Silver is a natural material that reacts when it comes in contact with air, resulting in oxidation. The speed in which oxidation takes place depends on, for example, humidity. Surfaces change over time; Shiny surfaces become somewhat duller while duller surfaces can get shinier.
Clean the surface with a soft cloth. For a deeper cleaning we recommend a gentle, non-abrasive polish. Avoid using brushes, steel wool and the like. Also avoid lengthy contact with oxides that can cause permanent damage. To avoid water damage, keep surfaces dry with a soft cloth.