Anish Kapoor's studio, a sprawling collection of warehouses in a quiet street in south London, is both artistic crucible and thriving workshop, the kind of place where charts stuck to the walls indicate what kind of hot beverages are preferred by the employees (should Kapoor ever pop round, offer him black coffee, not tea). Twenty-five people labour here, either coolly in the fresh white offices above, or in the sweat of masks and overalls below. As we meet, Kapoor has just designed two stage productions: Pelléas et Mélisande for La Monnaie in Brussels, and a dance-theatre piece called in-i for Akram Khan and Juliette Binoche at the National Theatre in London. He is also preparing to open an exhibition at the Royal Institute of British Architects in London; and he has a clutch of projects, ideas and commissions simmering away - not least Britain's largest piece of public sculpture, for the Tees valley, and a retrospective at the Royal Academy of Arts next year.
Kapoor stands alone in the British art world: he is a touch younger than the so-called New British Sculptors such as Richard Deacon and Richard Wentworth, though he has often been spoken of in the same breath since he is represented by the Lisson, the gallery indelibly associated with that group of artists. And, when he won the Turner prize in 1991, aged 37, he was the senior artist on the shortlist, a decade or so older than the other contenders, who included Rachel Whiteread - this was a new generation of Young British Artists snapping at his heels. But he also stands apart because his work is entirely sui generis: strongly voiced, unmistakably his.
Kapoor conducts a vigorous tour of his studio. The first thing he shows me is an enormous, deep resin tube, on which one of his masked accomplices is working amid the noise and dust of the workshop. It looks like an enormous semi-erect penis, I cannot help observing. "But it isn't!" Kapoor laughs . "It's the opposite. I'm interested in the opposite, and in the not-opposite. It's both. It will have a very, very dark interior. The idea is that a person will be able to walk in ... I have always been interested in antiphallic form - the opposite of which, of course, is deeply phallic. Hahahaha! It's not onwards and upwards ..."
It is inwards and downwards: Kapoor is a psychic tunneller and excavator. Seeing his sculptures en masse in the studio, it makes one almost queasy sensing how many of them are concerned with feminine holes, clefts, entrances, slashes - often sculptures in a deep, primal red that screams of female human flesh, menstrual blood, the blood of childbirth. But Kapoor has never been interested in anything other than abstraction. The exploration of the feminine bypasses a lurid fascination with the specifics of the female form; it is more, perhaps, to do with an investigation of the dark places of the imagination. Not for him the world of art as commodity; not for him irony. He says: "Donald Judd used to say that art doesn't get made, it happens. The implication of that is that the studio is a place of a certain kind of practice. And there things occur that hopefully have deep quotidian recall - but are not directed by the quotidian world. So the post-Warholian notion that everything in the world is all art - it's fine, but what it avoids is the truly poetic, or the poetic of a slightly different order. And it's that order I am interested in."
Figures such as Damien Hirst represent one branch of post-Duchampian art; Kapoor, however, takes on that history in a rather different way. "If Duchamp declared that all the objects in the world are art," he says, "then I am interested in the next stage of that argument, which may have been prompted by Beuys in some way - that all the objects in the world are symbolic. Now Duchamp, to be fair, was very careful about what was the found object; the found object was always deeply symbolic. So the arguments in fact come together and they don't get confused by the idea that you can put anything in a glass case and it's art. It isn't. It is the artist's duty to find poetic meaning in things." More baldly, he declares of the Hirsts on sale recently at Sotheby's: "It's just stuff , you know. It's not an artistic challenge. it's just stuff ... It's completely irrelevant." Later he adds: "It's almost not art. I'm going to go as far as to say it's not art."
From the Hephaistian busyness of the artistic forge, Kapoor now ushers me into another large warehouse, silent and empty but for a number of tables covered with maquettes and models for large-scale sculpture. This is normally Kapoor's private space, the thinking place. The models are laid out in preparation for display at the Riba headquarters in London. "All these projects are about a certain kind of architecture," he says. "Many of them are thoughts about a certain kind of almost religious space. This, for instance, is a very crude model of a piece made in a museum in Japan - a void in the floor - called L'Origine du Monde, for obvious reasons." He laughs. This, he says moving on to a model of a silvery bridge that resembles an elongated bead of mercury, "is a bridge that we've been working on for years and years and years. Whether it happens or not is another matter. It's a kissing bridge. It opens in two halves, and both parts open and slide across the channel." Where is it? "I'm not allowed to tell you. I'm not going to tell you because it's still a bloody confidential pain in the backside, but anyway. It's quite a heavy shipping route in the UK."
