part of a well known private collection.

In theory, if each of these steps is perfectly performed, there should be no way to expose the painting as fake. It will be a work of art in every way save one. But the world of today, the world in which the forgery is being created, is likely to fix itself in some form within the painting – as radioactive dust, perhaps, or as cat hair, or a stray polypropylene fibre. When that happens, only the scientist can hope to nab it.

• This article was amended on 18 June 2018. A previous version stated that the last art history A-level was cut in England in 2016, but in fact a campaign of protest led to a new version being introduced in 2017.

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