Foxing is the age related browning, or brown-yellowish spots, that can occur to book paper over time. When this aging process happens to the paper in a book it is referred to as "foxed".

The term may come from the rust brown color of the paper aging process or from a chemical used to coat paper called ferric oxide. Foxing may also be caused by fungal growth on the paper, chemical reactions, or high humidity.

See also: foxing.

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