When you make a study of silent cinema, you develop a relationship with mould. Invariably, it's an abusive one. You order up a title from the vaults, lug the reel out of its can, hoick it on the Steenbeck editing table, thread the film around the rollers, and throw the switch. On the tiny screen, long-dead actors gaze into the lens, their mouths moving silently, their skin glowing white with Leichner No 5. Then, their world is invaded by a sudden chaos of blotches, and you realise that time and fungus have reduced this portion of the negative to a mass of blistered emulsion; that what you wanted to see has been lost for ever.
For anyone with an interest in pre-1930 movies, Bill Morrison's film, Decasia, ought to be torture. Its director, a 38-year-old Chicago-born artist, toured the archives of North America, poring over their most dilapidated lengths of celluloid, grubbing about in their preservation departments, making fresh prints of frames too corrupted ever to be screened in public. The result, however, is neither depressing nor infuriating. Faces melt and buckle in the frame; figures flicker from positive to negative. A boxer smacks at the coursing vortex into which decay has transformed his punchbag; a man propels a spinning wheel that, through a freak of decomposition, has become a matrix of light. Watching Decasia is like attending a seance, or looking through a window into your great-grandparent's dreams.
Initially, some archival institutions were suspicious of Morrison's interest. "It was," he recalls, "like asking to see their dirty laundry." They were reassured, he suspects, because they realised that by showcasing the ferocity with which time eats up old nitrate film stock, his compilation might stir donors and funding bodies into helping.
Morrison's own view of the piece is rather different. "It's more about the acceptance of decay, which is the natural state of things. You have to appreciate things not only when they're pristine, but also as they're decaying, and when they're gone. And if, by chance, we get to see a ghost again, as we do in Decasia, then we can appreciate that while it's around.
"We have to do the best we can to preserve them - I don't mean that the archival industry is a waste of time, but as we get deeper into cinema history, it's becoming clear that we can't bring its entire legacy with us. Perhaps if Spielberg wanted to leave his estate towards film preservation, that would take care of a certain portion..."
Indeed, Morrison's film presents the decay of celluloid as something as beautiful and inevitable as a fall of autumn leaves. Its subjects seem engaged in a mysterious dialogue with the physical collapse of the medium in which, long ago, their images were fixed: a mass of ants appear as agitated by the degradation of their close-up as the stick with which the cameraman breaks up their nest; sea crashing against a beach seems to be showing that it can reduce itself to an abstraction of squiggles equal to the ruinous state of the film. You have the sense that nature is reclaiming materials borrowed by these film-makers - camphor, silver, alcohol, sulphur - and returning them to their original state.
For me, attending a screening of Decasia at the British Film Institute - just down the corridor from the cubicles in which I have spent hours watching the remains of our own silent period - the film also felt like a kind of exposure. I found myself asking whether I would find this material so intoxicating if I didn't know that 80 per cent of our silent cinema had been lost to decay, ignorance and critical indifference. I wondered to what extent my responses to watching a picture by, for example, the pioneering British director Henry Edwards, were shaped by my knowledge that, during the Second World War, most of his back catalogue was thrown on to a bonfire.
"When we lose a very old relative, it's not always tragic," Morrison reflects. "I went to the funeral of my 102-year-old grandmother, and it was a great family party." The same attitude, he argues, might be extended to the films that we will lose to mould and entropy. "The spirits, the dreams of these films are what is captured in the image. The films take the spirits and dreams with them as they go, but they're returning to the base elements, and may be reborn as something that new spirits and dreams can find their way towards."
'Decasia' screens at Tate Modern, London SE1 (020-7887 8000) from tonight to 7 December.