Oxidation

Oxidation, once called long ago Sulphuretting, is a color change caused by various chemicals, either accidentally or purposely.

A common change is with orange/vermilion or red engraved stamps turning brown or even black. This could be caused by sulfur and sulfuric acid in the home atmosphere from coal-fired heating systems. So if the entire stamp color has been changed, it may not immediately be clear that the stamps was originally red or orange. Stamps with such complete changes are called changelings.

Stamps that are partially brown or black are clearly oxidized. There may also be a metallic sheen associated with the effect. It really cannot be reversed without causing eventual destruction of the paper.  File:US 6c fist air x2.jpg|Normal (left) and oxidized stamp 