The most common problem facing the collector who has metal object is corrosion. While you may have no objection to using a commercial brass polish on your new brass bed, most collectors of militaria would shudder at the thought of applying such a product to a 19th century European parade helmet or fine sword details. In fact, the most commonly observed mistreatment of historic metal objects comes not from nature but from cleaning and neglect. Over-cleaning often results from a desire to have metals bright and shiny, especially brass and silver objects on display. Items such as swords, daggers, firearms, and medals are obvious victims of this well-meaning but misguided attempt at preservation.
Neglect too can often cause irreversible damage to a metal object.
Until recently, the green and black corrosion products commonly observed on bronze sculptures were mistakenly believed to be protective by some people. There are certainly instances in which the corrosion layer is protective, aluminum oxide being one example. Unfortunately, most historic metals do not form protective corrosion layers. Historic objects that have corroded are best left untreated until a metal conservator has had an opportunity to examine them. This is far better than the risks of over-cleaning, and the consequences of higher corrosion rates on freshly exposed metal surfaces.