Watercolour, Tate Britain, London

While the portability of the paintbox suits nature painting, watercolour can do much more. But is that really news?

Alan Bennett recalled once ringing Peter Cook to ask him to dinner, at the couple's request, with Prince Andrew and the then Sarah Ferguson.

Cook, unfazed, said, "I'll just have to look at my diary, Alan." There were off-phone noises, as of pages being turned; then Cook came back on the line. "Oh dear," he drawled. "I find I'm watching television that evening." And so to Watercolour at Tate Britain.

Watercolour. Well. The assumption behind the bald title of this show is that we all know what it means, or at least what it implies. It means vases of physalis – no, of primulas – painted by Welsh spinster sub-postmistresses for shows in village halls. It means Prince Andrew's older brother doing runny views of lochs and everyone saying how good they are. It means jam jars of clouded water with hairless brushes in them, and ... so on. An evening in watching the telly would be infinitely preferable to a show called Watercolour. Even an evening watching Friends.

Of course, we all know this isn't true, really, because we've been to art galleries and seen Turner and Blake. You can't come away from Blake's etched-and-washed Ancient of Days thinking that watercolours are the painterly equivalent of lavender sachets. This knowledge rather spoils the surprise.

For the hope at Tate Britain is that we will go around the show squealing, at intervals, "A watercolour! Surely not!", to which end the curators have gone for works in some way untypical of the medium. One of the points of watercolour is portability. You can't put an easel and palette in your pocket and walk up a hill; you can a box of watercolours and a sketch-pad. As a result, watercolours are often small and of hills. Alive to this problem, the curators of Watercolour have gone to great lengths to avoid small, hilly works. There is only one picture of primroses, by William Henry Hunt, and no physalis at all that I could find.

Instead, there are epically big pictures, done in that grand, finished style we more obviously associate with oil paint. A case in point is John Varley's The Suburbs of an Ancient City which looks, at first glance, like a Poussin. Why Varley should have chosen to paint a classical scene in watercolour is an interesting question: from the look of the work, he was captivated by Poussin's lemon sun, and watercolour is famously good for rendering light. From the point of view of this show, though, Varley's picture defies our expectations of watercolour in being big and formal. Or at least it defies the Tate's expectations of our expectations, which is not quite the same thing.

So, too, with Arthur Melville's luscious Blue Night, Venice. Lusciousness is not a quality we are expected to associate with watercolour, any more than we do Varley's grandeur. The miracle of Melville's work is in the blue saturation of its sky, a depth and intensity of colour it is hard to imagine coming out of a watercolour box. How did Melville do it? I have no idea. In the context of this show, though, the point of his work is its oddness, its status as a watercolour that doesn't look like one.

Other things we don't expect of a show called Watercolour are grittiness and contemporaneity. Herbert Cole's Private Green records, in livid detail, the surgical reconstruction of the face of a trooper wounded in the Great War. As to modernity, the exhibition's last room is given over to works by the likes of Tracey Emin and Peter Doig. For both, the appeal of watercolour seems to be that its translucence and unfashionableness evoke times past, a sense of loss. Both in its look and in the way the artists think of it, the medium encompasses death.

The problems with all of this are pretty obvious. The Tate tries to set up watercolour as an Aunt Sally, a thing old and lacy and smelling of 4711. Like any good Aunt Sally, the aim is to knock it down – to reveal, through untypical works, that many watercolours are not like watercolours at all; that they are rich and modern and big. This sends out the unfortunate message that typical watercolours are thin and archaic and small, also rather boring.

To compound the error, the curators play fast and loose with their definitions, classifying as watercolour any medium in which water plays a part. Is gouache really watercolour? Acrylic? Washed ink? The real problem, though, is that this show assumes that people who go to galleries are a bit thick – that they won't have seen watercolours by (say) Turner and Blake and worked out for themselves that they can be grand and experimental. Not very flattering, really.

To 21 Aug (020-7887 8888)

Next Week:

Charles Darwent sees the National Gallery's Jan Gossaert show

Art Choice

See John Stezaker's scissor skills at the Whitechapel Gallery, east London (to 18 Mar). His precise, subversive, image-slicing takes cut'n'paste to a new level. The Tate's Art in Yorkshire project makes its first big splash with David Hockney's 40ft-long painting, Bigger Trees Near Warter (that's be a village, not a typo) at York Art Gallery (to 12 Jun).