why some metal loses their shine and apperance dull ?

Some metals often lose their shine and appear dull, because of the action of air and moisture on them. Some metals as a result of oxidation with the atmospheric oxygen form oxides and by which their appearance becomes dull. As in iron which reacts with moist air and oxidizes into rust (iron III oxide) and aluminium reacts to form its oxide. Some metals react with hydrogen sulphide in air as silver forms silver sulphide (Ag2S) which is responsible for the dullness on silver. Similarly by the reaction between copper and air borne sulphur compounds and others results in formation of copper sulfate, carbonate which causes dullness on copper.