It's not every day that someone rediscovers a major work by Leonardo da Vinci. In his lifetime, this archetypal Renaissance man did not execute more than 20 finished artworks — the vast majority of what we have today are preparatory sketches for reams of unfinished projects — and no more than a few are known to be lost. That is why it is something of a miracle that Martin Kemp, the Oxford professor emeritus of art history and renowned Leonardo scholar, has been involved in bringing two of what seem to be the artist's lost masterworks back into the public eye.

One of these, the "Salvator Mundi," has been embraced as authentic by Kemp along with a formidable team of Leonardo experts, and will be included in the landmark "Leonardo: Painter at the Court of Milan" exhibition opening at London's National Gallery next month. The other, known as "La Bella Principessa," has been a more contentious matter. Sold at Christie's in 1998 for a mere $21,850 — the auction house's experts judged it a German pastiche in the Italian style — it was purchased in 2007 by the art collector Peter Silverman, and has since been the subject of a concerted, if sometimes messy, campaign to prove it to be a real Leonardo. Many scholars turned their backs. 

When Kemp became involved, however, the portrait — in colored chalk, pen, and ink on a page of vellum, or calfskin — gained new traction. Last year he co-authored a book, "La Bella Principessa," with French scientist Pascal Cotte, presenting evidence to support the Leonardo attribution. Now, the art historian has come forward to reveal a stunning new puzzle piece: he was able to locate the book, made for the court of Milan, from which the page was taken. To learn the story behind this remarkable turn of events, ARTINFO spoke to the scholar — who, something of a Renaissance man himself, has a wide-ranging book on iconography coming out this fall, called "Christ to Coke" — about the portrait, the tragic life of its subject, and why the work could only have been made by Leonardo.

Tell me, how did you first come to see the portrait?

Initially it was sent to me as a digital file. Normally, in the old days, I would be sent photographs of prospective Leonardos about once every six months. Now it's down to one or two a week, in digital files. Of course that's how they come through now, because it's easy to find my email address. This one came through almost three years ago now. Peter Silverman wrote to me and said, "My wife and I have something that you might be interested in." You generally look at these things and you think, oh my god, here we go again, and you reply politely. But this time I thought, this is not routine. Of course, with a digital file, you don't jump at anything. It almost looked too good to be true. Silverman had already shown it to a number of people, including Nicholas Turner, who was keeper of the drawings at the Getty and previously the British Museum. Nicholas said, "Oh, I think this is by Leonardo, but youshould ask Martin, because he's actually a Leonardo specialist." And I thought, this is worth following up and going to Zürich, where it was kept, to take a look at it.

What was it in that digital file that grabbed your attention?

The subtlety of the drawing. The characterization of this young woman. Leonardo has a particular way of drawing profiles which have a living sense — they're not just descriptive profiles but have a kind of life of their own. Also, the way the shading is clearly left-handed, abutted against the profile. It looked very like other drawings of Leonardo's — in particular, oddly enough, the skull studies from 1489 with that particular handling of edge. That itself isn't enough, though, and when showed something you always say, well, I'll keep trying to disbelieve it. Because once you believe something, you then see what you want to see. So all the time you're kind of pulling away at it.

Well, I never take any money, not even travel expenses, so I'm not beholden to the owner. But about six months later I was going Zürich to film a television program, so I stopped off at this special storage facility, and I saw the piece in the flesh, spent a lot of time looking at it, using magnification, et cetera. Pascal Cotte of Lumiere Technology in Paris, who's very remarkable and has probably the world's best system of multispectral scanning — very high-resolution image scanning — had already done quite a lot of work on it, so some of the science was already there, including infrared and ultraviolet. So it began at a high level right from the very beginning. Also, they'd already shown it to a number of people, including Nicholas Turner.

When you saw the portrait in the flesh, what about it began to convince you that it may, in fact, be a Leonardo?

There are various things. The first, and in a sense the last, is it has to have that frisson that you find in all of Leonardo's drawings, especially the Windsor drawings. Connoisseurship still plays a role. It's much denigrated and criticized, but ultimately, without connoisseurship, we really wouldn't know Leonardo's work at all. It's still a fundamental tool in establishing what was done by him and when it was done, since none of it is signed, none of it is dated, and, apart from "The Last Supper," nothing has a continuous provenance. So you still have a lot of that rather old-fashioned judgment by eye to do.

