City folk: Exuding a rugged, sculptural power through form and materials, a museum of folk art in manhattan is a luminous backdrop for the exhibits.

The new Museum of American Folk Art is housed in an eight-storey building, by Tod Williams Billie Tsien & Associates, at 45 West 53rd Street in the heart of Manhattan. This is the first new museum in the city since the Whitney opened in 1966, and is also the architects' first important civic building in New York City. As a museum, it provides a permanent home for the study and appreciation of a vital art that is eloquent of America's cultural diversity, of its varied history and traditions. Artefacts -- created out of materials to hand and, often, at critical moments in American history -- testify to the personal creativity and communal effort that laid the foundations of American statehood.

Long and thin -- only 12.2m wide -- the museum faces an open piazza so that it can be seen full-frontal from 52nd Street. It is flanked on three sides by sites owned by the Museum of Modern Art. In consequence, its exterior (not quite completed) is an assertion of its independence from MoMA and of its presence in the city. The building is clad with panels of Tombasil (a form of white bronze) which is used for fire nozzles and ship propellers and has never before been used architecturally. Panels are textured, their variations made deliberately by using steel or concrete moulds, and their fissures a natural consequence of the casting process: each one is different. Catching the glow of morning and evening sun, the building's exterior changes with weather and seasons. The front south face itself is a faceted, sculptural composition of changing light, folded slightly in an echo of origami.

Inside the museum, exhibits range from objects drawn from the traditional folk art -- paintings, sculptures, weathervanes, flags, quilts -- of the eighteenth and nineteenth centuries, to the works of contemporary self-taught artists. The four upper floors are devoted to galleries for permanent and temporary exhibitions. Collections are elegantly displayed by straightforward means in exhibition cases and in less traditional ways. Below the galleries are the ancillary facilities so essential to modern museums. A mezzanine level, between ground and first floor, contains a small coffee bar, and has views out to 53rd Street and into the main ground floor hall and two-storey atrium. Underneath it, next to the reception is a museum shop (open outside museum hours and reached by a separate door). In the upper basement are an auditorium, lecture hall, lavatories and cloakroom; and below, a library, rare books room, audio-visual facilities, offices and plant rooms. In general, the west side of the building is lined by services such as a fire stair, lift and stairs.

But in other respects, design refers to the grand traditions of nineteenth-century galleries like the Soane Museum. The museum is capped by a skylight over a grand central staircase, washed in light, leading from second to third floor. Openings on each level allow natural luminance to filter into galleries and down to the lowest levels. Sectional changes subtly enlarge space, and progress through the building is enlivened by devices that allow you to see the same object from different angles, to see through a dividing wall from one gallery to another, to look from level to another. Objects mounted in niches throughout the building or on walls lining the light slots provide a continually changing landscape and recall the ways in which progress was often enriched in the old museums. The result is a museum that feels very much larger than it is.

Materials and their coexistence are a continual source of pleasure. Floors are the simple slab, terrazzo ground, and Ruby Lake Fir. Set against the toughness of concrete (bush hammered in places) the Fir boards, up to 5m in length, are opulent. Other notes, warm against glass and concrete, are added by cherrywood handrails, and furniture designed by the architects and made by a Japanese American cabinet maker.

In a review of a Williams and Tsien house on Long Island (AR September 1999), Margaret Seal observed the architects' 'careful and intelligent use of materials and craftsmanship in pursuit of a gentle, sensuous and thoughtful arrangement of spaces revealed by light'. They are, she continues, 'undoubtedly American in the range of their responses to the very different cultures, climates and landscapes of their country'.

The truth of these observations is borne out in previous works, each with its special relationship to the context. Design of this museum celebrates with grace, without pomposity or condescension, the inventions of ordinary people. And, by the manner in which the objects are displayed and illuminated, it acknowledges the powerful fascination of folk art.