Christie’s auction of Salvator Mundi at its postwar and contemporary evening sale at Rockefeller Center on Wednesday is turning into a true media event. With the painting being touted as the last Leonardo da Vinci painting in private hands and said to be estimated at $100 million, hype for the auction has hit epic levels. Correspondingly, reactions to the work have been intensifying—ranging from the enraptured to the merely fascinated, the mildly curious, the skeptical, and the downright disgusted.

New York magazine critic Jerry Saltz appears to be firmly in the latter category. He went on an extended rant today, expressing “big doubts” that the work is actually by the hand of Leonardo. While admitting that he was “no art historian or any kind of expert in old masters,” he argued that the work lacks the compositional dynamism associated with the Renaissance master.

“The painting is absolutely dead,” he opined. “Its surface is inert, varnished, lurid, scrubbed over, and repainted so many times that it looks simultaneously new and old.”

In a later tweet, he posted the work with Donald Trump’s face, captioning it “Salvator Turdi.” (As of press time, Saltz was set to appear on NBC Nightly News to talk about the painting on the eve of the sale.)

Saltz was not alone in expressing criticism of the work. On his Instagram account, art advisor Todd Levin posted a (since deleted) shot of the 500-year-old painting on Tuesday, calling it a “SHAM” and a “MOCKERY.”

At a book signing for his newest title (Rogue’s Gallery: The Rise (and Occasional Fall) of Art Dealers, the Hidden Players in the History of Art), author Philip Hook, who is Sotheby’s senior international specialist in Impressionist and Modern art, gave his own opinion on the authenticity question. He said that it is generally accepted that somewhere on the panel there is “quite a lot of painting by Leonardo, but, over time, it has had to be restored, and now quite a lot of it is later restorers’ paint, so it’s not in an absolutely pristine state. There are passages of it by Leonardo; enough passages for it to be sold as a Leonardo.”

One thing Hook was unequivocal about was admiration for Christie’s salesmanship. “I have to take my hat off to them,” he said. “They have done the most brilliant marketing job with this Leonardo.”

Given the pre-sale estimate, the stakes are incredibly high. In addition to flogging the painting on CBS This Morning and cutting a trailer that features awed candid camera reactions to the work, the auction house is taking efforts to discredit the painting seriously. When a recent report in the Guardian zeroed in on a chapter of Walter Isaacson’s new Leonardo biography, linking his questions about the mysterious orb in Christ’s left hand to chatter surrounding questions of authenticity, Isaacson immediately took to Facebook to make his view clear:

Just to be very clear, this article leaves a bit of a false impression. In my new book, I state clearly and unequivocally that this painting of Salvator Mundi is by Leonardo. And I explore the reasons that he did not show the crystal orb distorting the robes of Christ. I say it was a conscious decision on Leonardo’s part. I do not say in my book, nor did I say in the interview, nor do I believe, that anyone but Leonardo painted this painting. I believe he made a decision to paint the crystal orb in a way that is miraculous and not distracting. All of the art experts I know agree, from Martin Kemp to Luke Syson.

The same Guardian story now bears a label across the top in italics reading: “This article is the subject of a legal complaint made on behalf of Christie’s International Plc.”

So, are there legitimate concerns about the painting? We spoke with expert restorer Jennifer L. Mass Ph.D, president of Scientific Analysis of Fine Art, LLC, about the questions surrounding Salvator Mundi‘s condition. (Mass has not reviewed the actual work, and spoke merely about general issues of conservation and restoration.)

“In general, when you walk through a collection, about 80 percent of what you see is the work of the artists and 20 percent is the hand of the restorers and the conservators who have worked on the paintings over the years,” Mass explained to artnet News. “It’s not unusual for paintings to have some degree of very appropriate in-painting and that is simply something that you would expect for a work of art that’s several hundred years old.”

In his New York essay questioning the work’s authenticity, Saltz quotes an anonymous “well-known expert in the field” who quipped that the reason this Renaissance painting was being included in a contemporary art sale is “[b]ecause 90 percent of it was painted in the last 50 years.”

We asked Mass about the issues that would be raised by a much lower amount of loss—say, 35 percent. At this level, she explained, “one potential problem in conserving a painting of this nature is decision making. If the paint is truly lost, one might have to interpolate between the islands of paint that are left behind (depending on the distribution of the losses). In that case, how are you making the decision about how the fully intact image should look? That in itself could be problematic, but whether such decisions are necessary depends on if we are talking about paint loss or paint damage. If the paint surface is damaged but not fully lost then the decision process is far more straightforward.”

