Close Examination: Fakes, Mistakes & Discoveries, National Gallery, London

Connoisseurship is a handsome word, calling to mind the dry lips of Sir John Pope-Hennessy and back copies of Apollo.

It is connoisseurship that gives art its moral order, allowing pictures in galleries to be tagged with the terms “attributed to”, “studio of”, “follower of”, “school of”, and, some way down the line, “after”. The difference between a follower-of and an autograph work may be many, many noughts, the number growing with the value of the artist followed. And since, at heart, connoisseurship, like art, is about money, its morality has bred an obverse immorality: a dark, shadowworld of fraud and overpainting run by clever forgers, glint-eyed auctioneers and, not infrequently, connoisseurs in art galleries.

And so to Lorenzo di Credi and Andrea del Verrocchio, at least one of whose works may be hanging in a new show at the National called Close Examination. Or not. The subject of Close Examination is forgery, although the show blurs that word to include all kinds of practices we might not have thought of as chicane. Among these is the Renaissance studio system. What any good studio wanted – and Verrocchio's in Florence was one of the best – was a stable of young artists who could turn their brushes to the style of their master. Which is to say, to forge him.

Among Verrocchio's more adept pupil-forgers was Domenico Ghirlandaio, to whom The Virgin and Child with Two Angels (c1476) was once attributed. It now seems more likely that another apprentice, Lorenzo di Credi, painted the work, or at least most of it. Verrocchio himself may have done the Virgin's right hand and the left-most angel; but, then again,he may not. The philosopher, John Locke, mused over the point at which a pair of darned socks became more darn than sock. If Verrocchio did paint half of The Virgin and Child with Two Angels, is it a Verrocchio or merely a studio of Verrocchio, or a bit of both? Of such vexations is connoisseurship made.

The question seems to be one of motive, although even this doesn't quite suffice. If some credulous Florentine was duped into buying The Virgin and Child with Two Angels in the belief that its every brush-stroke had been applied by the master's hand, then it is dishonest. If the imprimatur of a master's studio was in itself enough of a selling point, then it probably isn't. But what of Delacroix's freewheeling little Portrait of a Man, left to the nation by Walter Sickert? Sickert was a huge Delacroix fan, so much so that it is now thought that he didn't merely own Portrait of a Man: he painted it. Imitation may be sincerely flattering, but does it cancel out forgery?

And so Close Examination goes on, muddying the moral waters with an energy that verges on the reckless. Pieter de Hooch's A Man with Dead Birds, and Other Figures, in a Stable (c1655) was certainly painted by Pieter de Hooch, or at least the man, the other figures and the stable were. However, the birds – the picture's most memorable element – are 19th-century additions, covering the picture's original focus (a wounded man) to suit the taste of the day.

And that little tinkering seems straightforward compared with one of the National's most celebrated works, Giorgione's Il Tramonto. If it has ever struck you that The Sunset seems an odd title for a picture whose main event is St George killing a dragon, then you have reason on your side. The saint and his lizard are Victorian additions; and they were put there not by a forger, but by a gallery restorer.

Such slipperiness makes this show infinitely more engaging than if it had merely been of fakes in the National's collection. (Although there are plenty of those. How could Madonna of the Veil, circa 1920, have been bought as a Botticelli? So much for connoisseurship.) You can't help feeling, though, that Close Examination has itself been lightly tweaked. The last work on the last wall of its last room is The Madonna of the Pinks, classed for centuries as a copy after Raphael – a status hotly defended by the eminent Raphaelist, James Beck, up to his death in 2007, and by many other art historians today. The Madonna of the Pinks is included in a section called "Redemption and Recovery", of great works once attributed to lesser masters. And who upgraded the picture to a genuine Raphael, for which the National Gallery paid £35m? Dr Nicholas Penny, director of the National Gallery.

To 12 Sep (020-7747 2885)

Next Week:

Charles Darwent sees Fiona Banner's installation in the Duveen Galleries at Tate Britain: RAF meets a-r-t