Tarnish

Tarnish preserves the underlying metal in outdoor use, and in this form is called chemical patina.

[2] Unlike wear patina necessary in applications such as copper roofing, outdoor copper, bronze, and brass statues and fittings, chemical patina is considered a lot more uneven and undesirable.

Tarnish is a product of a chemical reaction between a metal and a nonmetal compound, especially oxygen and sulfur dioxide.

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,[4][5] or it may be removed with a special polishing compound and a soft cloth.

Tungsten rods with evaporated crystals, partially oxidized with colorful tarnish
A Morgan dollar demonstrating a colourful form of toning on its reverse. While tarnishing on other metal objects is generally cleaned off, some toning on coins may be considered aesthetically pleasing or beneficial to the coin's value, and is therefore kept in situ . [ 3 ]