<P> Tarnish actually preserves the underlying metal in outdoor use, and in this form is called patina . The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings . Patina is the name given to tarnish on copper based metals . </P> <P> Tarnish is a product of a chemical reaction between a metal and a nonmetal compound, especially oxygen and sulfur dioxide . It is usually a metal oxide, the product of oxidation . Sometimes it is a metal sulfide . The metal oxide sometimes reacts with water to make the hydroxide; and carbon dioxide to make the carbonate . </P> <P> Using a thin coat of polish can prevent tarnish from forming over these metals . Tarnish can be removed by using steel wool, sandpaper, emery paper, baking soda or a file to rub or polish the metal's dull surface . Fine objects (such as silverware) may have the tarnish electrochemically reversed (non-destructively) by resting the objects on a piece of aluminium foil in a pot of boiling water with a small amount of salt or baking soda, or it may be removed with a special polishing compound and a soft cloth . Gentler abrasives, like calcium carbonate, are often used by museums to clean tarnished silver as they cannot damage or scratch the silver and will not leave unwanted residues . </P>

If a metal changes color over time from exposure to air