Foxing, those small brown or yellowish-brown spots scattered across old paper, can often be reduced at home with gentle surface cleaning methods. Complete removal, however, typically requires professional conservation treatments involving chemical solutions that are beyond what most people can safely do themselves. The approach you choose depends on how valuable the paper is and how severe the staining has become.
What Causes Foxing
Foxing has two interrelated causes, and understanding them helps explain why the stains are so stubborn. The first is chemical: tiny iron particles and other metal impurities embedded in paper during manufacturing slowly oxidize over time, producing rust-colored spots. The second is biological: fungi colonize the paper fibers, often targeting areas already weakened by that chemical attack.
The fungi involved don’t need to be alive to keep causing damage. Research on Leonardo da Vinci’s self-portrait found that compounds left behind by fungi, particularly organic acids and metabolic byproducts, continue reacting with cellulose long after the organisms have died. These residues undergo a browning reaction (similar to what happens when food browns during cooking) that deepens the stains over decades. Fungi also produce oxalic acid, which directly degrades cellulose fibers and leaves behind tiny calcium oxalate crystals sometimes visible under magnification. This is why foxing isn’t just sitting on the surface. It’s woven into the paper’s chemistry.
How to Tell Foxing From Mold
Before treating anything, confirm you’re actually dealing with foxing and not active mold. Foxing spots are small, roughly circular, and tend to be distributed fairly evenly across the page. They’re flat to the touch and range from tan to reddish-brown. Active mold, by contrast, usually clusters along the outer edges of pages or near the spine where moisture collects. Fresh mold may look fuzzy or powdery and can feel slightly raised. If you see fuzzy growth, the paper needs to be isolated and dried before any cleaning, since brushing active mold will spread spores.
What You Can Do at Home
For light foxing on paper that isn’t particularly valuable, gentle dry cleaning is the safest starting point. You won’t eliminate deep stains this way, but you can improve the paper’s appearance and remove surface grime that makes foxing look worse.
Two tools work well for this. Vulcanized rubber sponges (sometimes called soot sponges or dry cleaning sponges) are soft, crumbly sponges originally developed for cleaning soot after fires. They pick up surface dirt without abrading the paper. Use a straight up-and-down dabbing motion rather than rubbing side to side. The second option is vinyl eraser crumbs. A white vinyl eraser like the Staedtler Mars Plastic, grated or crumbled into small pieces, can be gently worked across the surface to lift dirt. You can also cut the eraser into a wedge or point to clean around specific spots.
A few important warnings: avoid colored erasers, which can leave pigment behind. Skip electric erasers entirely, as they’re too aggressive for old paper. And stay away from erasing compounds marketed for architects or graphic designers. These are formulated for modern drafting paper, not fragile historical documents.
Sunlight Bleaching
Some collectors use indirect sunlight to lighten foxing. Placing the paper in bright but indirect light for a few hours can reduce the appearance of mild stains. This carries real risks, though. UV exposure weakens cellulose fibers over time and can fade inks and pigments. If you try this, limit exposure to short sessions and never place paper in direct sun.
Hydrogen Peroxide
Dilute hydrogen peroxide (the 3% solution sold at pharmacies) is sometimes recommended online for foxing. It can lighten surface stains when dabbed on with a cotton swab, but it also bleaches the surrounding paper unevenly and may weaken fibers. On anything with handwriting, printing, or watercolors, the risk of damage is high. If you try it on low-value paper, test on an inconspicuous corner first and work in small areas.
What Professional Conservators Do
For valuable books, prints, maps, or artwork, professional treatment is worth the cost. Conservators use a multi-step process that targets the different components of foxing: the iron oxidation, the fungal residues, and the acid degradation.
The most effective professional protocol starts with a pre-rinse in pH-adjusted deionized water to pull out water-soluble degradation products. Next, a solution combining a chemical reducing agent with a chelator is applied. The reducing agent reverses some of the oxidation that causes browning, while the chelator binds to metal ions (like iron) so they can be rinsed away. Research has found that specific combinations of these chemicals outperform others. After an intermediate rinse, an enzyme solution is applied to break down the biological component, specifically the fungal cell walls and residues embedded in the fibers. A final rinse removes all chemical residues.
This protocol can be done as a full immersion bath for sturdy paper, or as a controlled gel application for fragile items. The gel approach uses agarose (a seaweed-derived gelling agent) to hold the active chemicals against the paper surface without saturating it. This gives the conservator precise control over which areas get treated and how much moisture the paper absorbs.
The Role of Deacidification
Removing visible stains is only half the problem. If the paper remains acidic, foxing will return. Deacidification neutralizes the acids in and on the paper, then deposits an alkaline reserve to buffer against future acid formation.
Calcium hydroxide is one of the most widely used deacidification agents because calcium bonds well with cellulose and remains stable over long periods. Conservators sometimes use it in nanoparticle form, where the extremely high surface area allows better penetration and more thorough neutralization than traditional solutions. Magnesium-based alternatives exist but have been linked to yellowing of treated paper over time, making calcium compounds the preferred choice for most applications.
For heavily oxidized paper, a chemical reducing step using sodium borohydride may precede deacidification. This compound reverses oxidation damage and can produce a noticeable whitening effect. However, it reacts aggressively with acidic paper, producing hydrogen gas that can damage fragile inks or media. Conservators neutralize the paper first to avoid this reaction.
Risks of DIY Chemical Treatment
The reason conservators train for years is that every step in these protocols can cause irreversible damage if done incorrectly. Water-based treatments on paper with fugitive inks will cause the ink to bleed or vanish. Uneven wetting creates tide lines, those dark rings at the edge of a wet area, which are essentially new stains. Over-bleaching weakens fibers to the point where the paper becomes brittle. And applying alkaline solutions without proper buffering can cause discoloration or chemical burns to the cellulose.
If the paper matters to you, even sentimentally, a conservator consultation is the safest path. Many will do an initial assessment for a modest fee and can tell you whether treatment is likely to help or whether the risks outweigh the cosmetic improvement.
Preventing Foxing From Getting Worse
Whether or not you treat existing spots, storage conditions determine whether foxing progresses. The Smithsonian Institution recommends keeping paper-based collections at 35 to 65°F and 30 to 50 percent relative humidity. The humidity threshold matters most: fungi need moisture to grow, and the chemical reactions that cause browning accelerate in damp conditions.
Practical steps that make a real difference include storing paper in acid-free folders or boxes, keeping items away from exterior walls and basements where humidity fluctuates, and ensuring some air circulation around stored materials. Sealed plastic bags trap moisture and create exactly the micro-environment fungi thrive in. If you live in a humid climate, a dehumidifier in your storage area is one of the single most effective investments you can make. Even paper that already has foxing will deteriorate more slowly in a cool, dry, stable environment.