Foxing, New York Mayor’s Court Petition of John Peeck and Peter Zenger, undated. (Courtesy of Geof Huth)n.small, irregular, brown blemishes on paperRoberts and Etherington 1982Stains, specks, spots and blotches in paper. The cause or causes of foxing, which usually occurs in machine-made paper of the late 18th and the 19th centuries, are not completely understood, but in all likelihood, it is fungoid in nature. ¶ Two significant differences between foxed and clean areas of a paper are the higher proportion of acid and iron in the former, although there does not seem to be any clear and definitive relationship between iron and foxing. Insofar as the acid is involved, it is not clear whether this is produced chemically or as a byproduct of the life function of the organisms present.Sung, Leonov, and Waters 1990, 312This would be necessary if the iron oxide in the water introduced enough metal contaminates to cause oxidation and hence foxing marks which eventually eat holes in the paper.Ritzenthaler 2010, 376Foxing: Small, usually reddish-brown spots appearing on paper; thought to be caused by the presence of trace metal (such as iron) in paper, as a by-product of fungal growth, or a combination of these factors.Harvey and Mahard 2020, 156Paper is also susceptible to foxing when stored in high relative humidity environments. Foxing usually occurs in machine-made paper of the late 18th and 19th centuries, and it is not completely understood. It is likely that its cause is fungal in nature. It will always be reddish-brown in color, will look like it is integral to the fibers of the papers, and will often appear in patchy locations on one page but not in the same patches on adjacent pages.UIUC 2023Foxing is a common form of paper deterioration that is frequently seen in books. Although the exact causes of foxing are varied and not easily identified, it is often the result of high humidity combined with fungal growth or metal impurities present in the paper. Foxing will appear as reddish-brown (rusty) flecks on the paper. They can appear across the surface or can be situated in clusters in either “bullseye” (small and round, dark center) or “snowflake” (larger, irregularly shaped blotches) shapes.