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 can actually preserve the underlying metal and may be considered desirable. Boiling water, aluminum foil, vinegar and baking soda work nicely. Smooth jazzy therapeutic.