"Objects of Desire", the Hayward Gallery's autumn blockbuster, raises interesting questions about the nature of modern still life. Questions such as: how do you put it together, how do you stop it decaying and should we care anyway?
Can life be still, or, as the French have it, nature dead? Not for long. The world is process, sustained by continual movement. You can arrest it for a moment to examine it, but one of the most difficult tasks is to stop things changing. Can then a definitive image ever be made? If you paint a still life of soft fruit over a period of time, the subject will alter radically. Which moment in its deterioration does the artist choose to paint? Which is the truest version? A magnificent new exhibition devoted to the subject of still life, "Objects of Desire" at the Hayward Gallery in London, seeks to answer some of these questions.

The show surveys the 20th century view of still life, from 1907 to today, through more than 100 paintings and objects, ranging from Picasso to Cindy Sherman. Perhaps most interesting are the sculptures. When household materials and everyday objects began to be seized on and reinterpreted as art, the question of how to preserve those things arose. Many had an inbuilt obsolescence or limited shelf life. After a time they are supposed to fall apart. Then there was the question of whether we should be reverential to sculptures made of cardboard or, quite literally, junk? I think, as you walk around this show, the answer is a loud affirmative. These objects are worth preserving because they embody the real mystery of art: how the ordinary can be translated into the magical.

Apart from the logistical problems of installing, say, an eight-foot high stack of precariously balanced giant plates, there are the daily conservation problems. Conservators rightly place a premium on careful handling and storage. When an object is allowed to travel, special micro- climate crates are constructed, to minimise the changes in ambient temperature and humidity. These, together with exposure to light, are the main potentially destructive agents.

The two objects on loan from the Tate Gallery to this exhibition are both very fragile. The Picasso still-life is a painted wood construction, which the artist effectively knocked together with rusty nails in 1914, with little consideration of how long he wanted it to last. Artists are not technicians. The creative process is an imaginative, risky business, not tied to rules of procedure or standards of technical conduct. The upholstery fringe which plays such a key part in this sculpture is held to the body of the piece by very fine thread. The point is to keep that thread in a stable condition, and not allow it to deteriorate. That is the job of the conservators. In the case of the Tate's Marcel Broodthaers sculpture, Casserole and Closed Mussels, light is the big enemy, as it could affect the green polyester resin with which the sculpture is glued together, rendering it sticky or brittle. In addition, the lid is not actually attached to the sculpture, so its security must be a concern.

Some materials are more vulnerable than others. Fur is food to quite a wide range of insects, and so Meret Oppenheim's Object (1936), a fur- covered cup, saucer and spoon, is at high risk. Particularly when it's in store, perhaps for months at a time. However, even if the worst came to the worst, and the moth (or some other mite) got in, restoration would be possible. Think of the taxidermist's skills in making good the most damaged pelts and hides. The fact is that all materials need some sort of maintenance, even stone or bronze kept indoors. The effects of oxygen and moisture, the mere fact of people breathing out vast quantities of carbon dioxide, all these things effect change.

Some informal constructions are not treated with the respect they merit, nor handled sufficiently carefully. Because they look humorous or disposable, they are treated as such and run the risk of being damaged (unless they have Picasso as a name tag). Yet most of these objects, however fragile, should last hundreds of years in proper museum conditions. There are exceptions, though not included in this exhibition. Dieter Roth (born 1930), who uses substances such as chocolate or cheese in his sculptures, intends them to change organically with the passage of time. The process of dissolution is part of the sculpture's meaning.

Some objects are made to be transitory. Andy Goldsworthy, for instance, collaborates closely with nature, making sculptures from ice or leaves. These will disappear with time, the only trace remaining the photographic record made by the artist. For an increasing number of artists, documentation is of the utmost importance. Some sculptures or installations are variable according to the venue in which they are placed. Sometimes the artist allows the curator of an exhibition to make decisions. When "Objects of Desire" was shown in the Museum of Modern Art, New York, Mario Merz's sculpture Spiral Table looked completely different to its London incarnation. The key to this piece is the perishable component laid out on the table top. In New York, the decision was made to dress the table very sparingly with fruit and vegetables. In London, however, the Merz has become a contemporary cornucopia, or horn of plenty. The restaurant in the Festival Hall, The People's Palace, has agreed to donate fruit and veg, to be changed every Monday depending on the state of decay. The list of organic exhibits is long, from a dozen each of red cabbages, green cabbages and aubergines, to 5lb of courgettes, 13 acorn squash and 9 limes. How fresh this particular exhibit looks will depend on which day you visit it.

The table part is made of glass on an adjustable aluminium frame, so the breadth of the spiral varies. The centrepiece is a bunch of brushwood which has to be obtained locally wherever the sculpture is installed. So the exhibition organiser will be off to the florist's, or to a friend's garden, or to the nearest forest. With a piece like this, everything depends on the instructions left by the artist. Merz leaves the organic component to the discretion of the exhibition's organisers, but others are more specific.

