Hepworth came to Cornwall in August 1939 with her husband Ben Nicholson and their young triplets, fleeing the imminent war. They settled first at Carbis Bay in cramped quarters that made serious sculpture almost impossible - she carved in spare bedrooms, on landings, anywhere a block of wood would fit. Trewyn Studio, when she found it at auction in 1949, was the answer. A modest 18th-century house on Barnoon Hill with a high-walled garden falling away towards the harbour, it gave her something rarer than space: privacy and direct light. She wrote: "Finding Trewyn Studio was a sort of magic. Here was a studio, a yard and a garden where I could work in open air and space." She would never live anywhere else again.

The terraced garden became a working part of the studio. Hepworth shaped it deliberately, planting bamboo, palms, magnolias and yuccas to give her pieces foils of leaf and shadow that the cooler British north could never offer. She was helped in the planting by her close friend, the South African-born composer Priaulx Rainier, whose musical sense of pattern shows in the way one bronze is placed against another along sight-lines down the slope. Sphere with Inner Form, Four-Square (Walk Through), Two Forms (Divided Circle) - works that elsewhere stand in plazas and university quads sit here at the scale and in the surroundings she intended for them. The garden remains, half a century later, the most concentrated outdoor display of her work anywhere in the world.

The lower studio, where she worked stone, is preserved exactly as it stood at her death. Block of Galway limestone, marble offcuts, unfinished pieces standing in a quiet queue along the wall - what one visitor called "still waiting for their moment in the shadow of her workshop." In 1950 she had hauled two huge blocks of Galway limestone in here to carve Contrapuntal Forms, her Festival of Britain commission; photographs around the walls track the progress from raw stone to finished form. Upstairs is the wood-carving studio where she worked her great African hardwood pieces, and beside it the plaster studio where, from 1956 onwards, she modelled the prototypes for bronzes that would be cast at the Morris Singer foundry in London. The arrangement reveals her method: stone, wood, plaster, each in its own room, each medium asking different things of the maker.

Hepworth was already a Dame, already lit by international honours, when she died at Trewyn on 20 May 1975. The fire that killed her began in her bedroom and was, according to the inquest, started by a cigarette she had left on or near her bedclothes. She had been suffering from cancer for some years. Her son Paul, an RAF pilot, had been killed in a flying accident in Thailand in 1953 - a grief from which, those close to her said, she never fully recovered. She left clear instructions in her will: the studio was to be opened to the public, as nearly as possible as she had left it. Her family carried this out the following year, and in 1980 they transferred the museum to the Tate, which still manages it as part of Tate St Ives. It is the largest collection of her work on permanent display anywhere.

Most artist's museums offer the finished work. Trewyn offers process. The clay she shaped is here; so is the half-cut stone, the polished bronze, the spectacles she wore, the apron she put on every morning. You can stand where she stood, look at what she saw - rooftops sliding down towards Smeaton's Pier, the slate sky over Porthmeor Beach - and see how those forms found their way into her work. Hepworth wrote that "sculpture is a three-dimensional projection of primitive feeling: touch, texture, size and scale, hardness and warmth." The museum is built to test that claim. It puts you in the working room and asks you to look as long as she did.