Tarnishing is the result of sulphur gases in the air (pollution) combining with the surface of silver. This causes the surface of the silver to turn brown and eventually black. It can be removed by using the chemical abrasive called silver polish, which rubs away the surface layer. In time, silverware can be worn away by repeated polishing.
An alternative is to remove it using simple chemicals such as a baking powder solution and a sheet of aluminium foil. Place the foil in a tray, add warm baking powder solution and place the tarnished silverware in the tray, making sure each piece touches the aluminium foil AND is covered with the solution. The sulphur will leave the silver and make a coating on the aluminium. The aluminium can then be recycled. Because the air in cities has more pollution than in the countryside, tarnishing happens faster in cities than in the countryside.