Except for a few, pure metals do not occur naturally. They occur in the form of minerals such as oxides and sulfides and these have to be refined to produce the pure metal. Most pure metals attempt to reconvert to the combined state. The process by which this takes place is called corrosion. One of the primary requisites of any metal that is to be used in the mouth is that it must not produce corrosion products that will be harmful to the body. The mouth is moist and continually subjected to fluctuations in temperature. The foods and liquids ingested have wide range of pH. All these factors make the mouth an extremely favorable environment for corrosion.