In the two decades between 1929 and 1949, sculpture in the United States sustained what was probably the greatest expansion in sheer technique to occur in many centuries. (46) There was, first of all, the incorporation of welding into sculptural practice, with the result that it was possible to form a new kind of metal object. For sculptors working with metal, earlier restricted to the dense solidity of the bronze cast, it was possible to add a type of work assembled from paper—thin metal sheets or sinuously curved rods. Sculpture could take the form of a linear, two-dimensional frame and still remain physically self supporting. Along with the innovation of welding came a correlative departure: freestanding sculpture that was shockingly flat. Yet another technical expansion of the options for sculpture appeared in the guise of motion. (47) The individual parts of a sculpture were no longer understood as necessarily fixed in relation to one another, but could be made to change position within a work constructed as a moving object. Motorizing the sculpture was only one of many possibilities taken up in the 1930"s. (48) Other strategies for getting the work to move involved structuring it in such a way that external forces, like air movements or the touch of a viewer, could initiate motion. Movement brought with it a new attitude toward the issue of sculptural unity: a work might be made of widely diverse and even discordant elements; their formal unity would be achieved through the arc of a particular motion completing itself through time. (49) Like the use of welding and movement, the third of these major technical expansions to develop in the 1930"s and 1940"s addressed the issues of sculptural materials and sculptural unity. But its medium for doing so was the found object, an item not intended for use in a piece of artwork such as a newspaper or metal pipe. To create a sculpture by assembling parts that had been fabricated originally for a quite different context did not necessarily involve a new technology. (50) But it did mean a change in sculptural practice, for it raised the possibility that making sculpture might involve more a conceptual shift than a physical transformation of the material from which it is composed.