Foxing is the term for small, scattered brown or yellowish-brown spots that appear on old paper. If you’ve picked up a vintage book, print, or document and noticed clusters of rust-colored speckles across the pages, that’s foxing. The spots range from tiny pinpoints to larger, irregular blotches, and they’re one of the most common forms of paper deterioration found in collections worldwide.
What Causes Foxing
The short answer is that scientists still debate the exact mechanism, but two main theories have held up over decades of research, and the real answer likely involves both working together.
The first theory points to tiny metal particles, particularly iron and copper, embedded in the paper during manufacturing. These metal deposits oxidize over time, producing the brownish discoloration you see. The iron may come from the water used in papermaking, from chemical reagents in the pulping process, or from the machinery itself. When those metal traces react with oxygen and moisture in the air, they essentially rust in place, staining the surrounding fibers.
The second theory blames microorganisms. Fungi and bacteria colonize the paper, and as they grow, they produce pigmented compounds that stain the fibers. Studies of foxed paper have identified several fungal species in the stains, including species of Aspergillus, Acremonium, and certain filamentous bacteria. A notable study examining Leonardo da Vinci’s famous self-portrait found the fungus Eurotium halophilicum colonizing foxing spots on the drawing, along with chains of bacterial spores less than one micrometer across.
Most current research suggests foxing results from both processes happening simultaneously. Microorganisms break down cellulose fibers and produce metabolic byproducts, while metal particles oxidize alongside them. The fungi may even accelerate the chemical reactions involving iron, and the iron deposits may create favorable conditions for microbial growth. It’s a feedback loop rather than a single cause.
How to Identify Foxing
Foxing spots are brown to reddish-brown, roughly circular, and scattered somewhat randomly across the page. They tend to appear in clusters rather than as a single isolated mark. The spots often have a slightly darker center with a lighter halo fading into the surrounding paper. A useful rule of thumb among collectors: brown spots are foxing, black spots are mold.
Foxing is flat against the paper surface. If you see anything fuzzy, raised, or furry, that’s active mold growth, which is a different and more urgent problem. Foxing spots also become more visible when you angle light across the page, since the stained areas reflect light differently than clean paper. Under ultraviolet light, foxing stains often fluoresce, which is one way conservators map and study them.
Why Some Paper Is More Vulnerable
Not all paper foxes equally. Paper manufactured in the 18th and 19th centuries is particularly prone to foxing, largely because of how it was made. As papermaking shifted from handmade rag-based sheets to machine-produced wood-pulp paper, the process introduced more metallic contaminants from industrial equipment. The chemical treatments used to break down wood pulp also left residues that accelerate degradation.
Environmental conditions play a major role in whether foxing develops and how quickly it spreads. High humidity is the single biggest accelerator. Moisture activates both the chemical oxidation of metal particles and the growth of fungi. Warm, stagnant air compounds the problem. Paper stored in damp basements, unventilated attics, or tropical climates is far more likely to develop foxing than paper kept in cool, dry, climate-controlled environments. Some books are also more susceptible than others based purely on the quality of paper the publisher chose, so age alone doesn’t predict whether foxing will appear.
Can Foxing Be Removed?
Foxing can be reduced but rarely eliminated completely, and the treatments involved are not simple DIY projects. Professional paper conservators use a multi-step process that targets both the chemical and biological components of the stains. A typical protocol starts with a pre-rinse to remove water-soluble degradation products, followed by a solution containing a reducing agent and a chelator (a chemical that binds to metal ions and pulls them out of the paper), then an enzyme solution that breaks down fungal residues, and finally a rinse to clear everything away.
For fragile or valuable works that can’t be submerged in water, conservators apply these solutions through specially formulated gels made from polysaccharides like agarose or gellan. The gels deliver moisture and active chemicals in a controlled way without saturating the paper. This approach has expanded treatment options for delicate items like thin printmaking papers or works with water-sensitive inks.
One important caveat: washing and chemical treatments are irreversible. The Smithsonian’s paper conservators note that while the field generally aims for reversible treatments, washing permanently changes the paper’s structure. Conservators weigh the visual improvement against potential risks before proceeding, especially with historically significant works. For a valuable piece, partial reduction of foxing that preserves the paper’s integrity is often preferable to aggressive treatment that might weaken or alter it.
How Foxing Affects Value
For book collectors and print buyers, foxing always lowers value compared to a clean copy. How much depends on several factors: the severity of the foxing, the rarity of the item, and the age of the paper.
A book that’s still in print or easy to find in good condition takes a significant hit from foxing. Collectors can simply find a cleaner copy. For a book with just a few scattered spots, expect to lose roughly a third of the price you’d get for a clean example. Heavy foxing can reduce a common book’s value to a fraction of its potential. One dealer described buying a foxed book, sanding the visible spots off the page edges, and still only being able to charge $15 for what would have been a $100 book.
Rare and antiquarian books get more forgiveness. A 300-year-old volume that’s hard to find in any condition won’t be penalized as harshly, because buyers understand that some deterioration comes with the territory. If most surviving copies of a particular title show foxing, the market adjusts its expectations. But even among rare books, a foxing-free copy will always command a premium over a spotted one.
Preventing Foxing in Your Collection
Since moisture is the primary trigger, controlling your storage environment is the most effective prevention. Keep books and paper in spaces where relative humidity stays between 30% and 50%, with stable temperatures. Avoid attics, basements, garages, and any room without climate control. Good air circulation helps prevent the stagnant, humid microenvironments where fungi thrive.
For individual items you want to protect, archival storage materials make a real difference. Acid-free, lignin-free, buffered folders and sleeves prevent acidic compounds from migrating between items. Polyester film sleeves provide a stable, chemically inert barrier for fragile or valuable sheets. When storing pamphlets or loose pages, wrap them in buffered paper before placing them in folders. The buffering agents in these materials help neutralize acids that develop in aging paper over time.
If you’re storing books on shelves, keep them upright with enough space for air to circulate. Books packed tightly together in a warm room trap moisture between their covers. Periodically inspect your collection, especially during humid seasons, since catching early signs of foxing or mold growth before they spread gives you far more options for preservation.