tar·nish | ˈtär-nish
noun
A dulled luster or finish caused by a thin deposit of dirt which discolors the surface of metal and is easily removed
Tarnish is a broad term that refers to the discoloration of metal from a number of sources. While it can generally refer to the dulled appearance metal takes on when there is too much dirt or other elements on the surface, it also refers to the chemical reaction between certain types of metal and chemicals that impact that metal. The most commonly cited type of tarnishing is silver, which reacts to sulfur that occurs in a number of surfaces and products. The resulting silver sulfide can result in a duller, colorless appearance in silver, but can be removed with a special cleaning solution and light abrasion applied to the surface.
