Art for art's sake has always been the philosophy of the well-fed. To avoid the Cork Street dole queues professional artists need to make a living out of their labours - even if they are constructing art out of perishables such as strawberries, sow's intestines, butterfly pupae and stir-fried Thai food that is going to rot away and leave collectors with nothing more than an acrid, vinegary smell. With the renaissance in the use of organic raw materials, by artists such as Helen Chadwick, Rirkrit Tiravanija and Anya Gallacio, the whole notion of vita brevis, ars longa has been turned on its head. The artists' notion may be to evade commercialism through dematerialisation, but the world of dealers and gallerists is learning new habits - and relearning some old ones - as they ditch the notion of art as eternal.

We are talking the heart and soul of conceptual discourse here. In 1991, Rirkrit Tiravanija flew Untitled (Artificial Flavoring), two suitcases of bacon yoghurt flavoured crisps, to a gallery in Warsaw and invited his audience to eat. In the words of this month's Frieze magazine, "By exposing the notion of American consumption for a few moments in the form of a brief public snack in Poland, the artist is also subtly addressing the inequalities that exist between the two economic systems". In Untitled (View), installed at the ICA in 1993, he cooked and served curry to the assembled crowds without so much as a cover charge. Subversive as the idea might have been, collapsing the distinction between art and life, Tiravanija had to pay the mortgage, and sold vacuum-packed originals for his public to take away, or warm through in the comfort of their own homes.

Helen Chadwick, shortlisted for the Turner Prize in 1987, is best known for making art out of the organic. Using everything from cervical smears to entrails, her work deals predominantly with images of the human body. For one installation she loaded an old font in Spitalfields market with pounded strawberries. But her reputation is based on meat. "When I was looking for material to work with, I just got out the Yellow Pages. Sheep intestines were too small - I'd had them out in Greece in Easter Soup - and I couldn't get cow's intestines because of BSE, so I went to an abattoir in Essex to get sow's intestines. I got on the bus in Bethnal Green, and sat next to this guy I know from The Late Show. There was this slight smell coming from my binbag. It was marvellous not to let on, chatting about his latest project, with a mile of intestines between my knees."

The tabloids lap it up. Hacks can dust off the piece they wrote once about Carl Andre's bricks. The artists still think, by making art out of everyday substances, they are talking a language that people will want to understand. But temporary conceptual art seems to manage, more than any other, to alienate a public fed its fine art by Athena. Anya Gallacio managed to incense the more painterly critics recently with Aurora, a swimming pool filled with oranges. "I know it sounds ridiculous, but you had to be there. The smell was intense, the pool was this incredible blue and one of the walls had mirrors down it so the reflection became like a moving painting. But the tabloid press encouraged cynicism from the general public because they think I'm getting paid loads of money swanning around the world throwing fruit about. It's just not true."

Temporary art is not a new phenomenon. Dieter Rot was working with chocolate in the 1960s, while Warhol was busy painting with piss. Piero Manzoni made a fair living out of his own excrement, selling it - ounce for ounce - at the same price as gold. Billy Apple made installations from his nose wipes. And land artists, out in the field, were busy treating the gallery as one site - and the world as another. Their work was designed to be part of nature, not ripped out of its environment and put against a white background or hoist up on a plinth. Now a new generation of dealers and gallerists is having to relearn old tricks to sell on today's temporary art. That means everything from residuals and spin-offs to detailed certificates allowing collectors to recreate the work to an artist's specifications.

Gallacio's Stroke is an installation made out of chocolate that discolours and crumbles with the passage of time. The certificate which accompanies the work details the height of the chocolate, techniques for applying it to a wall, and even the brand name of chocolate used (with the concentration of cocoa solids, just in case our buyer finds himself abroad and in need of a local substitute). The certificate for In And Out Of Love, an installation by Damien Hirst, is equally detailed. Butterflies hatch out of pupae stuck onto white canvases, hung above a construction of steel shelves and potted plants. "Damien has to change the plants every two or three days if the piece is to be remade" says Jay Jopling, Hirst's dealer. "The canvases have to be restretched, the pupae attached, and whole exhibition presented again. Those are the conditions."

