A chemical reaction of an electrolytic cell is set up when metals are exposed to water. Moisture, even that in the air, can accelerate corrosion and over time will cause considerable damage. Archaeological metals are usually contaminated with salts (ions) that accelerate corrosion, especially upon exposure to the open air (oxygen) after excavation. These objects continue to be very much at risk until they can be stored in a permanently dry environment. Even decorative and historical metals deteriorate as a result of high humidity.
Another factor to consider is that of pollution. The mixture of pollutants around us in the 21st century poses a real threat to metals; these can be produced by materials within the museum, as well as from outside. Sulphur pollution will result in the tarnishing of silver, whereas acidic gases such as that released from the wood of oak cupboards will eventually reduce lead objects to powder. The effects of pollution are aggravated when the humidity rises.
Gloves should always be worn when handling metal objects because salts, acids and oils emitted through the skin are deposited on the surface. The cell process outlined above can then cause corrosion. A highly polished or freshly cleaned metal will react most quickly and can lead to visible fingerprints being etched on to the surface.
Exposure to light is a less important factor in metal deterioration. However, ultraviolet light has been shown to oxidise metals over a long period of time.