Foxing is a term describing the age-related spots and browning seen on vintage and antique paper documents such as books, prints, 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."
Various conditions are attributed to the occurence of foxing: impurities in the paper or inks which become oxidized over time, exposure to humidity or moisture which may result in mold or fungus, etc.
A competent paper conservator or restorer can "clean" the foxing in most cases. This is done using special diluted bleaches, sunlight, lasers, or other such techniques. Attempting to remove foxing is best left to a professional if the print is particularly valuable.
Many collectors do not mind the various signs of ageing on prints and find them part of the charm of rare prints!