Temporary art is a growing trend. And probably as reliable a monitor of the recession as any basket of European currencies. "Because the bottom dropped out of the market in terms of buying works of art at the end of the 1980s, the idea of making something that would last forever seemed less crucial" says Martin McGeown, of Cabinet Gallery in Brixton. "Artists went back to making things that were far more fragile and ephemeral. But it wasn't just about materials that artists started using. It was also that there were fewer galleries, so you were going off and doing things for yourself in temporary locations. And so the work was of a temporary nature. The fact was there was no-one around who wanted them anyway, so selling the work really wasn't important."

Making unpurchasable art might have begun as a very political statement, but the establishment has now embraced its radicalism, and the collectors are starting to cotton on. According to Richard Salmon, a dealer in 20th century art, what began as a reaction of today's artists to the over-inflated prices fetched by artists of earlier generations is, ironically, starting to fetch high prices of its own. "Initially it might have been a statement on the part of contemporary artists about not wishing to be involved in the market. They may have wished to go out of their way to create something that wasn't permanent. But if you look at the whole process of the history of art, that became a very interesting thing in its own right, and therefore very desirable to a collector. Even in the form of a certificate."

For the moment, the market in temporary art is uncertain. As Karsten Schubert, Anya Gallacio's dealer says, "It's hard to tell because you are talking about work from very young artists which will automatically be less expensive than more established people. A work's increase in value has nothing to do with it being a certificate or not - it's to do with how that artist's work is perceived at some later stage. On the whole, there are still less people around prepared to take that step. That's easy enough to understand because it's intellectually quite a brave thing to do. But the people prepared to take that step make up for their lack of number by being incredibly committed."

While the average art lover wants something to fill a space on the wall, collectors in temporary art are happy just to trade in paper. They are a different breed who feel they are collaborating with the artist, in the role of patron rather than purchaser. They don't necessarily feel the need to possess something in the same way. "They are paying to participate in the work of a serious contemporary artist" says Salmon. "And they are paying to be part of the history of art. Anyway, a certificate allowing you to recreate temporary art over and again is more eternal than anything solid. From that moment on the solid work is going to deteriorate. Whereas if you wait 300 years and then cover your walls with chocolate, and display a certificate from Anya Gallacio, you have got a completely fresh work of art."

One of the buyers of temporary art prepared to identify himself is collector Duncan Cargill, who bought the right to recreate Gallacio's Flowers Behind Glass, retailing at between £2,000 and £3,000. Every time he wants to instal the work, Cargill has to press fresh flowers between a 1m square of glass and his wall. He has not only got a work of art, he's bought himself a new hobby. "It does take a bit of care and attention, yes, but so does every piece of art. You can't forget about a Rembrandt. Anya's work needs me to replace the flowers, but it's a privilege to keep going back and doing it. It would be different if I never installed it, and just had a certificate in a cupboard. But I want the responsibility of recreating it myself and looking after it."

For Gallacio, that's part of the motivation of her work. "A buyer has to decide when to have my works installed. You can't just get them out of the crate and put them on the wall. It requires effort. You have to make a decision. You have to be active. You can still buy my work as an investment, but you can't show it off without first doing something. The story goes that one very well known buyer would order a work to be sent round to her house, have a dinner party, and then send it back because it was the wrong size. She never paid for anything. It was a standing joke. I resent that abuse of power. If someone wants to do that to me it's going to cost them - they have to buy the glass, buy the flowers and then spend three or four hours installing it."

Forgers have long since abandoned art for the bank note. As Schubert says, "With this kind of work, there isn't an original in the first place. At the gallery, that was one appearance of Anya's flowers piece. It's like a Shakespeare play - there is no original Hamlet, there are just performances." Given that Flowers Behind Glass bears no artist's signature, and is easy enough to re-install, had Cargill not thought of ripping off the idea, popping down the glazier, and setting to on a bit of DIY one Bank Holiday Monday? "What good is a Picasso on your wall if you've ripped it straight off?" he says. "You can't pretend that Picasso made it. Anya created the flowers piece, and I really wanted it. I didn't think of it. In the same way that you can make hundreds of drawings of a Picasso, but you didn't think of the original image so they aren't your images at all. They would mean nothing to me."