Sarah Lucas made a sculpture consisting of two fried eggs and a kebab on a wooden kitchen table, a metaphor for the female body, perhaps even a self-portrait. This piece forms part of the Royal Academy's current "Sensation" exhibition, and has to be remade every other day by an Academy conservator in collaboration with a local kebab shop in Piccadilly.

Traditionally, the best artists have not always been the best craftsmen. Some painters make pictures that will last, in theory, forever. A senior contemporary painter such as Karl Weschke (born 1925) has deliberately used earth colours for most of his career because they make the most stable pigments. Others are more cavalier with their materials, or simply aren't interested in technique. It wouldn't have suited the macho wild-man image of Jackson Pollock to spend time in patient investigation of how paint behaves. As a consequence, the paint is constantly threatening to flake off his canvases. Paintings that contain other matter besides paint - such as sand or cinders - have additional problems. Certain paintings by the Spaniard Tapies can no longer travel for exhibitions, they are too fragile. With the texture strongly reminiscent of an old wall, they are too likely to crumble.

Even Leonardo, ever the experimenter, used unstable paints, which had a tendency to darken rapidly and deteriorate. And Sir Joshua Reynolds was so fond of adding bitumen to his canvases that many of his portraits have darkened into unrecognisability. Today, with the decline in the quality of teaching at art schools, many young artists are simply ignorant of techniques. One young artist using house paint was surprised to have to repaint virtually a whole exhibition when the paint flaked off the wrongly- prepared ground.

The Tate Gallery takes considerable pains over conservation and establishing with accuracy the materials used in a mixed media sculpture. If the artist is alive when the piece comes into the Tate collection, then he or she is obviously the most important source. When, in 1983, the Friends of the Tate purchased the Angel of Anarchy, a blindfold surrealist head by Eileen Agar (1899-91), it was to the artist that researchers applied for information. The Angel is a plaster cast swathed with various kinds of fabric, decorated with beads and feathers and given a diamante nose. From the artist, we now know that the feathers are ostrich and osprey, the silken drape Chinese, and that it is African tapa (or bark) cloth around the neck. The detailed list of materials would assist any restoration required should time take a noticeable toll on the object. There is, in fact, a rubber glove in among the head feathers, which has now perished. However, it appears to form part of the substructure rather than the visible decoration, so it does not need to be replaced.

It's interesting to note that many of what we now think of as the classic early Dada or Surrealist objects are in fact later re-makes of the lost original. Or, if you want to put it more practically if not commercially, few artists could resist making an edition. Certainly Duchamp couldn't. The original Bicycle Wheel of 1913 and the second version were both lost, but Duchamp remade the piece several times (1951, 1961 and 1964) before authorising an edition of eight signed and numbered replicas in 1964. But as recently as last year, the Bicycle Wheel in the collection of the Museum of Modern Art in New York (in fact, the actual exhibit on loan to the Hayward), was stolen and deliberately disassembled. Reduced to a mere metal wheel and a separate wooden stool, the magic had gone. It was presumably done by a disaffected artist making a statement about "modern" art, nearly a century after the fact. Of course, the work was recovered and re-assembled, and it looks fine today. But has it been altered by its experience? Has its charge or physical presence been diminished? That's almost beside the point; because it is a conceptual sculpture, the idea behind it is supreme, not its physical manifestation.

One of the most demanding exhibits in "Objects of Desire" is Wolfgang Laib's Milkstone. It consists of a large slab of marble with a slight dip in its top. For the duration of the exhibition, on each of its 88 days, this sculpture must be carefully "filled" with milk every morning and "emptied" at night. Laib has left detailed and specific instructions; the milk must be poured gently on to the centre of the stone and then delicately teased to the edges with glass rods. It should form a trembling meniscus but not flow over the edges or it might dry and harden into casein glue, and bond the marble block to the floor. No jewellery may be worn by the technicians who tend this piece as the slightest scratch would damage the marble. At night, when the crowds have departed, a supersponge is used to remove the milk, and the marble is carefully washed down with water.

One-Ball Total Equilibrium Tank by Jeff Koons looks simple: a basketball in a fish tank on legs. In fact, it will take two days to install. The ball, which is filled with mercury, floats in the middle of the tank on a bed of distilled water saturated with reagent grade sodium chloride. In other words, it's like floating on the Dead Sea. It's left to settle overnight, and then, the next day, the tricky process begins of filling the top half of the tank with pure distilled water. Koons provides two pages of instructions for the chemistry lesson, but if the ball sinks, the process must begin again. If all goes well, the ball should remain suspended for between four and five months before slowly sinking. Like most of these artworks, it is broken down into its component parts to be stored.

Legend has it that the packing room of Sotheby's once received a wrapped sculpture by Christo to prepare for departmental cataloguing and eventual auction. The unpackers on duty duly began to dismantle the parcel, which seemed to them a well-wrapped bouquet of flowers. The paper continued to unfold until there was nothing there at all. Realising their mistake rather late in the day, professional restorers had to be summoned immediately, and the piece speedily remade. In this case the packing was the sculpture. I trust it reached its reserve