The spare steel modernism of Anthony Caro's early work has given way to a more earthy, expressive awkwardness. And his new sculptural interpreta tions of some of his favourite paintings actually sit rather comfortably in the opulent, history-laden spaces of the National Gallery. By David Cohen
Some wag in the art world once said that sculpture is what you trip over in a gallery when you step back to get a better view of a painting. Whoever made this wisecrack (variously attributed to Ad Reinhardt and Charles Baudelaire) would have felt at home in London's National Gallery: the collection is devoid of sculpture, indeed of anything except framed paintings hanging from walls. The monopoly of flat depictive art is finally being challenged, however, with an exhibition of Anthony Caro opening there this week, the first by a living sculptor.

The show is of Caro's interpretations in sculptural form of some of his favourite paintings. Images by Giotto and Mantegna, Manet and Van Gogh, Matisse and Goya, are translated into his own distinctive abstract language. Giotto's Ognissanti Madonna, for instance, a picture in the Uffizi Gallery, Florence, finds her introduced to the third dimension in the form of a polished wooden cylinder, her new throne a frame of red painted steel.

For its initiation into sculpture, the National Gallery has jumped in at the deep end. Caro's international reputation dates back to the 1960s, when he gave British sculpture a radical shake-out. His sprawling works in welded steel, painted in loud and daring colours, tested the boundaries of what sculpture was supposed to look like. They were, indeed, easy to trip over, and could do one serious mischief. His pieces were wide-open, formally speaking, and dispensed with plinths. The idea was to get away from the polite conventions of the statue as a self-contained entity, and instead to confront viewers with an all-embracing and decentred sculptural experience. He favoured clunky, brutal materials - I-beams and tank tops straight from the factory or scrapyard - neutralised and shed of all prior associations in their coloured state.

Caro belongs to the modernist tradition in that he stresses the physicality of sculpture, its working "in the round" - abolishing any sense of a single- point perspective - and in that he uses modern, experimental materials. But he would have nothing to do with the traditional modernist notion of primitivism. Not for him the sculpture as surrogate totem. His breakthrough works don't invite touch or elicit empathy. Instead they are about internal formal relations, and in that respect are supremely optical. The experience of looking is more pictorial than sculptural, in other words, which makes him a winner for the National Gallery.

Asked about the sense of physical interaction at the time, Caro made the point that his works are "for the eyes only". The ideal viewing situation for his coloured steel works, he has always insisted, is the pure white cube of a modernist art gallery. That way the forms float, relatively unburdened by gravity and their relation to the rest of the world, defining their own internal spatial dynamics with freedom and detachment.

Since the 1960s, though, Caro's work has moved on from its purist beginnings. Bright colours and open forms have given way to richer, darker, heavier materials and handling. Works now tend to be more grave and expressive. He started out being so reductive he looked like a minimalist avant la lettre, but since the real minimalists stepped forward to play their conceptual games with the meaning of art in light strips and piles of bricks, Caro has gone in the opposite direction. In reconstituting the expressive base of his sculptural language he has even taken to visiting the Old Masters for inspiration and ideas. (Transcriptions, though, account for a fraction of his gargantuan output.)

Caro himself prefers to account for the shift in his work in a way that stresses continuity. In the early work, he has written, "We were trying to find ways to make art with clarity and economy, to establish our grammar. Now we can write fuller sentences. We can allow for more weight and pressure without throwing overboard the gains that were won then." It is true that the recent work is still unmistakably in the same handwriting as the radical work of the 1960s. There are the same tensions between rough material and effete handling. The same upfrontness and unexpectedness and tight economy of means. But in recent years a richness and robust awkwardness has entered his sculptural language. The metal is more beaten about, the finish rusty. There are earthy, expressive contrasts of different materials: metal against wood, wood against ceramic. Once Caro wore Modernism with a capital "M" on his sleeve; now his sculptures increasingly have an Old- Master feel about them.

