Extensive foxing on a postage stamp of Nova Scotia.

Foxing (from Ferric OXide) is the effect on certain papers of the oxidation of iron or other substances in the pulp or rag from which the paper was made. It is responsible for the spots and browning seen on old books, documents, postage stamps, and so forth.

While unsightly, and a negative factor in the value of the paper item for collectors, foxing does not affect the integrity of the paper.

There is a product called FOXIT to remove foxing.