Oxidation is the chemical reaction that occurs when a material combines with oxygen, usually in the presence of moisture, causing it to change colour and surface texture. In antiques it is most often seen on metals, as the rust on iron and steel, the dark tarnish on silver, the brown or green surface on copper and bronze, and the dulling of brass.
Not all oxidation is unwelcome. The slow, even oxidation of copper alloys over many years forms the patina that collectors prize on bronzes, brass candlesticks and copper vessels, and stripping it can greatly reduce value. Wood, too, darkens and mellows through oxidation and exposure to light, contributing to the warm colour of old furniture. By contrast, active corrosion such as flaking rust or powdery green bronze disease can cause lasting damage and needs treatment. Conservators distinguish between stable surfaces that should be preserved and active corrosion that should be arrested, and favour gentle cleaning and protective waxes or lacquers. Keeping objects in stable, dry conditions away from fluctuating humidity is the most effective way of slowing harmful oxidation, and abrasive polishing should be avoided on important pieces.
