Unpolished brass will become darker over time from exposure to air. This change can be beautiful. If the surface is affected by water, grease such as fingerprints or by other corroding substances, it can eventually be damaged and more difficult to polish away. This can happen especially in a humid environment.
Clean the surface with a soft cloth. For a deeper cleaning we recommend a gentle, non-abrasive brass polish. Avoid using brushes, steel wool, polishing cloths, sponges and the like since the surface of brass is sensitive to objects that can scratch. Also avoid lengthy contact with oxides that can cause permanent damage. To avoid water damage, keep surfaces dry with a soft cloth.