Foxing is the term for small brown or reddish-brown spots that appear on the pages of old books and other paper documents. These spots are typically round or irregular, range from pinpoint-sized to a centimeter or more across, and tend to darken over time. If you’ve picked up a used book and noticed scattered tan or rust-colored blotches on the pages, especially near the edges, you’re almost certainly looking at foxing.
The term has been used by book collectors and conservators since at least the mid-1800s, though its exact origin is debated. Some attribute it to the fox-like reddish color of the spots. Despite centuries of study, the precise mechanism behind foxing is still not fully settled, involving some combination of fungal activity, metallic impurities in the paper, and environmental conditions.
What Causes Foxing
There are two main theories, and growing evidence suggests both can be true at once. The first is biological: fungi land on paper as microscopic spores, germinate within hours under the right conditions, and send thin filaments (called hyphae) into the cellulose fibers. These organisms feed on organic matter in the paper, secreting acids, pigments, and enzymes that physically break apart the fibers and chemically darken the surrounding area. The result is soft, circular spots that grow outward from the point of colonization.
The second theory is chemical, centered on tiny particles of iron and copper embedded in the paper during manufacturing. Paper made in the 18th and 19th centuries often contained metallic impurities from the water supply, the machinery, or sizing agents. Over time, these metal particles oxidize, essentially rusting inside the page. When researchers examined foxing on a late 18th-century manuscript using X-ray fluorescence, they found high concentrations of iron and copper at the center of each spot. Under UV light, these metal-driven spots often show a distinctive “bullseye” pattern, with a darker core surrounded by a lighter halo.
What makes foxing tricky to study is that these two causes frequently overlap. A landmark analysis of Leonardo da Vinci’s famous self-portrait found no living microorganisms on the foxed areas, but did find iron deposits and remnants of past fungal activity. The leading interpretation: chemical degradation from metal impurities may come first, weakening the paper fibers in a way that later attracts fungal colonization. The fungi then leave behind organic acids and metabolic byproducts that continue browning the paper long after the organisms themselves have died, through a chemical browning reaction similar to what happens when food turns brown during cooking.
What Foxing Looks Like
Foxing spots are usually easy to identify once you know what to look for. They appear as scattered, small, roughly circular spots in shades of yellowish-brown to dark rust. Unlike a water stain, which tends to create a single large area with a distinct tide-line edge, foxing produces many separate spots distributed across a page. Unlike active mold, foxing spots are flat and dry to the touch. Active mold often looks fuzzy or powdery and may have a musty smell. Foxing spots feel like the paper itself; they’re embedded in it rather than sitting on the surface.
Foxing can appear on blank margins, within printed text, and on endpapers. It tends to be worse on pages exposed to more moisture, so you’ll often see heavier foxing near the edges of a book or on pages adjacent to the covers. The spots don’t wipe off, and they gradually darken with age.
Why Some Books Fox and Others Don’t
Paper composition is the biggest factor. Books printed before the mid-20th century are far more susceptible because their paper was made with less refined processes, leaving behind metal particles and acidic compounds. Rag-based papers from the 18th and 19th centuries are especially prone. Modern acid-free papers, by contrast, are manufactured with fewer impurities and resist foxing significantly.
Environmental conditions act as the trigger. Fungi require moisture to grow, with different species needing water activity levels between 0.65 and 0.98, roughly corresponding to relative humidity above 60 to 70 percent. High humidity also accelerates the chemical oxidation of iron and copper in the paper. A book stored in a dry, stable environment may carry the same metallic impurities as one stored in a damp attic, but only the damp attic book develops visible spots. Temperature matters too: warm, stagnant air encourages both fungal growth and chemical reactions.
How Foxing Affects a Book’s Value
For book collectors, foxing is one of the most common condition issues in antiquarian and vintage books. Light, scattered foxing on a book from the 1800s is considered normal and reduces value only modestly. Heavy foxing that obscures text or illustrations, or foxing on key pages like a title page or frontispiece, has a more significant impact. Booksellers typically note foxing in their condition descriptions, using terms like “light foxing to margins” or “foxing throughout.”
Foxing alone does not make a book worthless. Many collectible and historically significant books have some degree of foxing simply because of their age and the paper they were printed on. A first edition of a major work with moderate foxing is still valuable, just less so than a clean copy.
Can Foxing Be Removed?
Professional paper conservators can reduce foxing, though complete removal is difficult and not always advisable. Treatment typically involves a combination of approaches: washing the paper to remove soluble acids, applying chelating agents that bind to metal ions and pull them out of the fibers, and using mild reducing agents that lighten the brown discoloration. Some conservators also use enzyme-based treatments to break down residual fungal material within the paper.
These treatments require skill and carry risk. Over-treatment can weaken already fragile paper, alter its texture, or affect printing inks. For rare or valuable books, conservation decisions are weighed carefully. The goal is usually stabilization and modest improvement rather than making the paper look new. For most personal collections, the cost of professional conservation outweighs the benefit unless the book has significant monetary or sentimental value.
Home remedies you might encounter online, such as bleach solutions or hydrogen peroxide, can cause serious damage to old paper and are best avoided. They may lighten spots temporarily while weakening the cellulose fibers and accelerating future deterioration.
Preventing Foxing From Getting Worse
You can’t reverse the metallic impurities already in old paper, but you can control the environmental conditions that activate foxing. The National Park Service recommends storing books at a stable relative humidity of around 45 percent (give or take 2 percent) and a temperature near 68°F (20°C). Avoiding wide swings in temperature and humidity matters as much as hitting the right numbers, since fluctuations stress paper fibers and create condensation cycles that promote both fungal growth and chemical oxidation.
Good air circulation helps prevent pockets of stagnant, humid air from forming around books on shelves. Keeping books away from exterior walls, basements, and attics eliminates the most common sources of excess moisture. If you live in a humid climate, a dehumidifier in your storage area is one of the most effective investments you can make for a book collection. Storing books upright on open shelves rather than packed in boxes also allows air to circulate around them.
For particularly valuable items, acid-free archival boxes and tissue interleaving provide an extra layer of protection by buffering against environmental changes and preventing foxing from transferring between adjacent pages.