He even looks comfortable in opulent, history-laden spaces. In another groundbreaking exhibition, Caro was the first contemporary artist to be shown in Rome's Trajan Markets back in 1992. His work held its own against the sumptuous, theatrical backdrop of the Forum, individual pieces framed by Roman arches and marble porticos, the metal offset against earthy Roman brickwork. The Trajan Markets exhibition opened 20 years to the day after Henry Moore's similarly apotheosic retrospective at the Forte di Belvedere in Florence. Caro had been Moore's assistant and protege, and still measures himself against the giant whose successor he is taken to be. Coincidentally, a centenary tribute to Moore at the National Gallery will overlap the Caro show.

Caro has always felt as much, if not more, kinship to painting as to sculpture. In a way, this might be part of his "anxiety of influence" towards his "fathers" in sculpture, Moore and the American welder David Smith. His debt to painters is necessarily tangential as influence cuts from one medium to another. Paintings are the perfect source for Caro as they give him the right balance of freedom and connection. He takes the energy from the paintings without being constricted by the formal means. He can be, at the same time, an innovator and part of tradition.

Caro's great champion at the outset of his career was the guru of post- war formalist art criticism, the American Clement Greenberg, who proclaimed him the greatest English artist since Turner, not to mention the leading sculptor of his day (the successor to Picasso, Gonzalez and David Smith). It is to seminal painters within the formalist canon that Caro turned first: to Manet and Matisse. His interpretation of Manet's Dejeuner sur l'herbe finds meanings as remote from Manet's original work (and probable intentions) as the radical and reductive readings by his formalist critic friends. It would take an astute eye to guess the source of Caro's transcription without the title, which anyway begs the question: are Caro's beaten metal shapes sprawling across the divide of a right-angled plinth the picnickers or their meal? The sculpture is not unlike the basket and provisions spilling out beneath Manet's nude, who fixes her gaze upon the viewer. As the eye adjusts to Caro's sculptural vision some shapes begin to read as touching, if not convincing, renderings of the voluptuous turn of a thigh or prop of an elbow. But the reading remains highly selective and subjective. It is an account of an energy in the original quite remote from its particulars.

Manet, Matisse and Mantegna, different panels of whose The Triumph of Caesar at Hampton Court have prompted a transcription, are all "cool" painters whose vision and touch suggest corresponding qualities in Caro's sculptural approach: a certain aloofness, in his case ensured by working with assistants, and lack of sentimentality. More surprising is that he should also tackle Rembrandt, Goya and most recently Van Gogh. When the idea of his National Gallery show was mooted it was noticed that none of his reworkings was of a picture in London, so he was asked to knock something out. What emerged were five versions of the Gallery's most popular picture, Van Gogh's Chair, of which three are included in the show.

Whereas the other transpositions take individual components within a picture and treat them as three-dimensional objects within the viewer's own space, in the Chair series, Caro respects the claustrophobic sense in Van Gogh's original of the chair bursting within the psychologically charged space of his room. Van Gogh painted this picture days before Gauguin's dramatic exit from his life which precipitated the nervous breakdown in which he cut off his ear. Caro makes the chair from stoneware which contrasts with the rusted steel that denotes the surrounding room. The chair is actually made up of "loaves" (a ceramicist's term for the received shape of blocks of material) that Caro has squashed and pulverised into expressive shape. They are awkward, heavy, almost gawkish forms, looking sometimes like elephants' feet, sometimes like arches from Stonehenge. It is ironic that, through engagement with the painting canon, Caro has finally entered his primitive phase.

In reworking such old masters as Rembrandt and Goya, Caro keeps company with some unlikely contemporaries. Frank Auerbach, the expressive realist painter who showed his Old Master transcriptions at the National Gallery three years ago, has also tackled a Rembrandt deposition (his in London, Caro's in Munich) while Goya's anti-war images have also found contemporary sculptural form as shop mannequins, thanks to the brothers Jake and Dinos Chapman whose tree-hanging mutilated corpses was a centrepiece of the recent "Sensation" show. But Caro's whole relationship to the Old Masters is of an order different from either the School of London painters or YBAs. There is no sense of Caro "taking on" the masters in some heroic, existential struggle to receive their reluctant blessing. Nor is there a whiff of irony or deconstruction in his approach. Tradition is neither a club he is desperate to join nor a pool of images to be raided but a place he likes to visit for assured aesthetic experience `Sculpture from Painting', National Gallery, London WC2, 25 February- 4 May.