Answer: Silver reacts with hydrogen sulphide and sulphur compounds in the air and forms a black coating of silver sulphide.
Air contains traces of hydrogen sulphide and other sulphur compounds (from burning fuels, rubber, etc.). Silver combines with them to form a black layer of silver sulphide on its surface:
\[\mathrm{4Ag + 2H_2S + O_2 \rightarrow 2Ag_2S + 2H_2O}\]
This is why silver ornaments and articles lose their shine and look blackish. The coating can be removed by rubbing with a paste of baking soda or by polishing.