So, in the flesh, you look at it. It's on vellum, and you can see the extent to which the surface is deteriorated, which you can't see, really, in a digital file, which smooths out the surface. You can begin to see where it's been restored — as you look at it in different light and from different angles, the physicality of it becomes apparent. But that's only your starting point. Then, all the heavy-duty research comes in, and we now have, of course, an enormous body of extra things we can look at. So the initial connoisseur's reaction merely tells you that something is worth looking at, but at any point one wrong thing can throw that all away — a later pigment, a bit of something that might come up about its history to indicate it was forged at some point, and so on. I was trained as a scientist, and if you have a scientific theory, you only need have one bit of the experiment that says, "this is not right," and the whole thing collapses. You always have to be looking for that one thing that is going to demolish the whole expectation that's being set up.

In terms of connoisseurship, you've handled an extraordinary range of Leonardo's work personally, including even the "Mona Lisa."

Yes, though I've not handled the "Mona Lisa" in the sense that I've laid my hands on her. I've been present twice when it's been out of the frame. It's carried by professional handlers, you know — the curators don't pick these things up. But they took it over to the window, and we had a look at it in different lights, and so on. That is an uncanny experience. A number of Leonardo experts were there the second time, but to see it twice is just extraordinary. It's a very anxious thing, you know, because that work is so famous. You think, is this going to be a letdown? And it absolutely wasn't. It has an extraordinary presence.

Is this something the Louvre does in order to give Leonardo scholars a frame of reference?

The second occasion was linked to their question as to whether they should restore the "Virgin, Child, and St. Anne" in the Louvre. So in part to diffuse controversy but also to seek opinions they invited a group of scholars in to look at all of their Leonardos out of the frame in the Salon Carré, to have a seminar, and talk about restoration. And they are, indeed, restoring the "Virgin, Child, and St. Anne," which is in worse condition than the "Mona Lisa." That was the second occasion. For the first, each year it's taken once out of its armored plating to run a conservation check on it, and I was invited along by Pierre Rosenberg, the former Louvre director, who said, "Do you want to see it when it comes out?"

With the "Bella Principessa," there are several things that are unusual about the portrait. For one, it's on vellum, and Leonardo is not known to have any extant works on that material. How do you explain that?

Leonardo, as I've pointed out in the book, discussed vellum — he discussed drawing in dry pigments on vellum, so he does talk about it. He knew vellum perfectly well. He also used oiled vellum for tracing, so he knew the materials. There's no problem there.

Also, the portrait is done in profile, unlike Leonardo's more famous portraits. Are there other examples of portraits he did in this cameo style?

Yes. The profile portrait was obligatory at that time for top members of the inner court circle, and the most spectacular example is the portrait of Isabella d'Este, which Leonardo did when he was in Mantua in 1500 after the collapse of the Milanese regime. That drawing survives in the Louvre, and a version of it was made that is now in the Ashmolean. This is 1500, so he'd already done the Cecilia Gallerani and "La Belle Ferronière," which is of a certain Lucrezia Crivelli. They were not in profile, but they were the king's mistresses. For formal court portraits it was not good manners to abandon the profile. That was the stock form. With Isabella d'Este he turns her slightly, like the "Mona Lisa," so it becomes quite strange, but clearly Isabella — at that stage in her career — knew that the profile was what you did for the inner member of the court circle, so it was obligatory.

Why was that the convention?

It was a matter of formality. It's more hieratic. It relates to Roman coins, of course, which also helps. We still do it on coins, don't we? And we still do it on postage stamps. So it's that kind of convention. The official portrait is a profile.

When you released your book "La Bella Principessa," in March of 2010, to claim the portrait as a true Leonardo, there was a certain amount of debate, with many experts dismissing the attribution out of hand. Then, you found the book from which, it seems, the portrait came from — providing tremendous weight to your claim. How on earth did you track down that book?

To say that there was a certain amount of debate is an understatement. A lot of people who hadn't seen it have an investment in it not being a Leonardo, particularly the people in New York who had all missed it. However, we knew it came from a book. It's on vellum. There are three stitch marks that are visible down the left-hand side, and a cut mark where the knife slipped when it was being cut out of the book. I thought, well, the chance of finding that book is pretty small. Where do you start? You start by going through library catalogues of manuscripts connected with the Milanese regime in the 1490s. It's a needle in a haystack, and I hadn't even tried such an endeavor, because I literally didn't know where to start.

