Uncatalogued paintings by recognised artists turn up in the market more frequently than you might think: there are a few ‘sleepers’ in almost any given Old Masters auction. Though such discoveries are a major topic of conversation among art dealers, they rarely make the news. But it is different when such a sleeper might turn out to be by Rembrandt; and now the art press is already buzzing about a little painting just sold at Christie’s, New York, featuring Rembrandt’s famous face.

It was listed only as ‘Manner of Rembrandt’, which in auctioneers’ jargon means the painting would appear to be by a later imitator of the master, not by one his students or close followers. And it was offered with an estimate of $15,000-$20,000, but with no reserve, meaning it could have sold for much less. In the end, it sold for a total of $675,000.

Its quality told, with the materiality of its surface strongly suggesting that it was a product of Rembrandt’s own school. Though could it really be by the master himself? Rembrandt made so many self-portraits; but his students would also reproduce them for practice and, strangely, they sometimes even invented their own versions.

Jan Six, a Dutch art expert who has previously discovered two Rembrandt sleepers for himself (and who also happens to be the direct descendant of the sitter for one of Rembrandt’s most glorious portraits), has already had his say on the new find. He believes it to be a fine work, but only by Isaac de Jouderville, one of Rembrandt’s early assistants.

It is a very plausible suggestion. He is absolutely right to point out that the elongated shape of the face is typical of Jouderville’s own style, in many of whose paintings we find such a droopy jawline, with the cheek seeming to dissolve away. But it is not too difficult to find similarly shaped faces in Rembrandt’s own oeuvre, especially during the 1630s. And, crucially, in our new example the face may actually be foreshortened, rather than simply elongated: notice that the ear is placed high, indicating that the head is angled down from our view, while in Jouderville’s portraits with comparable faces the ear tends to be set very low. It is possible that Jouderville’s quirky stylisation even evolved from copying such examples by his master, without fully comprehending their perspective.

But in the Christie’s picture the nose is also more regularly proportioned than we should expect, without the bulbous projection that would become something of a Rembrandt trademark. And then the gentle, rather passive expression in the eyes might again seem more typical of Jouderville than of Rembrandt, who at this stage was mostly painting with an intensely energetic realism. Yet in Rembrandt’s depictions of his wife, Saskia, we certainly notice similarly gentle expressions.

Even if, stylistically, Jouderville seems the much likelier candidate, when you zoom into the image and notice the free, painterly touch, and the extraordinarily impressive orchestration of tone and colour temperature, it suddenly all seems worthy of Rembrandt. Jouderville was at best a diligent painter whose way with his materials appears constricted, despite all attempts to imitate his master; and his organisation of light and shade was, at best, conventional – uninspired and unexciting. So either he surpassed himself here and, for once, reached rare new heights, or this modest little canvas really happens to be by Rembrandt caught in a milder moment, accidentally approximating his sidekick’s style.

As soon as I clicked on the image I was wondering how anyone other than Rembrandt could have made the shadows

Style versus quality: which, ultimately, gives the most important clue? This is often a serious dilemma in the field of attribution, and there is no simple answer, especially since studio practice could sometimes be more interactive than our categories would allow for. What if, for example, Jouderville began the little painting and, for whatever reason, Rembrandt later came to paint over it? It is easy to speculate, but impossible to know. However, a category for students’ works corrected by the master already exists in scholarship on Rembrandt’s drawings.

The painting’s authorship may well appear more obvious after cleaning and restoration, as new detail should be discovered in the hair and the full contrasts in the lighting should come out. So for now, given that we have known the work only briefly and superficially – only from a photograph – it is best to stay cautious. Yet we all have an instantaneous reaction – there is no point in pretending otherwise – and mine was positive: as soon as I clicked on the image I was wondering how anyone other than Rembrandt could have made the shadows under the eyes, in such broad and bold strokes, glow with such warmth. Or how anyone else could have drawn the shaded side of the chin silhouetted so beautifully against the back wall, now turning a pearlescent grey as it is struck by the light. That is an astonishingly apt passage of paint that is typical of Rembrandt, and I see no equivalence to it in any known work by Jouderville (nor for that matter do I see any equivalence to it in another painting which recently sold at auction for many millions with a full attribution to Rembrandt). I could easily be wrong; but in cases as tricky as this, a feeling one way or the other usually trumps argument. Here, for me quality begins to seem more important than style, because Jouderville can only resemble Rembrandt from afar – from up close his work is disappointing – whereas the Christie’s painting only resembles Jouderville from afar, and it resembles Rembrandt all the more in its details.