He's off again, to another model. This seems to be pure fantasy: it is an enormous circular hole in a valley in a mountainous landscape, a deep void leading to nothing and nowhere. "It's massive. It's kilometres across - and completely dark," he says. "One of the things that has emerged out of my work over all these years is this idea of the non-object, the absent object, the immaterial part of the material."
Kapoor is very interested in negative space, in spaces filled with a nothingness that is, paradoxically, deeply present. He recalls: "I made a work at Documenta [the five-yearly sculpture exhibition at Kassel, Germany] many years ago. You walked inside a building like a bunker, and inside there was a hole in the ground. It was completely dark - so dark that the hole resembled a carpet on the ground. One person was let in at a time. And there was a man who waited for 45 minutes, and when he went in he was absolutely furious. 'I've done many things in the cause of contemporary art,' he said, 'but I have never stood in line for all that time to look at a piece of carpet.' And he took his glasses off and flung them on to the carpet - and, of course, they disappeared down the hole. And then he was truly terrified. That's what I am interested in: the void, the moment when it isn't a hole, it is a space full of what isn't there."
Kapoor has also investigated this notion by way of his mirror pieces - a large group of sculptures of varying scale that include concave, circular wall-mounted mirrors several feet in diameter, and the huge Sky Mirror that was mounted near the Rockefeller Centre in New York in 2006. His Cloud Gate, a 110-tonne sculpture with a reflective surface for the Millennium Park in Chicago, also perhaps falls into this category, though it is a three dimensional piece, shaped somewhat like a kidney bean. He talks about these objects in terms of painting: the effect, he explains, of a traditional painted surface is to draw the viewer into a space that apparently recedes beyond the picture plane. In the mirror pieces, by contrast, "the space doesn't recede - it comes out at you ... a new sublime that's forward of the picture plane."
We are now in a quiet warehouse, where various sculptures sit awaiting their fate and the scrutiny of their creator. One mirror has a surface made up of a tiled pattern of squares and hexagons. Looking into it, parts of your reflection seem to dance in the space between you and the mirror; others seem to float back in the distance. Kapoor says: "I'm interested in the almost idiotic phenomenology of this. On one level you might say it's not art, it's a silly game. But I think there's something in that little edge which is interesting and problematic. There is something going between the meaningful and the banal."
Anish Kapoor was born in 1954 in Mumbai, his mother an Iraqi-Jewish daughter of a rabbi, his father a hydrographer in the navy. He went to the prestigious Doon School, in Dehra Dun. "The good thing about education in India in the context I knew it was that it was really cosmopolitan. We learned just as much about Jahangir and Akbar as we did about Louis XIV and Elizabeth I." His parents were "modern, wonderfully modern"; and for Kapoor, there was a deep sense - as a Jewish boy with a white mother growing up in a household speaking English in India - of being an outsider.
Kapoor's parents were keen for both their sons to see something of the world. "In those days, a plane ticket cost more than my father's salary for a year. So we were both encouraged to emigrate to Israel, because they paid for the plane ticket." It was there that "being an artist became not only an option, but something I could actively do something about".
He moved to London, where he studied at Hornsey College of Art and at Chelsea School of Art, as it was then known. College was "a total liberation" - partly from a state of deep psychological disturbance that afflicted him in his late teens. "I was seriously fucked up, full of inner conflict that I didn't know how to resolve." To tackle this turmoil, he eventually went into psychoanalyis, which lasted for 15 years and ended just before he met his wife Susanne (they now have two children, and have just built a new family home in Chelsea).
Art college was a "kind of respite" from his psychic turbulence. He recalls his student work, much of which was performance-based. "The pieces were all very symbolic and they normally involved interaction between two people. They were non-narrative, but there was a process; and it was normally left to the two people to enact the piece with props. The props were really important - quite sexual. All the language was there already, dammit!"
