Most people never take into consideration the surface of a piece of sculpture and the damage that can occur in ordinary day to day life. The patina (or surface of a sculpture has occurred due to acids or chemicals being placed on the bronze surface either cold or while being heated. The different colors occur due to whatever mixture of acids are used.
This outside surface is vulnerable to sun being reflected through glass (such as a large window where the sun shines directly through it). The surface can also become scratched by rings, watches or any abrasive and must be corrected because with time it can enlarge.
Our modern cleaning agents can not be used on any bronze...for instance never use windex or dust spray such as Pledge even if you are trying to do the base only. If it is a Stone base it has been sealed with stone sealer but should need nothing but a feather duster. Feather dusting occasionally will prevent dust from accumulating and as a last precaution salt water or even fresh water tanks put different chemicals in the air and humidity that in the same room can destroy any metal in time.
Usually a piece will need to be checked every few years for scratches or damage or have the need to be re- waxed and buffed...usually the artist will do that for you or a nearby foundry.
Art pieces are a lifetime investment that need to be carefully checked through the years in order to keep its quality monitored.