[...] A knowledge of, and sympathy with, the qualities of the materials used are essential to proper treatment.
Stone, the most ancient, should be kept massive, not cut into ribbons. The strength must be retained.
Bronze, cast, serves well for slender, attenuated shapes. It is strong even when very slender.
Wood has a grain which must be reckoned with. It can be slender in one direction only.
Wire, rods, sheet metal have strength, even in very attenuated forms, and respond quickly to whatever sort of work one may subject them to. Contrasts in mass or weight are feasible, too, according to the gauge, or to the kind of metal used, so that physical laws, as well as aesthetic concepts, can be held to. There is of course a close alliance between physics and aesthetics.
Strength and durability in sculpture are highly desirable. However, fineness and delicacy may be even more essential to the general concept, and it will then be necessary to decide which is to control the design.
Also there is the possibility of using motion in an object as part of the design and composition. The sculpture then becomes in one sense a machine, and as such it will be necessary to design it as a machine, so that the moving parts shall leave a reasonable ruggedness. Even those sculptures designed to be propelled by the wind are still machines, and should be considered thus, as well as aesthetically.
However the mechanical element must never control the aesthetic. Much better a poor machine and a good sculpture. [...]
Alexander Calder
A Propos of Measuring a Mobile - Upubl. MS, 1943, Calder Foundation Archives, New York.