Then professor D.R. Edward Wright, an American scholar [at the University of Southern Florida] who had read the book and was slightly upset by some of its reception, said, "Have you thought of one of the 'Sforziad's?" These were copies of a eulogistic book written by a court functionary in Milan called Giovanni Simonetta, a humanist, in praise of Francesco Sforza, who was the father of Ludovico, the duke of Milan. Since Francesco was the founder of the dynasty, this book eulogized the Sforzas and provided the foundation for their claim to rule Milan. David claimed that there were three surviving versions on vellum made for specific Sforza events — for births or marriages you might customize a book with all the heraldry and allegories relating to the event. One of these versions had clearly been dismembered. So he said, "Well, what about the one in Warsaw?", which had already been connected to Bianca [the duke's daughter] and her wedding to Galeazzo Sanseverino, the commander of the duke's forces.

And there was something that already suggested the woman in the portrait was Bianca, is that right?

She has the authorized court hairstyle, so she was part of the Sforza court maidens. I looked through the Sforza court women and ended up with probably four or five possibilities, and Bianca was the right age at the right time. So it was quite hypothetical. But Galeazzo Sanseverino was a Leonardo patron. Leonardo drew horses in his stables.

The clothes, too, placed her in 1490s Milan?

It all placed her. The clothes are very simple, and, in fact, as David Wright has shown, young brides wore rather simple clothes after marriage. It was only when they became older and they produced children that they wore some of the more luxurious garments again. So the relative simplicity of the costume actually helps tell us that it's an immediate post-marriage portrait. It is also bound up with the romantic story where she died tragically four months after marriage.

How did she die?

It was what they called a "passione di stomaco," which is literally a "passion of the stomach," but I suspect it was…. She was married when she was 13. These girls married at 13, 14. They died very early — all the Sforza princesses, the inner ones, they all died early. They basically had babies before they should have, particularly given child-bearing circumstances at the time and medicine at the time, so I suspect she had an ectopic pregnancy or some similar kind of problem. One can only speculate.

So you go to Poland and find the book, which is missing a page.

At that point National Geographic was filming a program on my research because they were interested in the process, the science, and I said to them, "Look, I'm going off. I want to go off to Warsaw to look at this." We got a grant from the expeditions committee of National Geographic, so we went off there with Pascal Cotte. He brought all his technical equipment, photographic equipment, spectral equipment, and we had the television crew in tow, so it was a little bit of a circus. But we did a very close analysis of the early pages of the book, which are the ones which are tailor-made and have all the special material in them. We also took to Warsaw an exact facsimile of the portrait that Pascal made, dimensionally accurate to a fraction of a millimeter, so that we could not only match the page size but more importantly we could look at the stitch marks. Three of the stitch marks, the ones that we can still see on the edge of the "Bella Principessa," match as well as they conceivably could — and they're irregular stitch marks. Pascal also did spectral tests on the vellum, detecting the spectral composition of the light coming off the vellum in places, which then matched that of the vellum of the "Bella Principessa."

Why do you think the portrait was originally cut out of the book?

There was a great fashion for cutting illuminations out of manuscripts at one point. People were very cavalier about things of that sort. If they saw something they liked, or if the book was being rebound, then they thought, "Oh, well, we're rebinding it, let's cut this out and take it." In fact, dealers still do it today.

What happened to it after it was taken out? Is there any provenance history?

Not much. We've got a reasonable idea of what happened to the book. There were very strong relationships between the Sforzas and the Polish royal family. There was lots of intermarriage, so there's no particular problem for imagining how it got to Warsaw, which, far from being an obscure place, was a major Renaissance city. What happened to the portrait when it was cut out — if, say, it was cut out in the 18th century — we don't know. We're still working on the rebinding of the books. It went into the greatest humanist library in Poland, the Zamoyski library, which had a lot of the royal books in its collection. I have a young Polish scholar looking at the rebinding of the books in the libraries to see what he can tease out.

But it came to Christie's at some point, and then it was purchased by the current owner. How did it get to Christie's?