But which Rembrandt does it resemble, painting when? Jan Six raised yet another serious concern, that the painting is not easy to slot into Rembrandt’s career, and it is true that the technique seems more advanced than it should for the age at which Rembrandt appears in the image. But could that not partially be explained by the small scale, and the informality of the depiction? Because the reddish earth colour used to draw the fold in the eyelids still seems entirely typical of Rembrandt, and so do the brushstrokes used here to sculpt the nose, which are at every angle identical to the brushstrokes sculpting Saskia’s nose in the portrait of 1633, at the Rijksmuseum.

An additional explanation might be that the Christie’s painting was left at a relatively early phase of working; and it will be fascinating to investigate what, exactly, is happening outside of the drawn circle in which the head is framed, because the rest of the square canvas has also been sketchily painted, rather than left blank as if it were to be ignored or covered up. The details there are difficult discern, and it might just be that the head was painted on top of a previous sketch; but if it was in fact an idea for a trompe l’oeil – as if the set up were a circular mirror in a gold frame propped against a dark wall – then Jouderville would seem the less likely candidate stylistically too, because there is no evidence that he was prone to such witty invention, while Rembrandt often enjoyed playing with his viewers’ expectations. For example, there is the portrait of Agatha Bas, in the Queen’s collection, in which the sitter’s hand appears to reach out and rest on the picture frame.

Besides the excitement of the discovery – or, the possible discovery – there is another story here concerning the vendors. You might imagine they would be happy to have made over thirty times more money than they expected; but the painting was actually being deaccessioned from a museum, supposedly to raise funds to buy more important works.

It is certainly an embarrassing mistake, but it may also be significant. The Bass Museum in Miami Beach was opened in 1964 with the donation of the private collection of John and Johanna Bass, and the Christie’s painting was part of that original donation. Its sale may have resulted from a radical redefinition of the museum’s “mission” which, as the website plainly states, is now just to “create connections between international contemporary art and the museum’s diverse audiences”. That would hardly leave room for a Rembrandt, whether its attribution were contested or not.

If, as the auction listing declared, the painting was really sold to raise funds, then why was it consigned at such a pitifully low estimate and offered with no reserve? The fact is that $15,000 is a trivial amount of money in the world of contemporary art, and the museum’s board members surely wouldn’t struggle to raise more significant sums in Miami Beach.

They dismiss old art as elitist and propagandistic, though secretly they are jealous of the superior qualities they fear it might possess

That they didn’t care what the painting sold for, suggests that they weren’t flogging it for all it was worth, but getting rid of it. There is a prevalent but carefully concealed disdain for old art, amongst those who work with modern and contemporary art. They dismiss old art as elitist and propagandistic, though secretly they are jealous of the superior qualities they fear it might really possess and the superior virtues they fear it might really embody. But they are also just fashionable people, beneath the pretentious posturing, and they are therefore instinctively repulsed by what is no longer ‘up to date’.

The very existence of that dusty old painting might have seemed an affront to them, despite its being hidden out of sight in their storerooms – I can imagine how it burdened them, at least. So, out with the old, and in with something new and spangly, and reassuringly expensive. If only they knew!

Whatever the verdict turns out to be on the Christie’s painting, whether it is given as Jouderville’s best effort, or it gets the blessing as an authentic Rembrandt, or it is given to someone else altogether – and arguments over Rembrandt attributions tend to go on and on – it is clearly a work of outstanding quality, deserving of greater study and, more importantly, appreciation. And anyone who is blind to that simple fact should have no business pretending to be interested in art, let alone in deciding which artworks a local community should get to see.