What causes tarnishing and what kind of chemical reaction is it?
Tarnish is a thin layer that forms over copper, brass, silver, aluminum, and other semi-reactive metals as their outermost layer undergoes a chemical reaction. Tarnish is mainly caused by chemicals in the air, such as sulfur. It often appears as a usually dull, gray or black film or coat over metal. Tarnish can easily be removed, however, and does not destroy the metal the way the oxidation process known as rust destroys iron. In the case of silver and copper tarnish actually preserves the underlying metal and is considered desirable. Tarnish is a product of a chemical reaction between a metal and a nonmetal or compound, especially oxygen and sulfur dioxide. It is usually an oxide, the product of oxidation.