Conceptual art remains an act of faith on the part of the dealers. After all, the gallery's electricity bill and council tax are down to them. So many undertake a delicate balancing act with the art that they show, where traditional underwrites the more conceptual. "There's an enormous amount of goodwill going on between gallerists, dealers and artists" says Salmon. "Being an art dealer in London in 1994 is an exceedingly expensive occupation because you spend your time ordering 40 tonnes of salt for somebody who puts it on a beach and have it knocked over by a wave. The upshot of that might be some very charming black and white photographs of the event taking place, but no-one really knows what is going to turn out from the process."

McGeown is currently working with a group of artists who have been making crop circles in Wiltshire. Not only do they wish to remain anonymous, but they refuse to say which circles they have been a-making. Their work has been subjected to analysis by UFOlogists and folklore researchers, but it's also subject to rain and, perish the thought, harvest. "Exhibiting goes totally against the grain, if you'll excuse the pun," says McGeown, "when the work is about being temporary. So we're looking at how somebody could add it to their collection. Maybe removing a segment of earth and having it freeze dried, but that seems very literal. So far it will just be documentation, rather than the object itself."

Artists can choose to make the lives of their dealers a bit easier. When Damien Hirst first conceived of A Thousand Years (twin glass cabinets, one containing maggots, the other a putrefying cow's head and a neon insectocutor), he didn't consider the sale of the work. But once he had considered the practicalities, he went to Pinewood and had the head modelled out of latex. It would still get smeared with rotting matter, and provide a safe haven for those maggots, but a fake head would mean the piece required less maintenance. "The original sculpture had to be changed every 72 hours" says Hirst. "The show was on for six months, and I was moving around so I simply couldn't have done it. But it made sense to me at the time - I see art as theatre anyway."

Most temporary artists now produce signed residuals - gestures, like momentos of larger works, that people can take home. Gallacio has mixed paint-pots full of chocolate and vegetable fat (after Stroke), and pastel crayons from the dust of Red On Green. "The work was 10,000 red roses, and when they dried on the floor it seemed to me to be the natural progression to force them to dust" says Gallicio. "The roses were really exquisite when they had dried, but I destroyed that by putting them in a blender. It was something about not being sentimental. It was quite a violent thing to do to them. The dust was the end of one thing, but the beginning of another. I've got to survive, and residuals help me make a living, but they are separate pieces that hopefully don't compromise my work."

Temporary artists such as Andy Goldsworthy took to photographing their work, producing beautiful, permanent, cibachrome images to sell and hang on the wall. But Gallacio and Chadwick have both refused. "For me, my work is about experiencing the real thing" says Gallacio. "It's a reaction against looking at things second hand through film and tv. It looks spectacular in photographs, but often there's a smell or a sound that you can't capture." For Chadwick, certificates and photographs are too displaced a form of ownership. "I'm always attracted to a more dynamic relationship between a viewer and an artefact. Nothing else has a physical entity that to me is real or exciting. It's crucial there is that physical encounter. That's why a framed photograph of an installation feels so unsatisfactory."

If we can get over our bourgeois fury at an art that seems so devoid of craft, and classical tradition, this stuff can have merit. We have to accept that this isn't art about art - like Athena's Great Masters range - but art that's about the washing-up. Or breast cancer. Or swimming in Brighton. The idea is everything, but then the same can be said of most twentieth-century paintings. They are actually canvas, paint and wood, so what you are buying is a notion of painting. Love it, hate it, or eat it, temporary art is at least attempting to give a flavour of what day-to- day life is really like in Britain in the 1990s. Whether some of the artists can escape the embrace of the fashionable, and become as timeless as the work of Andy Warhol, only time will tell.