Global president of Christie’s Jussi Pylkkanen views Salvator Mundi on October 24 in London. Photo by Carl Court/Getty Images.

Many of the questions about condition are covered extensively by conservator Dianne Dwyer Modestini, who did the restoration, in an essay called “The Salvator Mundi by Leonardo da Vinci Rediscovered: History, Technique and Condition” in the 2014 collection Leonardo da Vinci’s Technical Practice: Paintings, Drawings and Influence. Given how intensely issues of originality and restoration are now being scrutinized in the lead-up to the sale, a few passages are worth quoting at length to give a sense of what in the painting suggests the original Leonardo, and what is the product of the restoration.

For example, regarding the all-important face of Christ, Modestini writes:

The irises of the eyes, painted directly over the imprimatur, are as thin as watercolor and seem to have been left in reserve until a late stage. The underlying white preparation shows through more in some areas than others, and is the only way glints, motes and reflections are rendered; there is no body color. This highly unusual technique lends a mystical element which is unsettling. It is a god-like gaze and this is one of the devices Leonardo utilizes to convey the divine. Since both eyes have been abraded, the left one to a greater degree than the right, the ambiguity between abrasion and highlight made the restoration extremely difficult and I redid it numerous times. As little as possible was done to the left eye. No attempt was made, for example, to emphasize the pupil, which is reasonably well preserved in the right eye. Carefully following the remnants of the original, which contain a line of drawing to place the lower lid, resulted in eyes of slightly different size; the left is small than the right. Imposing a more logical or definite shape caused the eyes to completely change character.

Regarding the controversial magical sphere in his left hand:

The rock crystal orb, symbolizing the cosmos, was painted with practically nothing, thin glazes and scumbles which unfortunately have been abraded, especially along the top of the wood grain. Originally the illusion must have been magical since simply toning down the lighter areas with translucent watercolor glazes rendered it convincing. The three large specks of white paint must represent reflections on the surface of the sphere from an outside light source, but since many of the original glazes have perished, even when toned down, they float without context. The lower right side of the orb contains carefully observed inclusions, characteristic of rock crystal. They are astonishing under the microscope. Each has been described by an underpainted middle tone, bracketed by a curlicue of white, and a dark shadow. They vary in size and disposition and are each somewhat different depending of the fall of the light. Only Leonardo, with his interest in the natural sciences, would have gone to such obsessive lengths.

And finally, regarding the background, which has been painted both green and brown before it was restored to its current deep black (a Leonardo signature):

The initial cleaning was promising especially where the verdigris had preserved the original layers. Unfortunately, in the upper parts of the background, the paint had been scraped down to the ground and in some cases the wood itself. Whether or not I would have begun had I known, is a moot point. Since the putty and overpaint were quite thick I had no choice but to remove them completely. I repainted the large missing areas in the upper part of the painting with ivory black and a little cadmium red light, followed by a glaze of rich warm brown, then more black and vermilion. Between stages I distressed and then retouched the new paint to make it look antique. The new color freed the head, which had been trapped in the muddy background, so close in tone to the hair, and made a different, altogether more powerful image. At close range and under strong light the new background paint is obvious, but at only a slight remove it closely mimics the original.

What will be interesting is whether all the second-guessing affects the outcome of the sale. A few years ago, there were reports that the experts who authenticated the work hoped to fetch a jubilant $200 million for it. That optimistic number has been slashed in half for the current sale. (Christie’s has also secured a third-party, or outside guarantee, for the work, so it will have a new owner regardless of the outcome tomorrow night.)

For his part, Hook thought Christie’s had executed its strategy well, speculating that it was being pitched towards new wealth in Asia, “which is not necessarily a terribly sophisticated market, but is going to be very susceptible to the romance and drama and the uniqueness of a Leonardo.”

This, Hook thought, explained the groaner of a marketing tagline: “the Male Mona Lisa.”

“They [Christie’s] have looked at the situation and they have decided what is against it in terms of getting a Chinese or Asian buyer is the fact that it shows Jesus Christ,” he speculated. “And so they have marketed it not as a portrait of Jesus Christ, they’ve marketed it as ‘the Male Mona Lisa.’ You have to take your hat off to them. I just wish we’d done it!”

Additional reporting by Sarah Cascone.