Watching the detectives: Fakes, mistakes and hidden masterpieces

If art isn't fun, all by itself, there are plenty of other ways to get us diverted. There is auction. There is theft. There is vandalism. There is destruction. All these art-stories will draw the headlines. And we shouldn't be snobbish about it.

None of these wild and worldly dramas are irrelevant to pure art. Money, damage etc... one way or another, they're all linked to its value and uniqueness.

But perhaps the most thrilling art attraction is forgery. It gets to the heart of it. It makes us look much more closely at art than mere beauty or shock. It's so near to the real thing itself, except that it seems to know more. The forger is the conjurer, the trickster, the great deceiver, the master-craftsman. The forger upstages art itself. Though of course forgery can only be enjoyed when it is undone.

So for a summer exposé, try a squinny at Close Examination: Fakes, Mistakes and Discoveries. That's the idea, anyway. Works from the National Gallery's collection, some of them very famous, some rather obscure, are put together to reveal their secrets. The scholarly staff emerge from behind the scenes, and onto the stage, explaining the history and the research. From Wednesday, you're being invited to look closely. And in the first room there are some genuine fakes to see.

But, well... not many, and not very thrilling. In terms of big lies and big eyefuls, don't expect much. It's not going to be The Hay Wain or Sunflowers. And Uccello's St George and the Dragon, which was quite recently queried as a 19th-century cook-up, has been safely confirmed as kosher. The most spectacular example is Dürer-ish. It isn't an all-through fake at all.

The Madonna with the Iris is not from the master's hand, no. But it's largely from his studio's team, at the time. And then parts of the painting were repainted in the 18th century, adding Dürer's signature. People were trying to bump it up. But the Dürer-esque bits survive, and they've got some lovely, piquant nature studies.

Maybe it's to the gallery's credit that it has collected so few conspicuous forgeries over the years. But for the purposes of this exhibition, it should have surely had more. No visitors will feel terribly astonished by this bunch of con artists; some will feel a little let down. And after that, it's a matter of innocent puzzle-work of one kind or another.

The show tries to keep up our spirits, accompanying each painting with bold and challenging captions. "A prestigious provenance is no guarantee of authenticity. Did Charles I receive an original or a copy?" "Portions in this painting are the work of a 19th-century restorer. What parts, and how can you tell?" "Will scientists and curators ever determine who painted this mystery portrait?" Ooh, er, missus!

And all these questions can be asked and answered, or not answered yet. But why do we need them asked? What do these technical and detective issues ultimately motivate? After all, there are many genuine and authentic works that are not worth looking at full stop. All these studious enquiries have to be underpinned by more vital responses. It's necessary to get things right, granted. You have to pursue names, dates, places, infrared, x-rays, the chemistry of paint or wood. But when close examination loses sight of delight or terror or strangeness, why bother with art at all?

In fact, these responses can't be excluded. They're merely lurking in the wings. The show manages not to mention them, but the visitor can't help wondering. For example, in 1874 a pair of wide horizontal paintings were acquired as "Botticellis". One of them, Botticelli's Venus and Mars, became a favourite in the gallery, beautifully languid, complex, half-funny. The other one is called An Allegory. It lost its Botticelli status. It's hopeless. It's hard now to see how this dud could ever have had a chance.

But they are both hanging here, side by side, and the comparison is handy. You may even call it the "Hans van Meegeren" effect. That famous 20th-century Dutch forger produced a number of Vermeers. They were at first greeted as great discoveries. And then they were exposed, and immediately nobody could see how they looked anything like Vermeer at all. Of course, An Allegory wasn't a fake. It was merely a weak Renaissance imitation. The "forgery" was all in the eye of the 19th-century beholder. But it's strange today how once it was taken for the finer work.

Anyone looking at this work wants to hear about it – and why it didn't then look fatally lifeless, and does now, while Venus and Mars is still life itself. In fact, when school parties are taken round, instead of telling them about stories (the usual approach) get them to consider this pair, and what makes one picture better than another. Introduce them to taste and judgment. It would be an easy and excellent lesson. It could be even more interesting than a forgery.

Or come to a more curious case. In 1937 the gallery bought a Giorgione, that wistful and mysterious Venetian painter. Almost at once it was unmasked. And people were very disappointed. The work was demoted to a minor Venetian, Andrea Previtali. But why were these people so sad? They could see it didn't really deserve the poetic qualities. Yes, but some of them wanted the name, all the same. The oeuvre of Giorgione is small and rare. They would have preferred obviously to have another one. But there wasn't one there to be had. Too bad.

Meanwhile you're left with this Previtali. And naturally, the mistaken attribution is taken out on this unfortunate painting. But in its own way it's an original and oddly modern work. It's like a cartoon-strip, with incidents illustrated through four almost identical scenes, legends about love and death. Its repeat-images recall a Magritte. So if people once failed to recognise a pseudo-Giorgione, let's not fail now to recognise a Previtali. Contemporary artists might be interested. The gallery shouldn't talk of "demotion". It should take a more grateful interest.

Or take the most extraordinary turn-around in the show. It's A Dead Soldier, his body flat out at an angle. It's a 17th-century picture. Nowadays it has no identified painter, but it's Italian. In the 19th century it was assumed to be Spanish, indeed by Velázquez, whose fame was beginning to triumph. This painting was the model for Manet's magnificent The Dead Toreador.

And the fascinating thing about this anonymous work is not its unknown authorship, which is never going to be very interesting. The fascinating thing is that a mistake can create genius. Manet probably wouldn't have been inspired by this old painting, couldn't have made his own great painting – a much greater painting, in fact, than the original – if he hadn't been wrongly inspired by the name of Velazquez. If scholars had helpfully informed him with what they know now, he might have just given up. In other words, some future artist, and the world, may benefit from some researchers keeping their eyes and mouths shut.

Now there's no really scandalous revelation in the National Gallery to compare with the Prado, where in recent years Goya's so-called masterpiece, The Colossus, has been finally reattributed to his pupil Asensio Julia. Scholars and the public are still coming to terms with it. Its mixture of visionary grandeur and crude handiwork is more than a conundrum. If The Colossus had always been credited to Julia, would he have been praised for this amazing one-off all along? Or did this work need its long Goyesque fame to give it a kick-start? History itself can be a forger.

Close Examination: Fakes, Mistakes and Discoveries, National Gallery, London WC2 (Nationalgallery.org.uk) 30 June to 12 September

For further reading:

'The Conman: How One Man Fooled the Modern Art Establishment' by Laney Salisbury & Aly Sujo (Gibson Square Books). Order for £7.49 from the Independent Bookshop: 08430 600 030