If you visit an archive with scrolls, illuminated manuscripts, or old books on display, you might notice a particular kind of round, yellowish, or orange-tinted stain on some of the paper artifacts. This color effect is called foxing, and paper so affected is said to be “foxed.” Foxing can occur even in your library.
There are two major causes of foxing: contamination from metals and from growth of microorganisms such as fungi. In the papermaking process, traces of metals like iron, copper, and copper alloys rub off from machinery and fall on the paper as dust. Exposure to humidity and air pollution causes these metals to react, forming spots, as if the paper is corroding.
When books move to libraries, fungal spores are always in the air and they easily land on the paper. If the environment is suitable to the spores, they can feed and grow for years. This sort of foxing is essentially cosmetic. The stains don’t compromise the structure of the paper, unlike mold or mildew, which are very destructive.
Artifact conservators often find both metal and fungal contamination are at work. Before the 1980s, detection using ultraviolet light or chemical testing was common. Today, electron microscopes allow for more precise testing.But responsible conservation is paramount if you want to protect paper. It is often impractical to reverse foxing. If you notice that a book of yours is showing signs of foxing, you can stem the growth of bigger spots by avoiding dust or shifts in humidity. Wash your hands before handling delicate or aged books. But keep in mind, foxing is one of many factors that make books unique and reminds us of how they age.