What is foxing?
“Foxing” refers to brown spots caused by one type of mold, a saprophytic fungus (living on dead organic matter). The fungus destroys the sizing in paper and discolors it. Although almost any dark spotting is frequently referred to as foxing in non-scientific condition reports on Japanese prints, some of these discolorations are actually spot staining from different molds or other sources. Technically, foxing requires that the paper has been infected by a saprophytic fungus. Sometimes analysis under a microscope or under UV radiation is required to confirm the nature of a mold. One difference is that some molds appear “furry” when viewed by eye, whereas foxing does not. The etiology of foxing is still not clear. It may be caused by chemical reactions between iron salts in the paper and organic acids released by fungi. It is also possible that decomposition products of cellulose have settled in the areas made spongy by the activity of fungi, after which the spots have then darkened with
