When I spoke to Whiteread at her studio on Friday, she remained cryptic about the actual form of the sculpture. "I think it will make people see the building in a completely different way," she says. "I hope it will make them look up and see something very much contained within a space. And there will be an amazing sense of light, which will be nice because it's a winter show." Asked to explain further, she says firmly: "You'll have to wait and see."
For anybody who knows Whiteread's past work, there's a certain amount that can be guessed at. She is most famous for casting the entire insides of a terraced house in London's East End and then demolishing the building to leave only its concrete ghost. The project won the Turner Prize in 1993 and established her as one of Britain's most prominent young artists. In her casts of baths, hot-water bottles and the undersides of chairs, she has since exposed many other hidden and overlooked spaces, including the interior of her own home.
"Before we moved in, I made a lot of new work here," she says of the converted east London synagogue she shares with her sculptor partner, Marcus Taylor. "It's a way of getting in touch with the building. It's a very thorough way of investigation - perhaps a bit too thorough, really."
Will the Turbine Hall project be a continuation of this exploration of the guts of things? Certainly Whiteread, now 42, has been rummaging around at Tate Modern since she accepted the commission nearly two years ago. So it seems likely that the sculpture could be site-specific, perhaps revealing some hitherto secret aspect of the building.
For several months, she says, she deliberated about whether to accept the commission. Did she have a moment of revelation? "It started with a thought," she says, "and then turned into a paragraph."
Something we can expect from this project, though, is that it will be fun. In contrast to the sombre Holocaust Memorial in Vienna, or Untitled Monument, her fourth-plinth project in Trafalgar Square, her new piece, says Whiteread, is a response to the "sense of play" generated by the Turbine Hall's atmosphere.
"It's much more like Covent Garden, more performance-based than the National Gallery," she enthuses. Whiteread also expects a high level of interactivity from the public. "You will be part of it. It's very difficult to make a work on that scale where you don't involve the audience. Touching will not be encouraged, but it's going to be very difficult to stop people, I think."
There is also a possibility, with this piece, that Whiteread will work in a material we haven't seen her use before. Always keen to explore a broad palette, she "started off as a painter in Brighton, but by the third year of my degree I was making things in cast aluminium, wax and papier mâché. I was always very interested in the touch of things and the making of things."
Whiteread says the material for the Tate piece has been formed using an industrial process, but she also reveals that it's "100 per cent recyclable" and will be dismantled rather than sold when the show finishes next year.
After the opening, the hard work continues with the installation of an exhibition of her new work at the Gagosian Gallery in King's Cross. "I'm hoping that people will see the show there too, because it will inform the work at Tate," she says. "The two things complement each other" How will she celebrate when it's all over? "I'm going on holiday for a week," she says.
The Unilever Project by Rachel Whiteread, Tate Modern, London SE1 (020 7887 8008), from 11 October to 2 April 2006. Rachel Whiteread, Sculpture, Gagosian Gallery, London WC1 (020 7841 9960), from 19 October to 3 December
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