Foxing is a term describing the age-related spots and browning seen on vintage paper documents such as books, postage stamps, certificates, and so forth. The name may derive from the fox-like reddish-brown color of the stains, or the rust chemical ferric oxide which may be involved. Paper so affected is said to be "foxed."
Although unsightly and a negative factor in the value of the paper item for collectors, foxing does not affect the actual integrity of the paper.
Aside from foxing, other types of age-related paper deterioration include destruction of the lignin by sunlight and absorbed atmospheric pollution, typically causing the paper to go brown and crumble at the edges, and acid-related damage to cheap paper such as newsprint, which is manufactured without neutralizing acidic contaminants.
Causes of foxing 
The causes of foxing are not well understood. One theory is that foxing is caused by a fungal growth 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.
Repairing foxed documents 
Foxed documents can be repaired, with greater or lesser success, using proprietary bleaches, dilute hydrogen peroxide or lasers. Each method risks side effects or damage to the paper or ink.
Foxing in biological specimens 
It is generally not advisable to repair study specimens, except perhaps for mechanical damage. Type specimens should, if by any means 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.
- E.g. Smithe (1974, 1975, 1981)
- Smithe, Frank B (1974): Naturalists' Color Guide Supplement. American Museum of Natural History, NYC. ISBN 0-913424-04-8
- Smithe, Frank B (1975): Naturalist's Color Guide (2nd ed.[verification needed]). American Museum of Natural History, NYC. ISBN 0-913424-03-X
- Smithe, Frank B (1981): Naturalist's Color Guide Part III. American Museum of Natural History, NYC. ISBN 0-913424-05-6
- Roberts and Etherington, Bookbinding and the Conservation of Books
- Antique Roadshow: 'Antiques Speak'
- The Library of Congress: 'Preserving Works on Paper'
- Chris Hicks, bookbinder: 'Caring for Your Books'
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