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. It is mainly caused by chemicals in the air and tarnished silver will normally have a bronze colour which can usually be easily removed using a 'silver cloth' or chemical solution. It does not cause any damage to the metal and can actually preserve it.
As the purity of silver decreases, the probability of tarnishing increases. Ironically silver itself is not very reactive, meaning it does not react with oxygen at ordinary temperatures so does not easily form a silver oxide or tarnish, however other metals in the alloy such as copper, may react hence causing this effect.
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