Philip Hensher: In our time, not even sculpture is made to last

Antony Gormley's Margate man had a curiously unfinished air, as if waiting for that final act

In January 1494, snow fell heavily in Florence and one of the strangest artistic commissions of all time took place. The new ruler of the city, Piero de'Medici, commissioned Michelangelo to make a snowman. It was the only occasion on which Piero asked Michelangelo to do anything. It might have been taken as a bit of an insult, but Michelangelo entered into the spirit of the thing and created something which, contemporary observers remarked, had to be the most beautiful snowman ever made.

What it looked like, we have no idea, and it was lost forever with the thaw. There are other lost works by Michelangelo, but this is the only one which was always meant to be lost.

The idea has just been revived, in a dramatic and perhaps shocking way, by Antony Gormley. As part of an arts project in Margate, a day of public performances entitled Margate Exodus, Gormley constructed one of his familiar human figures, on a giant scale - at 25 metres, I think it is, or was, taller than his celebrated Angel of the North. It was constructed out of rubbish and detritus collected over two months by volunteers, and on Sunday, with a nod to The Wicker Man, it was set on fire, destroyed in half an hour. The splendid spectacle was captured on film - he had a beautiful day for it - but the sculpture itself was only briefly in existence.

When we think about sculpture, we tend to think of something preserved from time's all-toppling wave, as Auden said. Whether marble or bronze, it is something which seems not to partake of the general idea of art as something fragile, which only survives by chance and a careful curacy. No particular care need be taken of it; it just survives against anything but the most determined intentions.

If you visit the Royal Academy's magnificent Rodin exhibition, you may be surprised, as I was, to find the wonderful Gates of Hell not in a carefully sealed and air-conditioned room, but simply standing in the courtyard under sun and rain. Plenty of sculpture survives for centuries under these conditions - the Baptistery doors in Florence, which Rodin had clearly studied, are still in perfectly good condition. And, just across the piazza, the monumental David of Michelangelo may be a replica - the original was long ago moved into the Accademia. But, under the loggia, exposed to the open air, those are the original Giambolognas and Benvenuto Cellinis.

Elaborate security provisions may now be made necessary, but sculpture, in essence, lasts for hundreds of years with the minimum of care.

Blake said that eternity was in love with the productions of time. Looking at Assyrian reliefs, or Egyptian funerary statuary, one can't help but be awed by an art which has preserved the appearance of a humanity over thousands of years, and which will go on being looked at long after all of us are gone. It's not just the prerogative of sculpture - it is heart-stopping, in Tate Britain at the moment, to look at such exact preservations of living, breathing people as Holbein contrived five centuries ago. But it is, uniquely, in the substance of sculpture that it intends to last forever, and perhaps in some ways that can prove a burden.

From time to time, the art turns away from its future long vistas and, instead of expressing its love for the ephemeral by preserving it, decides to join in with that ephemerality. The result is almost always extremely touching.

In the last 50 years, sculptures have been made from such strange materials that the technical staff of galleries are having to acquire a huge range of skills to try to slow down a work of art's fast decay. Joseph Beuys's fat and rust pieces are rotting under the eyes; Eva Hesse's sculptures are approaching a state of fragility where they are not very likely to be able to travel much more; even Damien Hirst's pickled shark is wrinkling and soon will begin to decay.

Artists have started to respond to this changing idea of sculpture by making works which aren't intended to last, such as Anya Gallaccio's pieces made of roses, or chocolate, or ice. Some, like Antony Gormley, have made pieces where the sculptural element is bound up with a theatrical finale, in which the substance of the work is destroyed - the Margate man, before its conflagration, had a curiously unfinished air, as if it were waiting for that final act.

And others, connected to performance art, have contrived events which by their very nature could only occur once, so that art enters fully into the utterly ephemeral nature of existence. One of my favourite artists, the Roman Signer, of Switzerland, contrives small, embarrassing events, often in the open air, which only happen once and which, quite often, no-one even sees. Jeremy Deller's magnificent recreation of The Battle of Orgreave was another event of this sort; and perhaps sculpture is heading towards the condition of performance, in which "you had to be there" to understand it at all.

Of course, most of these works are not truly ephemeral any more. Thanks to film, they are all just as carefully preserved as any marble by Bernini. But I would rather like it if an artist working in this way refused even that method of preservation, contriving works of art which existed once, like meals, and for ever afterwards survived only through memory, retellings and rumour, becoming more and more extraordinary and fabulous with wishful thinking. Michelangelo's snowman didn't need to be filmed to take on a most peculiar immortality.