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The plate, or metal frame, of a piano is usually made of cast iron. It is advantageous for the plate to be very massive. Since the strings vibrate from the plate at both ends, vibrations absorbed by the plate will result in energy loss to the desired (efficient) channel of sound transmission, namely the bridge and the soundboard. While some manufacturers now use cast steel in their plates, cast iron remains the preferred metal. Cast iron is easy to cast and machine, has flexibility sufficient for piano use, is tremendously resistant to deformation (unlike steel that can bend), and is especially tolerant of compressive forces. The casting of the plate is a delicate art, since the dimensions are crucial and the iron shrinks by about one percent during cooling.
The inclusion in a piano of an extremely large piece of metal is potentially an aesthetic handicap, which piano makers overcome by polishing, painting and decorating the plate. Plates often include the manufacturer's ornamental medallion and can be strikingly attractive. In an effort to make pianos lighter, Alcoa worked with Winter and Company piano manufacturers to make pianos using an aluminum plate during the 1940s. The use of aluminum for piano plates, however, did not become widely accepted and was discontinued.
The numerous grand parts and upright parts of a piano action are generally hardwood, e.g., maple, beech, or hornbeam. However, since World War II, plastics have become available. Early plastics were incorporated into some pianos in the late 1940s and 1950s, but proved disastrous because they crystallized and lost their strength after only a few decades of use. The Steinway firm once incorporated Teflon, a synthetic material developed by DuPont, for some grand action parts in place of cloth, but ultimately abandoned the experiment due to an inherent "clicking" that invariably developed over time. (Teflon is "humidity stable" whereas the wood adjacent to the Teflon swells and shrinks with humidity changes, causing problems.) More recently, the Kawai firm has built pianos with action parts made of more modern materials such as carbon fiber reinforced plastic; these parts have thus far performed reasonably, but it will take some decades to know if their longevity and performance can match that of wood.
The part of the piano where materials probably matter more than anywhere else is the soundboard. In quality pianos, this is made of solid spruce (that is, spruce boards glued together along the side grain). Spruce is chosen for its high ratio of strength to weight. The best piano makers use quarter-sawn, defect-free spruce of close annular grain, and make sure that it has been carefully dried over a long period of time before making it into soundboards. In cheap pianos, the soundboard is often made of plywood.