Before long he moved on to using kitsch printed fabrics instead of canvas, superimposing cartoonish figures on to their patterns of spaceships or forest animals. He would also doctor his own photographs, using chemicals such as uranium to alter prints in unpredictable ways. The aesthetics of all this were largely secondary; Polke did not make art to express a viewpoint. "It is the procedures in and for themselves that interest me," he said. "The picture isn't really necessary!" While this might make for conceptually interesting work it doesn't necessarily make for pictures that are always rewarding. It is why he has long been a favourite of curators and other artists rather than the public.

Polke stated that he wanted "to show how dependent we are on existing forms. We are continually resorting to what already exists ... consciously or unconsciously." Whereas Richter and Kiefer have found a way to twist the pre-existing to their own ends, Polke never quite did. It means that almost all his art can seem a work in progress rather than an idea conceived, considered and expressed. Indeed, the most fully formed — and beautiful (not a word Polke elicits too often) — of his pieces are those where chance did most of the work. When, for example, he mixed materials such as pigment, lacquer and varnish together and poured them on to a canvas to congeal haphazardly. Or when he used the smoke from an oil lamp to trace sooty trails on sheets of glass. Or blew dust from a meteorite across a resin-coated surface to form a haphazard cloud.

Polke himself had an explanation for the heterogeneous nature of his work. He was, he said, interested both in the instability of vision and in the conflicting claims of representation and abstraction: his double-exposed films and pictures painted with unstable photographic chemicals examine these borderlands. Of course such vagaries can also be seen as a get-out clause to explain his flibbertigibbet experimentation and equally unsettled enthusiasms. "Solutions," he said, "are the product of a lack of freedom multiplied by a complacent satisfaction."

Because of his variety Polke exhibitions usually concentrate on one particular medium — his photographs, his films, his drawings. This show, however, has a bit of everything. Whether it will enhance his reputation is another matter. His art is fascinating for the way it thumbs its nose at bourgeois conformism but nothing ages as quickly as the avant-garde.