Just in time for the latest round of heated debate about the status of the most expensive painting in the world—Leonardo da Vinci’s portrait of Christ, Salvator Mundi—renowned scholar Martin Kemp has released a new book that delves into the Renaissance master’s paintings. Leonardo by Leonardo, published by Callaway Arts & Entertainment, is an oversize book with hundreds of stunning color illustrations that presents a comprehensive survey of the paintings, preparatory drawings, and dozens of excerpts from the artist’s notebooks. The publication also marks the 500th anniversary of the artist’s death at the age of 67.

Few people are better positioned to opine on Leonardo than Kemp; he’s devoted nearly his entire career to the Renaissance master. Kemp is an emeritus research professor at Oxford University and worked for more than 25 years at the University of Glasgow and the University of St. Andrews. We sat down with the historian—a soft-spoken gentleman whose easygoing affect belies his strong opinions—during a brief visit to New York last week to discuss the inspiration for his latest book, Leonardo da Vinci’s signature moves, and what the public doesn’t understand about Salvator Mundi.

What prompted you to put together a book specifically on Leonardo da Vinci’s paintings?

After 50 years, I’ve probably written enough about Leonardo—probably too much for many people. I then thought, I’ve actually never done a book specifically on the paintings as a group. Which surprised me a bit. I wanted to introduce what you might call the period voice—that is to say, the documents, the contracts, and Leonardo’s own voice through quotations and other sources like his poems. It’s called Leonardo by Leonardo not out of personal grandeur. It’s to say that the illustrations mean that Leonardo is talking to us with incredible vividness; the quotations mean that he’s speaking to us in his own voice.

There has recently been a resurgence of debate about whether or not Salvator Mundi, which is included in your book, is, in fact, by Leonardo. What’s your take?

It’s been driven by two things. One is people wishing to sell their own books saying, “Oh well, there’s a $450 million painting and it’s not by Leonardo, it’s mainly [by the] studio.” The other is of course that debunking this picture—particularly since its ownership is mysterious—becomes very fashionable and you get column inches. So people who normally are limited to the ghetto of the arts pages end up being on the front page. That’s a dynamic which is very typical of Leonardo because he’s such a figure in the public mind.

Leondaro da Vinci, Salvator Mundi, ca. 1500. Courtesy of Christie’s Images Ltd.

How do you respond to some of the specific doubts or criticisms? For instance, art critic Jerry Saltz says the frontal view of Christ is out of step with Leonardo’s style and represented a major stylistic step backward. 

Well, what Jerry Saltz is saying is actually ill-informed. I think he doesn’t understand the nature of Renaissance painting or Renaissance image-making. The Salvator Mundi is not a very common subject, but it’s a standard subject and to be a Salvator Mundi you have to do three things. First, Christ has to look at the spectator; his stare becomes unavoidable. In fact, that’s commented on at the time—that the point of Salvator Mundi is this ubiquitous view of God. The second aspect is that Christ should be blessing. This is part of the subject. And the third aspect is he should be holding a globe. Now in Leonardo’s case, he has done something very remarkable which none of the boys or copyists could do or would understand. He’s turned the globe into a crystalline sphere. It’s a rock crystal sphere with what in geology are called inclusions, gaps. They’re not air bubbles like you get in glass. I did some geology at Cambridge when I was there doing natural science. As soon as I saw the sphere, I thought it looked like rock crystal. So you’ve got these three absolutely required elements of the Salvator Mundi if it’s to be a Salvator Mundi. It was certainly a commissioned painting.

In his book on Leonardo da Vinci, Walter Isaacson focuses on the orb in Christ’s left hand and explores why the artist chose not to realistically depict the visual distortion it would have caused. Isaacson does not necessarily see this as a sticking point in the argument for or against authenticity, but he does question why an artist like Leonardo, who was so immersed in visual experiments and observations, would not have gone with a realistic portrayal. What do you think?

The argument has been made a number of times that the globe should either invert the objects behind it or present huge curvilinear distortion. But this is to misunderstand Renaissance painting: it’s not a photograph. Leonardo is using his knowledge of the appearance of things to make devotional images when he’s doing a religious subject or non-devotional images in the case of non-devotional subjects. It’s not naturalistic in that sense.

So would it be fair to say that the artist chose creative license over what a scientist would consider a realistic depiction?

