Silver is a highly unreactive metal so it does not react with the oxygen of air easily. But air usually contains a parts per billion concentrations of pollutant gases, such as HCl,H2S,SO2,NO2 and ozone. The exposure of silver to H2S gas at 25∘C and air containing 75% relative humidity produces a black coating consisting of silver sulphide (Ag2S) on its surface. Similarly, the exposure of silver to NO2 gas at 25∘C and air containing 75% relative humidity produces a thin film of AgNO3 (transparent) and Ag2O (black) on its surface. Thus, silver ornaments gradually turn black due to formation of layers of Ag2S and Ag2O.