In dentistry, metals represent one of the four major classes of materials used for the reconstruction of damaged or missing oral tissues. Although metals are readily distinguished from ceramics, polymers, and composites, it is not easy to define the word metal because of the wide variation in properties of metallic materials. The Metals Handbook (1992) defines a metal as "an opaque lustrous chemical substance that is a good conductor of heat and electricity and, when polished, is a goodreflector of light."1