This is the least elegant position I've ever adopted at a private view, as far as I can remember - my cheek brushing the dust, my rear in the air - but I can see Maclean's micro-sculpture now: a tiny piece of resin in which a flailing aluminium figure isfrozen head downwards. "You're only the third person to see it," he grins, helping me to my feet and hurrying me away as a security guard returns.
The Tate is not alone in unwittingly housing the unknown Maclean's work. The Saatchis have a micro-sculpture, so has the Metropolitan in New York. There is one in the Pompidou Centre in Paris and Tokyo's Spiral Building has two. Berlin, Los Angeles and Amsterdam are among the other oblivious hosts whose exclusive cultural parties Maclean gatecrashed last May in a whirlwind 22-day trip around the world.
"I call it `involuntary acquisition'," the 33 year-old Glaswegian explains. "I got the idea in 1989 after painting for several years and getting nowhere. I didn't go to art school, which is how some people get picked up on by dealers and critics, but pe o ple locally liked my stuff. I started to take slides of my paintings down to commercial galleries in London, but no one would even bother to look at them because they'd never heard of me. I used to think, if I'd exhibited in world-famous public galleries , you'd all be dying to look at my work and people would pay thousands of pounds for it. So I decided to steal the space I needed. I wouldn't do any grovelling at all - I'd just go into the biggest galleries in the world, place the work and walk out."
Maclean is part of a growing band of cultural terrorists who mount regular challenges to the art establishment. The K Foundation turned the 1993 Turner Prize announcement on its head by simultaneously awarding the winning artist Rachel Whiteread a £40,000 award for the worst entry. Another group called FAT (Fashion, Art, Taste) created confusion at the last Royal Academy summer show by placing red dots beside every painting.
With no group backing or sponsorship, Maclean financed his odyssey himself. He worked for a year teaching English in Tokyo to save the £5,000 he needed to place an uninvited exhibit in each of 13 selected "metropolitan cultural cathedrals". The sculptures - "tiny people I made without thought from aluminium chocolate wrappers while I was doing something else" - were set in resin and packed in a suitcase with three shirts and a toothbrush. Maclean bought two round-the-world tickets and set off with a collaborator (a member of the self-styled "Faculty of Transient Identity") who filmed each "opening" with a Super-8 camera.
Maclean maintains that it doesn't matter whether the sculptures are visible. "I had to hide them because otherwise they'd be taken away," he states with perfect logic. "My sculpture at the Saatchi Gallery has actually been painted over with grey emulsion, but I can still say I have work there. People may see it unconsciously while strolling.
"I don't think going to a gallery is very different for most people to watching television or buying a newspaper - they hardly notice it," Maclean muses as we pass a young woman checking her reflection in the glass over a Picasso. "I asked some kids in LA why they were in the Museum of Contemporary Art and they just said, `Oh, where did you get your clothes?'."
Maclean takes a childlike delight in carrying out each operation unnoticed. "I check the security in each space thoroughly before I place the micro-sculpture," he recalls. "The guards are always walking around so you need split-second timing. In the Mus e um of Modern Art in New York I had a tricky moment when I accidentally superglued my finger to a window frame along with the sculpture. When the guard came in, I leant against the window trying to look casual and then wrenched myself free."
In Berlin's Daad Gallery, Maclean came across an exhibition consisting entirely of potted plants lying around the floor. "It was an `installation' by Damien Hirst," he says, laughing somewhat bitterly. "That's the epitome of the art market: interest is generated in names so that they can sell work for money. There's no technique, no meaning, nothing creative about those plants or a pile of bricks in the Tate. ... People sometimes say to me: `Maybe you don't get exhibited because your work isn't very good.' I don't mind because I just think of Damien Hirst's plants."
On his return to London, each gallery was sent a fax that read: "Jack Maclean is now exhibiting a micro-sculpture in Violation of your Art Space." He considers the project finished now that it has been "exhibited" and has already started on new work, yethe is not planning to give away the exact locations. "I like the idea of people looking for the micro-sculptures. ... It might make galleries more interesting."
While Maclean says his main purpose is to "make people think", he admits he would quite like to be famous himself: "People would give me about £15,000 for each sculpture, which would fund new projects; I could then become unknown again by changing my name to Neon Glassbender or gaining weight or starting to wear flares," he suggests, with an air of artlessness of which Andy Warhol would have been proud.
Some time after Maclean has taken the plane back to Tokyo, where he has decided to live, I speak to Simon Wilson from the Tate Gallery. Ironically perhaps, he is complimentary, if academic, about Maclean's "violations".
"He is challenging the acquisition practices of the world's museums," says Wilson. "The challenge is the point and the whole project is a work of art carried out with determination. Fabulous! I take my hat off to him."
Wilson's attitude towards the Tate's very own micro-sculpture, however, is rather less accommodating. "If we knew where it was, we would take it down and return it to the artist," he says. "It would be deeply unfair to other artists to accept this sculpture just because Maclean wants us to have it. He has not gone through the proper procedures. Are you going to tell us where it is?"