Judith Keeling talks to Leonardo da Vinci expert Martin Kemp about how uncovering a new Old Master is fraught with difficulties.

As the world’s leading expert on Leonardo da Vinci, Martin Kemp is used to receiving a regular stream of emails from art owners everywhere claiming to have discovered a “lost” masterpiece. Indeed, after 40 years in the “Leonardo business”, Kemp is equally adept at patiently pointing out the myriad technical and historical reasons why his correspondents are destined for disappointment.

“Leonardo was copied like no other painter,” says the Oxford Emeritus Professor in the History of Art. “The best copies are those done by a host of his followers and imitators. Most are no-hopers that bear no close relationship to Leonardo at all. “There are also the deliberate forgeries. With Leonardo drawings fetching a minimum of £10 million, the attraction is obvious.”

Originally trained as a scientist (he was a natural sciences undergraduate at Cambridge University before switching to history of art), Kemp’s natural instinct as he puts it “is to look for the falsification”. So it is partly Kemp’s very familiarity with fakes and those he dubs “Leonardo loonies” that makes the professor’s own recent identification of a lost Leonardo masterpiece so dramatic. That, and the fact that Kemp’s discovery took place around the same time as another lost Leonardo – the Salvator Mundi – was discovered.

“Nothing like these two major discoveries has emerged for over a century. I will forgo the joke about London buses!” says Kemp.

But while the Salvator Mundi was largely accepted by a fascinated art world (and displayed in the National Gallery in London in 2011), Kemp’s discovery, a drawing he christened La Bella Principessa was to prove highly controversial. It’s a profile portrait on vellum of a young lady, identified by Kemp as Bianca Sforza, illegitimate daughter of Leonardo’s great patron, Ludovico Sforza, who was the Duke of Milan. The drawing first crossed his path in 2005, “when I got the inevitable email with a digital file”. The email came from the drawing’s owner Peter Silverman, an art collector living in Paris. The picture had already been shown to a distinguished art authority who believed it to be a Leonardo and recommended that Silverman contact Kemp, now an emeritus at Trinity.

Intrigued, Kemp started to investigate. What followed was a magisterial piece of historical sleuthing: an international needle-in-a-haystack search that finally ended in Poland’s national library following furious hostile opposition from powerful art world interests. It’s an extraordinary real-life quest that could easily eclipse the plot of any Dan Brown novel.

“I asked myself what I always do: ‘Is it real and, if so, what does it tell us about Leonardo?’” says the former Slade professor, who stresses that he never has any financial involvement with any works he studies. Several factors struck him immediately about the portrait: the extraordinary subtlety and delicacy of the execution (which few artists would be capable of) and the background of the picture, demonstrated by another leading expert to be the careful parallel shading of a left-hander (Leonardo being famously left-handed).

Then there were the historical details. The portrait has a series of stitch-holes in the left-hand margin, suggesting it was a page ripped from a book. The image of Bianca sports a distinctive ponytail (a “coazzone”) involving hair extensions and supported by a net caul, which was a specific fashion at the Sforza court during the 1490s. This meant that the girl in the picture was almost certainly a Sforza; the hunt to match a name to the picture was on.

Kemp’s breakthrough came when he was contacted by American scholar Professor R Edward Wright, who suggested that the book the drawing could have been torn from was a version of the Sforziad, a book illustrated to celebrate the achievements of the Sforza family, and currently in the National Museum in Warsaw. It had been commissioned to celebrate Bianca’s wedding to Ludovico’s military commander, who was a known patron of Leonardo.

So Kemp travelled to Poland with a specialist who had undertaken scientific analysis of the Mona Lisa. “Lo and behold, we could identify that there was a page clearly removed,” Kemp says. “The stitch-holes matched, the vellum matched. It is indeed 1496, it is Bianca and indeed for her marriage. It’s uncanny. You could say: ‘Stitchholes are always the same distance apart.’ But the irregular stitching was spaced by eye, not precisely measured.”

Technical analysis revealed the vellum to be a match for the vellum on the book. The volume even bears an incision where the blade that removed the sheet slipped.

For Kemp the evidence was overwhelming. But he found himself facing a torrent of objections and even abuse from powerful art world voices when the news of his discovery reached the press.

As Kemp says now, “The portrait of Bianca Sforza emerged in a way that was guaranteed to alienate a crucial group of Leonardisti. It was trumpeted in the media before the basic research was complete and the press latched on to the story as a controversy, which they proceeded to stoke. A specialist telephoned out of the blue by the press asking about the ‘great new Leonardo’ is likely to react badly.”

By contrast, there had been a consensus among scholars over Salvator Mundi before it was unveiled.

There was also another massive problem: the work had been seen and missed by some very powerful experts who should have recognised it. It had been sold by Christie’s New York, who identified it in 1998 as an anonymous German 19th-century work. It fetched more than $21,000 (£11,400) for its owner Jeanne Marchig, widow of a notable Swiss picture restorer: a high price for a picture of that type, but nothing compared with the £10 million price tag it could expect to command as an authentic Leonardo. As a result the auction house found itself involved in legal battles with Marchig, one of which was recently settled for an undisclosed sum.

Moreover the buyer in 1998 was a leading dealer in Old Master drawings who also failed to identify it as a Leonardo before selling it on to its current owner for the same price she paid for it.

As Kemp remarks: “The auctioneer, the dealer who bought it and did not reattribute it before it was bought by its present owner, and the New York scholars (who would have seen it at least once) all have powerful reasons for not accepting the Leonardo attribution.”

Today La Bella Principessa is still privately owned – and for Kemp, who has never had any financial interest in the discovery, it’s been an instructive experience to reflect on.

“The serious point is that it shows, very dramatically, that what we accept as knowledge, right and wrong, is hugely coloured by how that knowledge emerges and who propagates it, particularly when such large sums of money are at stake,” he says.

Now working on an internet commentary on the 72-page Leonardo manuscript Codex Leicester, which is owned by Bill Gates, and planning a new book called Living With Leonardo, which lifts the lid on some of the forgeries and thefts he has come across over the years, Martin Kemp is commendably phlegmatic.

“For me, all of this is a small price to pay for getting to know the great genius that was Leonardo da Vinci,” he says.

Martin Kemp is Emeritus Professor of the History of Art at the University. He has written and broadcast extensively on imagery in art and science from the Renaissance to the present day, and is one of the world’s leading authorities on Leonardo da Vinci. Leonardo by Martin Kemp is published by Oxford University Press, £10.99. The revised edition (2011) is the first book on Leonardo to include the Salvator Mundi and La Bella Principessa.