The person who brought it to Christie's, who is still engaged in a legal action against them, is Jeanne Marchig. She is the wife of a very accomplished Swiss conservator and restorer called Giannino Marchig who worked for major museums, major galleries, major dealers, and so on. He was working in Italy, and he collected — particularly when he was in Florence — some very nice things. Old Master drawings, smaller works of art. And Jeanne Marchig, who's now an elderly lady, had been systematically selling these things via Christie's New York in aid of her animal charities. She's quite a significant benefactor. This was the last significant work she had left.

Is there any idea where Marchig bought it, or is that where the provenance trail ends?

It dries up. We don't know where he came across it, but If he got it in Florence the chances are he bought it in the early 1940s, or acquired it by gift, or whatever. But Jeanne Marchig said he thought it was perhaps a Ghirlandaio, which is not daft. He was an exact contemporary of Leonardo, and there are beautiful portraits of Florentine women by Ghirlandaio. So Jeanne Marchig said to Christie's, "Well, Giannino thought this was by Ghirlandaio." They said, no, it was German 19th century — a German 19th-century pastiche, basically.

They weren't the only ones to differ on the attribution of the painting, and when you first announced that you believed it to be a Leonardo, a lot of people disagreed. One museum director even told the Telegraph's Richard Dorment, anonymously, that it was a "screaming 20th century fake, and not even close to Leonardo himself." Has there been any reversal since then?

I don't know who this anonymous person was, but we carbon-dated the parchment, and that eliminates it from being a screaming modern forgery. If it were a forgery, it used things that we've only recently discovered about Leonardo's technique in the last 20, 30 years. The fact that it was owned by Giannino Marchig takes it outside the period when it could be a forgery, knowing what we now know, so that's not an option. The ultraviolet turns up retouching, and it's very clear that it's been heavily restored, but most objects 500 years old have been heavily restored, including the "Salvator Mundi," which is the new picture being shown in the National Gallery.

Can any pentimenti, or telltale signs of compositional changes during the artistic process, be found in the portrait?

Yes, there are a number of pentimenti. You can even see it in the back of the head where some chalk drawing has been rubbed out. Probably with bread. You can see the stripes where some pigment has been rubbed out. There have been some quite nice pentimenti, not big ones, because it's a rather formal image, so you're not moving the head around in a big way. But it also showed a hand-print, which is very characteristic of Leonardo. When he was painting left-handed he would press down the side of his right palm into the composition, using flesh to make flesh, as it were. Bellini did a bit of that, too, and Dürer did a bit of that. But Leonardo did it very extensively. Some years ago, I was in the National Gallery in Washington, D.C., with fingerprint experts from the F.B.I., looking at Leonardo's portrait there, the "Ginevra de' Benci," and there were marks of the hand all the way across the flesh tones. A very unusual technique. That is very clear in the "Principessa" from Pascal Cotte's imaging.

None of the dubious scholars have been brought around by this new evidence?

I haven't had any direct feedback, but I sent the full study to all the major Leonardo scholars I knew — hopefully I haven't left anybody out — so they've got the data. They may choose not to believe it, and I'm not going to great lengths to convince them other than to produce the evidence.

Is there any plan to bring the book and the painting together?

Not at the moment, but that would be terrific at some stage, to actually have the book and the painting together. That would be the next obvious phase, to do an exhibition with those things brought together. But, one step at a time. First we're going to write up the initial results, the full technical report and everything, in the bulletin of the National Library in Warsaw. We're also doing an Italian edition of my book, so that will have all the new material in there as well. As for the reception of it, I'm completely relaxed. If people choose not to believe it, I've done my best with it, I've produced evidence which I think is very, very strong — about as strong as it gets. If somebody wishes to disbelieve that, that's their right. I won't feel greatly insulted, and it doesn't make me a stupid person. Nobody has died. It's a matter of opinion.

The National Gallery in London stands by their refusal to include the "Principessa" in next month's landmark Leonardo exhibition?

All I've seen so far is a New York Post report saying they won't change their stance. Well, they rejected it from their show. If I were a curator there, I wouldn't show it in the exhibition either.

Why not?

Because they wouldn't have clear guarantees it's not going to be sold out of the exhibition, as it were. I've been a trustee of the British Museum, the National Galleries of Scotland, and I know they have to be very careful in public galleries as to what they show. If the work is thought to be on the market, then you're not wishing to be seen as part of the commercial presentation of it.

How does that apply to the "Salvator Mundi," which, when it was rediscovered earlier this year, was immediately touted as being a $200 million painting.

You'd have to ask the gallery about the assurances they had. They have apparently been given assurances that they regarded as reasonable that the work is not presently for sale.

