To make a structure that will stand the test of time, architects choose materials that are strong and durable, as well as pleasing to the eye. Kahn preferred simple forms and natural materials. To achieve a sense of serenity and elegance in the Kimbell, Kahn selected materials that complemented each other in tone and surface: travertine, concrete, white oak, metal, and glass. Simple and unadorned, each of these materials shows its innate character by its variation of texture.
Concrete, according to Kahn, was “a noble material if used nobly.” Revolutionizing the modern use of materials, Kahn viewed concrete as both an aesthetic and structural choice. In the Kimbell’s galleries, concrete vaults shimmer with light to create a subtle luminosity that Kahn compared to a “silvery powdered moth’s wing.” Reinforced concrete also supports the weight of the structure in the form of vaults, walls, and piers. Creating the right look to the concrete was a matter of serious importance to Kahn, who went to great lengths to select the proper color (soft gray with lavender tones) determined by the mixture of sand and cement. Numerous wall tests were poured and allowed to cure in the Texas sun until they found the right surface qualities and perfect match for the soft tones of the travertine. Kahn believed that buildings should tell the story of how they were made and that incidents of the construction process should be left as a visual record. Accordingly, when they occurred, marks from plywood mold forms, bits of rubber, and air pockets remain for all to see (although the workmen practiced to attain perfection).
Travertine, on the other hand, acts only as “in-fill” material. Kahn even called it wallpaper. (Glass and wood are also non-weight-bearing materials in the museum.) The travertine (a type of colored limestone) used for the Kimbell was imported from Tivoli, near Rome, Italy. This material is riddled with irregularly shaped holes left by gases and pieces of vegetation trapped in hardened layers of calcium carbonate. Despite its “Swiss-cheese” texture, travertine is a durable material and has been used since antiquity for countless buildings. Kahn was deeply influenced by monuments and ancient ruins that he studied as a student and sketched on his travels through Italy, Greece, and Egypt. In his own buildings, Kahn used such materials as travertine to emulate the timeless and monolithic qualities he so admired in those ancient structures. Over one million pounds of travertine sheath much of the Kimbell’s interior and exterior walls, gallery floors, porches, and stairs. These thin, rough-hewn pre-cut slabs (5/8 inches thick) were shipped from Italy in 17 boatloads over nine months. Fissures and openings were not filled. Every attempt was made to retain the material’s natural appearance.
Lead was selected for the roof cover for its color, dull sheen, and discreet, natural appearance. Because this soft metal ages quickly, Kahn believed that it would look consistent with the travertine and concrete. In keeping with his palette of warm and cool tonal harmonies, Kahn also selected white oak for the gallery floors, doors, and cabinetry; anodized aluminum (a light-weight metal noted for its high reflectivity that has been covered with a protective oxide coating) for the soffits and reflectors; and mill-finished steel for windows and door frames, elevators, and handrails, as well as in the kitchen, conservation studio, and darkroom. The Kimbell’s uniquely shaped handrails are made of folded metal, because Kahn preferred emphasizing the sheet quality of the material instead of pretending that it was worked like a solid material, such as wood.