What Is Foxing? Causes, Prevention, and Treatment for Brown Spots on Book Pages

Foxing is the term used for the small, brownish or reddish-brown spots that appear on the pages of old books, prints, and documents. The word likely derives from the colour’s resemblance to fox fur, though some etymologists trace it to “ferrous oxide” (iron staining). Foxing is one of the most common condition issues encountered in books from the eighteenth through early twentieth centuries, and understanding its causes, prevention, and treatment is essential for any book collector.

What Does Foxing Look Like?

Foxing appears as small, irregular spots ranging from pinpoint-sized to several millimetres in diameter. The spots are typically:

• Brown, reddish-brown, or yellowish-brown in colour
• Scattered randomly across the page (not following text or image lines)
• Often more prevalent near the edges of pages
• Sometimes denser in specific gatherings or sections of a book
• Visible on both sides of the affected leaf

Foxing ranges from light (barely visible) to heavy (obscuring text or images). In severe cases, spots may merge into larger brown areas.

What Causes Foxing?

Despite extensive study, the exact causes of foxing are not fully resolved. Two primary mechanisms are recognised, and most foxing probably involves elements of both:

Fungal Growth

Microscopic fungi — particularly species of Aspergillus and Penicillium — can colonise paper under conditions of elevated humidity. The fungi produce metabolic byproducts that stain the paper brown. Fungal foxing tends to be:

• Circular or roughly circular spots
• More common in books stored in damp conditions
• Sometimes accompanied by a musty odour
• Potentially active (continuing to spread) if conditions remain favourable

Iron Impurities

Iron particles present in the paper from the manufacturing process (from water supplies, rags, or machinery) can oxidise over time, creating small rust-coloured spots. Iron-related foxing tends to be:

• Smaller, more pinpoint-like spots
• Related to specific paper batches (some pages heavily foxed, adjacent pages clean)
• Not dependent on storage conditions (though humidity accelerates oxidation)

Contributing Factors

Paper composition. Certain paper stocks are more susceptible to foxing than others. Paper made from rag or containing sizing agents with organic components may be more vulnerable to fungal colonisation. Paper manufactured with water containing dissolved iron is more susceptible to iron foxing.

Humidity. Elevated humidity (above 65% relative humidity) promotes both fungal growth and iron oxidation. Books stored in damp basements, attics, or climates with high humidity are much more prone to foxing.

Air circulation. Stagnant air promotes both humidity buildup and fungal growth. Books tightly packed on shelves with no air circulation are at greater risk.

Paper pH. Acidic paper (common from the mid-nineteenth century through the mid-twentieth century, when wood-pulp paper and alum-rosin sizing were standard) is more susceptible to degradation processes including foxing.

Effect on Value

Foxing reduces a book’s value, with the severity determining the impact:

Light foxing — scattered minor spots confined to endpapers or preliminary pages. Impact: modest (5–15% reduction in value). Described as “minor foxing” or “occasional foxing.”

Moderate foxing — noticeable spots throughout the text block. Impact: significant (15–30% reduction). Described as “foxing throughout” or “moderate foxing.”

Heavy foxing — dense spotting affecting readability or appearance. Impact: substantial (30–50%+ reduction). Described as “heavy foxing” or “pervasive foxing.”

Foxing on plates or illustrations is particularly damaging to value, as visual material is affected more directly than text.

For very rare books (incunabula, early printing, unique copies), foxing is more readily tolerated because clean copies may not exist.

Prevention

Environmental Control

The single most effective prevention is maintaining proper storage conditions:

• Temperature: 65–70°F (18–21°C)
• Relative humidity: 30–50% (consistently; fluctuation is harmful)
• Air circulation: Ensure air moves around bookshelves; avoid packing books tightly
• Light: Store away from direct sunlight and fluorescent light

Dehumidification

In humid climates or seasons, a dehumidifier in the storage area prevents the conditions that promote fungal foxing.

Isolation

If you acquire a book with active mould (fuzzy, powdery spots that can be rubbed off), isolate it from the rest of your collection until the mould is treated. Active mould can spread to adjacent books.

Periodic Inspection

Check stored books periodically for early signs of foxing or mould. Early detection allows you to address environmental conditions before damage becomes severe.

Treatment

What You Can Do

Improve storage conditions. If the foxing is environmentally caused, moving the book to proper conditions will halt further development.

Dry active mould. If you see active mould (fuzzy growth), isolate the book, allow it to dry in a well-ventilated area, and gently brush off surface mould with a soft, dry brush outdoors.

What a Professional Conservator Can Do

Washing. A paper conservator can wash affected pages in deionised water, which reduces foxing stains and removes soluble degradation products. Washing also deacidifies the paper, slowing future deterioration.

Bleaching. Controlled chemical bleaching can further reduce foxing stains. However, bleaching must be done carefully — over-bleaching damages paper fibres and can affect printing ink. Only a trained conservator should attempt bleaching.

Deacidification. Treating acidic paper with alkaline solutions neutralises acids and creates an alkaline reserve that protects against future degradation.

What You Should NOT Do

• Do not attempt to bleach pages yourself. Household bleach and peroxide solutions damage paper and can cause irreversible harm.
• Do not attempt to erase foxing spots. Rubbing creates paper damage.
• Do not use commercial cleaning products. These are not designed for historical paper and may cause chemical damage.
• Do not ignore active mould. It will spread to adjacent books and worsen over time.

Describing Foxing in Book Trade Terms

Standard descriptions for foxing in bookseller catalogues and listings:

• “Light foxing to endpapers” — minor spots limited to the endpapers
• “Scattered foxing” — intermittent spots throughout
• “Moderate foxing to prelims” — noticeable spots in the preliminary pages
• “Foxing throughout” — spots across most or all pages
• “Heavy foxing” — dense spots that significantly affect appearance
• “Some foxing as usual for this title/period” — acknowledges that the specific book or paper type is characteristically prone to foxing