Six shapes in concrete are sited along the rim of an otherwise barren mesa 65 miles north of Phoenix, Arizona. The shapes one cube, two apses, two barrel vaults and one parallelepipedon are the initial, partial embodiment of Italian-born Paolo Soleri's vision of the city of the future. They are the first structures of Arcosanti, the beginnings of what some day may be a community of five thousand persons living in a new, energy-efficient environment.
The appearance of the structure, the site-cast panels particularly, is a function of a casting process many believe was discovered by Soleri. It is called silt-casting and is derived largely from Soleri's experience in earth casting of bells and wind chimes. In his desire for concrete that would exhibit the sort of texture, sculpturing and coloration inherent in the earth-casting process, Soleri arrived at silt-casting. This consists essentially of casting a wall panel, for example, on a flat bed of fine silt brought up from the river bottom below the mesa. This rusty-pink-colored silt, leveled and compacted, serves as both a casting bed and coloring agent. As the panel is tilted up and away from the bed, a fine layer of silt, which is a grain or so deep, adheres to and is embedded in the bottom of the surface, thus imparting natural desert color and texture to this surface that eventually forms part of the exterior of the building.
A further refinement of the silt-casting technique is apparent in the barrel-vault structures. They were cast on curved beds of river silt, with designs in concrete mineral colors painted onto the silt. Thus, as the arched segments were lifted away from the beds, the bottom surfaces had transferred to them, in brilliant color, the design laid onto the silt. In some instances a further refinement was hand tooled into the sand bas relief designs, thus imparting raised sculpturing as well as color to the interior of the segment.