We may be living in the age of virtual reality and 3-D printing, but for the ultimate test in design, carmakers still rely on clay, one of the world’s oldest and most tested materials, reports Phys.org.\nEvery major car manufacturer from GM to Mercedes, and even Tesla, makes a life-size clay mockup of a new model before it goes into the manufacturing chain. Specialized craftspeople sculpt scale and full-sized models down to their most elusive creases and lines. Interiors are so painstakingly detailed that focus groups sometimes don’t know they’re looking at a clay version.\nClay is special. Carmakers have tried other materials and other approaches — plaster, for example, and computer design – but without the clay hand-sculpting process, “designs tend to look stiff and stale,” Bob Boniface, the design director for global Buick exteriors at General Motors, told Phys.org.\nInitiated by GM in the 1930s, clay hand-sculpting has become increasingly crucial because design now constitutes the key selling point in the showroom. On all other factors, such as fuel economy, road behaviour and crash performance, cars are just about equal. Design is the major differentiator. \nThe whole process moves from computer to clay and back, in an iterative loop where sculptors and engineers go back and forth to fine-tune details.\nFinally, the model is coated with a film resembling paint to meet its ultimate test: bringing it outside to see what its curves and lines look like in daylight. “Sometimes, you take it outside and it looks horrible,” says Boniface. So, back to the drawing board... or screen.