Foxing is an age-related process of deterioration that causes spots and browning on old paper documents such as books, postage stamps, old paper money and certificates. The name may derive from the fox-like reddish-brown color of the stains,[1] caused by the rust chemical ferric oxide which may be involved. Paper so affected is said to be "foxed".
Foxing is rarely found in incunabula, or books printed before 1501.[2] Decrease in rag fibre quality may be a culprit; as demand for paper rose in later centuries, papermakers used less water and spent less time cleansing the rag fibres used to make paper.[3] An early work of art to have been affected by foxing is the Portrait of a Man in Red Chalk, a drawing on paper by Leonardo da Vinci.[4]
Foxing also occurs in biological study skins or specimens, as an effect of chemical reactions or mold on melanin.
Foxing can also be intentionally added to clothing to give it an aged appearance.
Aside from foxing, other types of age-related paper deterioration[5] include destruction of the lignin by sunlight and absorbed atmospheric pollution, typically causing the paper to become brown and crumble at the edges, and acid-related damage to cheap paper such as newsprint, which is manufactured without neutralizing acidic contaminants.
The causes of foxing are not well understood. One theory is that foxing is caused by a fungal growth[6] on the paper. Another theory is that foxing is caused by the effect on certain papers of the oxidation of iron, copper, or other substances in the pulp or rag from which the paper was made. It is possible that multiple factors are involved. High humidity may contribute to foxing.
Foxed documents can be repaired, with greater or lesser success, using sodium borohydride,[7] proprietary bleaches, dilute hydrogen peroxide or lasers. Each method risks side effects or damage to the paper or ink.
Another method is to scan the image and process that image using a high-level image processing program. This can usually remove the effects of foxing while leaving text and images intact.
It is generally not advisable to repair study specimens, except perhaps for mechanical damage. Type specimens should – if at all possible – not be altered in any way. If foxing affects the study value of a specimen (e.g. in bird or mammal skins or in insects, where it might affect diagnostic coloration), this might rather be remarked on the specimen label. Color standards can provide a means of documenting coloration before or in the early stages of foxing.
Foxing may also be applied to clothing through intentional scuffing of shirt collars, cuffs and shoes, to make items of clothing appear older, used and well-worn.
It also has an older meaning of older, worn, comfortable and prized clothing that is worn despite frays, wear etc.