Removing Foxing
There are many types of mildew and all are commonly known as "Foxing", the mildew gets its name from the (F)errous (Ox)ide or the iron it is attracted to in the paper, usually chemical wood pulp, and all are activated by the same cause...relative humidity.
Paper and leather are hygroscopic, a hygroscopic material is one that attracts moisture from the atmosphere.
If not protected from contact with a damp atmosphere, books left in damp conditions will eventually dissintigrate
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