He left art school in 1977. At that time, Kapoor reckons there were perhaps 10 artists in Britain who were able to make a living just from selling their art: he assumed he would find some kind of teaching post. He rented a studio in Wapping, and for a while scraped money together by making furniture for the society decorator Nicky Haslam.
"Then," he says, "in early 1979 I went to India . . . and I suddenly realised all these things I had been making at art school and in my studio had a relationship to what I saw in India." And this relationship? "It was a certain attitude to the object. I was making objects that were about doing, about ritual. It was that 'doingness', that almost religious doing, that I saw everywhere ... It felt like a huge affirmation."
It was from this idea of ritual that his first significant works sprang: his pigment pieces — bright shapes set on the ground covered in pure pigment. "I would almost ritually lay on the pigment; they are very much performed," he says. "I made them all in three years. It was an incredible time of discovering something new every single day. I really didn't know where they came from; I felt that they had tumbled out into the world."
This feeling of unforced creation still happens, once in a while, for Kapoor. He shows me something he has been working on over the past couple of days: a model for a huge installation for the Grand Palais in Paris for 2011. What eventually gets seen in that vast space may bear no relation to what he shows me, he says, but what has come to him is a scenario in which the glass roof of the space is covered over with red gel, JCBs busily tunnel into the ground, and a large inflatable sphere, 25 metres in diameter, hovers over proceedings. There is something hellish about it.
"It's got a sense of being an excavation of the interior. I would never have made a model like this 10 years ago. I would never have allowed this kind of apocalyptic moment." The mess and the chaos of it have an imaginative relationship with the recent works he has been making out of gunky, visceral, blood-coloured Vaseline and wax, such as My Red Homeland (2003), 25 tonnes of the stuff in a circular container constantly formed and unformed by a large steel arm. "After years and years of looking for a kind of wholeness in my practice, I find myself over the past couple of years dealing with tragedy and anxiety - with things that are fragmented," he says.
The sculpture for which Kapoor is most famed is Marsyas, the trumpet shaped structure, like a flayed skin stretched over a framework, that occupied Tate Modern's Turbine Hall in 2002. It was seen in some quarters as a triumph of size over substance. "Every idea has its scale," he says. "Marsyas wouldn't be what it is if it were a third of the scale. The pyramids are the size they are because they are. Scale is a tool, a tool of sculpture."
While vast, Marsyas will seem small in comparison to Temenos, the first of a five-part installation known as the Tees Valley Giants, which will be the largest public-art initiative ever, the design for which was unveiled in July. Has public art become a clichéd response to the urge to regenerate post-industrial cities? "I think we've gone totally public sculpture mad," he says with vehemence. "I hate public sculpture." So why are you doing it? "It's really a problem, I've got to say it's really a problem. Public sculpture ... oh God, even the phrase makes me feel tired. Why I am engaged in it? Well, I think, as a sculptor, that is something of one's lot. Because scale is a tool of scultpure, and it needs to be worked with."
He shows me the model for Temenos, which will be 100 m long. It is a tube of nylon cut from a pair of tights stretched between two rings, one of them propped on a pole - blissfully simple. Temenos is the Greek word for a sanctuary, a place set apart; and Marsyas is also a reference to ancient Greece: he was a man flayed for daring to challenge Apollo to a musical contest. "It is a myth of conceit, the conceit of art," Kapoor laughs. "The conceit of the artist!"
Kapoor on Kapoor
There had been a long tradition in sculpture that said that materials have to be what they appear to be — this thing of truth to materials. I couldn't deal with that. Even as a student I didn't know what that meant. It seemed to me that art's all about illusion and the unreal. "Truth to materials" ran, and runs, contrary to everything I want to do. Quickly I realised that when you make an object and place pigment on it, the pigment falls to the ground like a halo around the object. And the implication is that it's like an iceberg: that most of the object is hidden, is invisible.
And so I became more and more interested in the invisible object. There was part of it that protruded into the world but the rest was really interesting. If you look at the pigment pieces, nearly everything else I have done is set there, and I keep going back to them. Sometimes I long for that kind of ebullient outpouring. That year I started making them was unbelievable. I didn't know where they were coming from. I didn't think them up, they popped into my head.