A love-Tate relationship

There was palpable nervousness inside the new Tate Modern in London when its architects, Pierre de Meuron and Jacques Herzog, took us on a brisk tour to preview the building. Were those gigantic chains lying coiled on the cement floor left over from building works - or were they a priceless work of art? Belly up, bundled onto a trailer awaiting a crane lift to the entrance, the latest sculpture by 87-year-old American Louise Bourgeois could have fooled even the most ardent art lover. The director of the Tate, Sir Nicholas Serota, was quite annoyed when I asked him to confirm that it really was the special commission by Unilever for the Tate Modern.

From the outside, London's familiar landmark looks pretty much the same. It has been stripped of its Art Deco fins, and topped with a two-storey glass box like a lantern to beam light into the cavernous building, but the architects kept the facade. They didn't have to be so respectful - the building was never listed. Inside is a whole new building that is equally respectful to the artists as it is to the public, but without being neutral. "Not a conversion, or a heritage preservation, but a new building," as Sir Nicholas says.

The west-side main entrance dips quite steeply down a ramp through the monumental portals. Think of the temple at Luxor and you get the shift in scale - as well as light - that accompanies a Harrison Ford descent into unknown territory. The same buzz, too. Once inside, the building opens out into a truly awesome space with a big vertical thrust: a glass ceiling soars 115 feet overhead and runs the length of the Turbine Hall, 500 foot long and 99 feet wide. The Tate Modern offers one and a half times the display area of the Tate Britain at Millbank. Proportions like this would have dwarfed even monumental sculptures like Richard Serra's industrial castings, without skilful intervention by the architects: opaque back-lit glass-clad balconies - or viewing platforms - abseil like window-cleaners' boxes up the towering gantries to delineate gallery floor levels and take the eye horizontally across the building, which immediately becomes comprehensible. You can see at a glance the gallery floors.

Stitching on a new steel structure allows the architects to add bridges and balconies, a new system of floors and walls, and to vary the ceiling-to-floor heights in galleries. Seldom have artists and curators had so much freedom. Peter Wilson, the Tate director who oversaw the architectural works, says, "The building has to be subtle to be great for art." The net construction figure for the Tate Modern was £134m. Minus architects' fees it was £70m.

De Meuron and Herzog reflect a new way of combining minimalism and old-fashioned modernism without being retro. They take the best of the purity of minimalism, and add the warmth of surfaces, textures and materials as robust as English oak and concrete floors rubbed to a silvery patina. Unlike most gallery architects, they are unafraid of colour. Deep in the building's core, the 260-seater auditorium - done out in what Jacques Herzog calls "Swiss flag red" - has crimson woven fabric on the walls, red seating and carpeting. Even the glass screens are screen-printed red with the weave pattern of the wall-covering, to keep the seamless look. "What appears extremely simple needed a lot of thought and work. How wide, deep, long? Where an active pattern? Which colour?" says Herzog. Just as the Inuit supposedly have 50 words for snow, so Herzog and de Meuron have hundreds of numbers to codify different shades of white from ivory to magnolia.

Their spatial manipulation with light allows the curator control. It was a trainshed in Basel, Switzerland, that convinced Nicholas Serota and his judges that Hertzog and de Meuron should be the architects of the new Tate. Overhead light wells bathe in light the confluence of all the railway lines which run through the shed. Nearby is the signal box, a windowless tower to exclude light so that computer and radar operators can read their screens. Clad in copper angled like gills, it seems to breath. "A sculptural object all of its own," enthused the art critic David Sylvester.

At the Tate Modern, daylight from the banked clerestory windows and the glazed two-storey box on top is reinforced at night by light boxes in every gallery ceiling with compact fluorescent in four shades to mimic daylight and allow changes in the lighting levels. So Rothko's six abstracts saturated in colour smoulder in dim cathedral-like recesses against walls painted in "Rothko grey" as the architects call it, while Joseph Beuys' gigantic Ich Bein steps out within a double-height white gallery bathed in natural light from the clerestory windows.

"Topography" is how the architects describe the landscape they have created, divided into "climactic zones" that Nicholas Serota says "suggest pools within the gallery of different periods and different artists who share related concerns". On three gallery levels, different artists explore different genres of art established by the French Academy in the 17th century, namely Landscape, Still Life, the Nude and History. So in "Landscape, Matter and the Environment" Richard Long's Rain, pelting down from a kinetic spiralling sky with huge splattering drops of rain in black and white, confronts Monet's reflective waterlilies on the other side. "Look for the conjunctions and analogies," Nicholas Serota says. "You make the connections."

In Experience or Interpretation: the dilemma of museums of modern art (Thames & Hudson) Serota argues that art galleries in the 21st century should become a site of experience in which the mind is engaged as much as the eye. Forget the curatorial-archival-encyclopedic art tour of a century ago, he urges, when galleries reflected the curator's view of history at the expense of differing viewpoints.

In the "History, Memory, Society" section, the pure abstraction of Carl Andre's sculptural metallic chessboard neatly laid upon the floor below Sol LeWitt's wall painting in a pastel grid, create an almost hypnotic calm. Juxtapositions and jump offs between artists like this makes the gallery provocative. All the isms are here - Fauvism, Cubism, Surrealism, Abstract Expressionism, Minimalism and Pop art with Conceptual art, but as Serota says about the New Tate, "If you want to follow art movements chronologically get an art book." If you want to be moved by art, visit the new Tate.

Just as compromise is a word that artists don't accept, neither will architects. De Meuron and Herzog prefer the word "dialogue" to describe how they got their finicky precision off the ground. Not formulaic, nor formatted and too elegant to be merely fashionable, their interest in boundaries - the points where walls and floors and ceilings meet - meant they insisted on spirit-level perfection, with neither a skirting board nor a shadow gap (the delineation in plaster that other architects rely upon to cover up imperfections). They insisted upon doors no taller than 2.5 metres (with removable panels which address the need to move larger artworks through them), no barriers between artworks and the public.

In a world of mighty egos and theatrical effects, de Meuron (too shy to appear at press calls) and Herzog (the slightly nervous presenter of success) are clearly a good team. They contemplate their promotion to the world of paparazzi and film with a shrug. Among the architectural critics there is considerable - and vulgar - interest to find out how rich they. When Herzog and de Meuron won the Tate job they employed four people. Now they employ 100, with commissions all over the world to design galleries, museums and houses. Yet Herzog, who will be 50 in a few weeks, admits that he still bikes around Basel, while de Meuron drives a Lancia. Now that their portfolio has expanded they talk of buying a jet - except they wouldn't know where to keep it.

Nicholas Serota is right when he says that they are not signature architects with a format or a formula, but that they respond to every project. Now their international modernism seems to be spearheading a renaissance of Swiss design. And Herzog and de Meuron's cool rationalism, as well as their punctuality, makes them as Swiss as cuckoo clocks, or so the country's ambassador, Mr Bruno Spinner, assures me.