It is one thing to discover a masterpiece by Gian Lorenzo Bernini, one of western art’s greatest sculptors, but it is quite another to find further “lost” works by this and other important masters of centuries past. As an international connoisseur-dealer, Andrew Butterfield has an extraordinary record in rediscovering Old Masters, having devoted decades to fine tuning his eye to recognise every quirk of an artist’s hand.

One of his discoveries had long been relegated to garden statuary, shivering outside but somehow surviving the elements. Others were overlooked as sorry-looking artefacts hidden beneath layers of paint or varnish.

With forensic research and restoration, he has breathed new life into such pieces, turning them into museum-worthy sculptures with values of seven- and eight-figure sums. He was able to prove, for example, that a bronze statue of the crucifixion that had failed to sell at auction in 1975 for $200 was in fact a lost Bernini now worth a staggering $50m.

By devoting sometimes years to researching each sculpture, Butterfield has discovered, authenticated and sold important works by Donatello, Ghiberti and Giambologna, among others. Based in New York, he sells to private collectors and museums worldwide, such as the Louvre in Paris and the Metropolitan Museum in New York.

Casting his mind back to initial inspirations, he recalls at aged nine being “really struck by sculpture” on a visit to Kyoto in Japan, but then, as a teenager, dreams of becoming a classicist were forgotten after a visit to the National Gallery in Washington. “I couldn’t stop going,” he says.

Until 1996, he was an academic — latterly with the Institute for Advanced Study in Princeton — before he entered the world of commerce with some trepidation. He worked at Christie’s and the Salander-O’Reilly Galleries in New York before becoming an independent dealer. He has, until recently, kept a low public profile, although he is known in the rarefied circles of academia, museums and the art market. He can be described, perhaps, as an “undiscovered discoverer” of Old Masters — an art historian detective.

One of his Bernini finds is a “Modello for The Moor”, one of the few large models by the artist to survive. It was a preparatory study, made about 1653, for the artist’s spectacular “Fountain of the Moor” in the iconic Piazza Navona in Rome.

It is thought to have spent time as a garden ornament, judging from marks indicating prolonged exposure to dripping water, which were subsequently removed by a restorer.

In 2002, it came up for sale at Sotheby’s. It was described as “attributed to” Bernini, rather than the more definite “by” and given an upper estimate of about $300,000. Butterfield was so sure of its potential that he bought it for $3.2m. Subsequent research enabled him to sell it to the Kimbell Art Museum, Fort Worth, for about twice that figure.

He sold the “The Crucified Christ (Corpus)”, a 1650s bronze statue, to Murray Frum, a Canadian collector, who bought it specifically to donate to the Art Gallery of Ontario because he felt that it belonged in a public institution. This is possibly the only sculpture that Bernini made for himself and is described by Butterfield as “one of the greatest bronze statues ever executed with a degree of naturalism far beyond that of any other known bronze of any period in the world”.

It is so lifelike, you almost forget that it is made of bronze. Once the restorer removed a non-original coating of black paint, “what emerged was an unmatched suppleness of the skin and a tenderness in the expression that was without parallel in the history of bronze sculpture”, Butterfield says.

Yet it had been misidentified over the decades. It was part of an estate sale of a woman associated with the American Kennel Club. While her statuettes of dogs were selling for $5,000 and $10,000, this bronze sparked no interest, Butterfield says in disbelief.

Butterfield bought Donatello's 'Spiritello' in 2012 in Turin

Donatello is one of the greatest of all Renaissance artists. Only a small handful of works by him have been found in the past 100 years. Butterfield’s finds include a two-and-a-half-foot tall wooden sculpted “Spiritello”, a cherub or putto, which he has shown to be a lost work from the 1430s and is now worth millions. For most of the last century, it was hidden away in a private Italian collection.

How can such great masterpieces be overlooked? “Often what you’re looking at is literally covered in paint from later periods,” Butterfield explains. The Donatello, for example, was covered in a “cloudy”, pigmented varnish, added in the past 20 years. “Varnish is a very common mode of second-tier restorers. It’s an easy and a cheap way of solving problems. You’re getting rid of any imperfections. But you’re also getting rid of the perfections.”

The struggle to see beyond the varnish is like “trying to see something through smoke”, Butterfield says. “It’s not so much that the varnish is hiding some clue, it’s that it’s distracting you . . . because the light is scattering.” While others might have been confused by the figure’s pose on one foot, subsequent research matched it to another Donatello sculpture.

Does he scour markets, such as Portobello Road in London, looking for possible finds? “I don’t. Other people do. There’s a network of people who talk to each other all the time . . . I know people who just drive around Europe [searching] . . . antique stores, galleries . . . They’re not interested in the authentication part. That’s often when I get involved.”