He is not following all the implications of what he knows about seeing. For instance, he says you should avoid showing blur. He knew about blur, but there was no way as a painter and a Renaissance artist he could cope with blur. So there were certain things he said: these are matters for the speculatori, for the philosophers of natural things, of physics and vision—but they’re not for the painter. It’s a question of good manners. If you can imagine a baptism of Christ in which Christ’s legs are refracted by the River Jordan—they just wouldn’t do that. It’s pictorial bad manners. So here, he’s saying, “this is a rock crystal sphere,” but he’s not following through on the implications of that to the bitter end.

Leonardo da Vinci, Salvator Mundi [detail] (ca. 1500). Courtesy of Christie’s Images Ltd.

Let’s look at how he portrayed hair. It’s very characteristic, this vortex hairstyle. Now the followers and boys could do curly hair pretty nicely. But Leonardo had a theory about the curling of hair. He said it’s like the movement of water, and with water, you’ve got the impeto, he calls it—the tendency to revolve against the impetus of the current, which creates a helix. If you look at the hair in Leonardo’s paintings, there is always a sense that there is a logic to the vortex formation. It has a kind of anatomy. It’s based upon his thinking about how hair curling occurs and its relationship to turbulent water. And none of the followers or boys really get that. They can do a superficial imitation. But if you look at the hair on Christ’s left side, our right side, it was very beautifully preserved. You can get a sense there’s a double helix going on there which follows its way through this complex tangle of vortices.

And was that something that struck you immediately the first time that you looked at it?

Yes. When I walked into the National Gallery conservation studios [in 2008], I think, “What do I do? Do I act cool? Do I say wow?” I got out my magnifying glass and looked at it carefully. You get that immediacy of fact. I noticed the rock crystal sphere and the quality of the hair and other aspects of the picture like the hand—they had a kind of zing.

What do you say to critics who question the work’s authenticity based on the amount of paint loss or conservation?

I would say anybody who comes up with a percentage [of how much of a work was done by Leonardo versus a conservator], unless it is a very clear case like The Last Supper, is actually not accurate. These are layered pictures. They have multiple layers in them from the gesso priming, under-drawing, and upwards. And in different areas, you get different levels of loss. You can get some quite superficial loss, for instance. The area of the beard has been abraded and his very fine technique, marks made with these fine-tipped brushes that he used, have simply been abraded off. That’s losing some of Leonardo, but all the original flesh tones are still there in pretty good shape. It’s a very complicated issue.

And how does scientific testing or analysis tie in to this? That has become such a big part of proving authenticity nowadays.

I somewhat challenge the idea of proving authenticity using a particular criterion. What I’m interested in is how you combine what I call judgment by eye, which is what is traditionally called connoisseurship, with all the other evidence we have—documentation, provenance, etc. In Leonardo’s case, we have very characteristic ways of proceeding. His under-drawings have a certain look to them. And we’ve got the hand print technique, which is very characteristic of Leonardo. Before 1500 particularly, but also to some extent after 1500, he would press the edge of his hand into the paint surface at various points and you can see the hand prints. Not fingerprints—it’s not forensic stuff. He did it to blend these very thin mixtures of pigment. The studio apprentices don’t really work this way.

Leonardo da Vinci, The Annunciation [detail] (1472-75). Courtesy of the Uffizi Collection, Florence.

Initially, I was quite relaxed about it because I’ve worked a bit with the Louvre Abu Dhabi. They’re quite secretive, which they’ve got a right to be. I was told by Christie’s it had been bought by the Department of Tourism and Culture. The Louvre Abu Dhabi is now saying that’s not the case. They don’t have it. So there’s been a silence, a vacuum, and a lot of speculation has come in about [Saudi Arabia’s crown prince] Mohammed bin Salman and so on. But until something concrete comes out, I’m believing nothing. If somebody has got a $450 million painting, they’re probably going to make sure it’s kept in a secure state. But it’s worrying in terms of public access.

The one thing that I should say very firmly is that it has been said that the Louvre have withdrawn their request for the loan or they don’t want the loan of a now questionable picture.

And?

It’s simply untrue. It is untrue because even if the Louvre was persuaded that there was studio participation in the picture, which would be feasible and not unknown after all, it wouldn’t stop them from showing it. It’s a major picture, an important thing. The story is sensationalized and inaccurate. You can quote me on that.