Silver tarnish. A pair of spoons, one recently polished and the other with a dark tarnish. The tarnish is a layer of silver sulphide, formed by a reaction with oxygen and trace amounts of hydrogen sulphide in the air. The reaction is describe by the equation 4Ag + O2 + 2H2S = 2Ag2S + 2H2O. Once a thin layer of tarnish forms it blocks further hydrogen sulphide reaching the silver below, thus tarnish is a self-limiting process. Tarnish can be removed electrochemically, but is normally removed by polishing with a very fine abrasive.