foxing

Foxing (sometime spotting) is a catch-all term for spots or stains -- usually reddish as with a fox coat.  It is most often found on the endpapers, where the pages are closer to the boards, and as the discoloured imprints from inserted papers (such as bookplates, clippings etc).  Foxing is typically caused by moisture content and can be prevented or arrested through effective dehumidification.  Books avoid fungus best in moderate temperatures.  However, as with sunning/tanning, certain books appear to be more at risk of foxing than others and those with a high acid content, such as machine-made books from the nineteenth century, tend to show stronger foxing due to the absorption of minerals like iron salts.