Most of the “finds” turn out not to be what the hopeful hunters thought they were, though. But, as often as not, his gut instinct has been proved right — sometimes after hard work lasting years.

Asked about his most-wanted list of missing sculptures, he says: “There is no doubt that, at the top of the list, are lost works by Michelangelo . . . He made numerous works which are now missing and possibly still out there.

“Number one on the list is the monumental sculpture of Hercules, last recorded in the royal gardens at Fontainebleau in the early 18th century. Of course, this might have been destroyed; but it still could exist and if so, it is likely standing outdoors in some garden in France or England, unidentified.”

Others include a kneeling angel that Michelangelo carved in his youth: “I am reliably informed that the work was last documented, although incorrectly identified, at an auction in Philadelphia in 1929. “There are other missing works by him as well that might still turn up.

Antonio Corradini's 'Endymion' was made for an 18th-century collector in Venice

“Indeed, Michelangelo often gave sketch models in wax and clay to friends and it would not surprise me if one of these were to be found . . . The wish list is almost endless . . . There were so many great works by great masters, tens of thousands of them, which have been lost or misidentified.”

Without expertise, though, the chances of future art historians being able to spot them will dwindle. About 30 years after Butterfield began his career, he worries that some lost masterpieces will never be found because future art historians will be unable to distinguish between the hand of a master, an assistant or a copyist — let alone a garden ornament.

He is so concerned that the next generation is not receiving adequate training, that he delivers a stark warning: “Art history is in trouble.”

In Butterfield’s opinion, universities are generally more interested in teaching trendy subjects such as social history “in the absence of looking at real works of art”.

Commenting on the demise of connoisseurship beyond Italy, which still offers traditional training, he says that “there’s very little of it going on”.

He laments a widespread assumption today that “there’s no kudos in looking at things”, exacerbated by the fact that “so much of art history is done from photographs”.

The study of sculpture also lags behind that of painting, he says. “There are far fewer experts. Paintings are considered to be more glamorous.”

Butterfield is not alone in fearing the worst. A conference, titled Art, Law and Crises of Connoisseurship, was staged last December by ArtWatch UK, the London School of Economics Cultural Heritage Law and the Center for Art Law at the Society of Antiquaries in London. Michael Daley, ArtWatch UK’s director, warns that “connoisseurship is in disarray”. He says: “The conference grew out of our realisation that scholars, increasingly, were falling down on the job of monitoring restorations and often on attributions because they’ve become too theoretical and insufficiently interested in artistic practice.”

The 'Modello for the Moor' is one of the few large works by Bernini to survive © Maggie Nimkin

He adds: “Younger art historians are not confident about judging art as art any more. They prefer to talk about feminism, social history, Lit Crit — any old subject that can be pegged on art — and to allow all questions of attribution and restoration to be left in the hands of restorers/conservators. This terrifying professional abdication would have been unthinkable in any other generation.”

The former director of the National Gallery, Nicholas Penny, also commented earlier this year that “the cataloguing of the permanent collection is not now often considered an essential part of the curator’s job”.

Further warnings are sounded by art historian Alison Cole, author of Italian Renaissance Courts: Art, Pleasure and Power. “It is remarkable how uncertain the art of attribution is,” she says.

“While we are equipped with the skills to interpret scientific and documentary evidence, we are rarely taught about how to really look at works of art and about the purpose of looking.”

Concerns about scholarship are also voiced by Jon Landau, a collector of Renaissance art, however he pays tribute to Butterfield as “a great teacher” who “brings a level of scholarship to the process, which is very rare”.

In the meantime, Butterfield and the ever-dwindling number of scholarly art experts will continue searching for those lost masterpieces.

Three scholars for collectors to watch

Daniel Katz Fine Art

Danny Katz, an Old Masters dealer in London, has rediscovered many sculptures through scholarly investigation. In 1980, he found a life-size marble figure of Bathsheba in Sweden and identified the hand of Florentine master Giambologna, before selling the work to the Getty Museum. Other sales to public institutions include the Victoria and Albert Museum, which, in 2011, bought an enormous 1790s relief by British artist John Deare depicting Caesar’s invasion of Britain.

Galerie J Kugel Brothers

Nicolas and Alexis Kugel are the fifth generation of dealers who began trading in the late 18th century in Russia. Today, the specialisms of their Paris-based gallery span medieval and Renaissance up to the 1850s, and include silver, furniture and sculpture. Such is their reputation that in 2008 the Prince of Liechtenstein lent them some of his finest pieces for a non-commercial exhibition.

Whitfield Fine Art

Clovis Whitfield, an art historian and dealer, established his London gallery in 1979. His specialisms include Italian pictures, with a Caravaggio among his discoveries. He has sold to the National Gallery, London, and Museo del Prado, Madrid, among public institutions.