Two of the critics who were most vociferous in differing with your attribution of the "Principessa" were Edward Fahey at the Metropolitan Museum of Art and Carmen Baumbach at Yale University. Baumbach was part of the group of scholars, which included you, that attributed the "Salvator Mundi."

I've never had any reasoning from Carmen why she's not prepared to accept it. I've only seen press quotes saying, "It doesn't look like Leonardo," which is not a satisfactory way to argue. Fahey, I've not heard anything from at all. I've heard nothing direct, and I've not even seen any reports, so I can't comment on that. Fahey is not a Leonardo scholar. Carmen is a very distinguished scholar of Leonardo drawings. She's been given an opportunity to see it, but so far hasn't. She's a friend of mine. If she chooses to think that it's not Leonardo, fine. We're still friends, and she's still a great scholar.

The top echelon of Leonardo scholarship seems to be a very small coterie.

It's a limited number, but, I think, not unduly fractious. I think there are coteries of scholars of other figures that are much more fractious. I think we get along quite nicely. No problem with them. The Italian Leonardo scholars have accepted it — I think Pietro Marani [the Milan-based scholar in charge of restoring "The Last Supper"] has, who had doubts. I've sent them all the evidence. That's all I can do. These are not personal matters.

One thing that critics of your "Principessa" attribution tend to bring up is the involvement in your research of Peter Paul Biro, a fingerprint expert whose credibility was questioned when he championed fake Jackson Pollock paintings. What is your opinion of him?

Well, Biro I knew of as someone who'd specialized in fingerprints and paintings, so we asked him to look at the fingerprint that is in the upper left side of the "Bella Principessa." I had data on fingerprints and finger marks in other Leonardo paintings, and he said one of these matched — not astoundingly, because it's just the tip of a finger, and one doesn't rely on fingerprints on vellum. It wouldn't convict anybody in the court of law. You need more than that. So he did a limited job there, and we didn't depend too much on that evidence. The press liked it, of course, because it was cops and robbers stuff.

I would not now probably say much about it at all, because on reflection I think we don't have an adequate reference bank of Leonardo fingerprints. I've talked to fingerprint specialists, and they typically require a full set of reference prints. We don't have that for Leonardo. My sense is — and this is Pascal's sense, too — that it's probably premature, given what we know about Leonardo's fingerprints, to come up with matches at all. But the job Biro did was perfectly straightforward. There were no grounds for dishonesty. Peter Paul Biro is saying the allegations are all nonsense and is suing the New Yorker, but I can't comment at all upon the court case because that's about things that I know nothing about, so it'd be totally improper. But he did work for us, which I now, let's say, place less reliance on, simply because, on reflection, I think the fingerprint evidence is rather slippery.

Because of the work you have done to bring the "Principessa" into the fold of acknowledged Leonardos, some say you have crossed from the realm of scholarship to something more like advocacy. How do you explain your passion for the portrait?

I would say that one of the differences between being a historian of art and being a scientist, as I was trained, is that you're dealing with objects that are deliberately communicating with something other than just our intellect. So, for me, it's not a dry process. You begin with the feeling that it's special, and if it stands up to the research, you end with the feeling that it's special, and I make no apology for that. I've been criticized as acting as an advocate for it, but if I'm writing, as I am in "Christ to Coke," about the "Mona Lisa," I'm an advocate for that too, because it's a miraculous picture. Also, when I'm writing about the Coke bottle, I'm not an advocate for Coke as a drink. I hate it. But it's one of the all-time great bits of product design, and I'm happy to say that.

In the newly released second edition of your book "Leonardo," you describe the "Principessa" as exhibiting "indescribable delicacy and tenderness." That passage is suffused with feeling. What draws you so personally to the work?

You can't ignore the story. "Mona Lisa," or Lisa Gherardini as we know her to be, was a blameless Florentine upper-bourgeois wife who did what wives did and gave birth to babies, then died quite late in the Convent of Saint Orsola — an exemplary life, as far as we can tell. I feel, for all the princesses, that these are pawns in elaborate dynastic games. They're married at absurdly young ages, expected to bear children, and die. By any standard, the way they were treated is horrible. In the "Bella Principessa," Leonardo has captured that tender fragile beauty of someone who is only entering puberty, and he's captured that spectacularly well. I think it's very moving.