Foxing

Foxing is an age-related process of deterioration that causes spots and browning on paper documents such as books, postage stamps, old paper money and certificates, and on textiles like clothing and artists' canvasses. The name may be a variant form of the English West country dialect term foust and Scots foze, to become moldy. Alternatively, it may derive from the fox-like reddish-brown color of the stains. Paper that is affected is said to be "foxed".