Foxing — Causes, Prevention, and Treatment of Brown Spots on Paper

Foxing — the characteristic brown, reddish-brown, or yellowish-brown spots that appear on the pages of books, prints, maps, and other paper-based materials — is one of the most common and recognizable forms of paper deterioration. The term has been used since at least the mid-nineteenth century, though its precise etymology is debated (it may derive from the fox-like color of the spots, or from an obsolete meaning of “fox” as a verb meaning “to become discolored”).

What Foxing Looks Like

Foxing spots vary in appearance:

• Size — from tiny pinpoint dots to spots several centimeters in diameter
• Color — typically brown to reddish-brown, sometimes yellowish or grayish
• Distribution — can be scattered randomly across a page, concentrated in specific areas (particularly margins and blank spaces), or appear in patterns related to the paper’s structure
• Shape — usually roughly circular but can be irregular
• Density — ranges from a few isolated spots to heavy freckling that covers entire pages

What Causes Foxing

Despite over a century of study, the exact causes of foxing remain debated among paper conservators and scientists. The current consensus is that foxing results from multiple interacting causes, not a single mechanism.

Fungal Growth Theory

One school of research holds that foxing is caused by microscopic fungal growth on the paper surface. Evidence supporting this theory includes:

• Foxing spots sometimes show fungal hyphae (thread-like structures) under microscopic examination
• Foxing is more common in papers stored in high-humidity environments, which favor fungal growth
• Some foxing spots fluoresce under ultraviolet light in patterns consistent with biological growth

Metal Particle Theory

Another well-supported theory attributes foxing to metallic impurities in the paper — particularly iron particles introduced during the papermaking process. Evidence includes:

• Iron particles have been identified at the center of some foxing spots using electron microscopy and X-ray spectroscopy
• The brown color is consistent with iron oxide (rust) formation
• Papers with higher concentrations of metallic impurities tend to develop more foxing

Combined Mechanism

The most widely accepted current view is that foxing typically involves both mechanisms working together: metallic impurities in the paper create sites where moisture accumulates and fungal growth is initiated, with the resulting biological and chemical activity producing the characteristic brown staining.

Contributing Factors

Several environmental and material factors promote foxing:

High humidity — the most important single factor. Papers stored in environments with relative humidity consistently above 65% are at much higher risk of foxing.

Temperature fluctuation — cycling between warm and cool temperatures causes moisture to condense on paper surfaces, promoting both fungal growth and oxidation of metallic impurities.

Paper quality — papers with higher levels of metallic impurities, sizing agents, or residual chemicals from the manufacturing process are more susceptible.

Air quality — atmospheric pollutants (sulfur dioxide, nitrogen oxides) can accelerate the chemical reactions that cause foxing.

Storage conditions — books stored in damp, poorly ventilated spaces (basements, attics, closets against exterior walls) develop foxing at much higher rates than books in climate-controlled environments.

Prevention

Environmental Control

The single most effective way to prevent foxing is to maintain stable, appropriate environmental conditions:

• Relative humidity — 45–55% RH is the recommended range. Below 40%, paper becomes brittle; above 60%, mold and foxing risk increases dramatically.
• Temperature — 65–70°F (18–21°C) is ideal. Cooler is generally better than warmer, but stability is more important than the exact temperature.
• Air circulation — good air circulation prevents the stagnant, humid microclimates in which foxing thrives. Do not pack books tightly against walls or in enclosed cabinets without ventilation.
• Monitoring — use a hygrometer to monitor humidity levels in your storage area. Inexpensive digital hygrometers are widely available.

Dehumidification

In climates or seasons where humidity consistently exceeds 60%, a dehumidifier is a worthwhile investment. For small collections, desiccant packets (silica gel) placed in enclosed bookcases can help control local humidity.

Material Quality

When acquiring books, be aware that:

• Nineteenth-century papers (particularly those made from mechanical wood pulp) are especially prone to foxing and acidic deterioration
• Handmade rag papers (common in books before approximately 1850) are generally more resistant
• Modern acid-free papers are highly resistant to foxing

Treatment

Professional Conservation

For valuable books, prints, or documents, foxing treatment should be performed by a qualified paper conservator (a member of the American Institute for Conservation, or equivalent professional body). Professional treatment options include:

Aqueous washing — immersing the paper in water (sometimes with added agents) can reduce foxing stains. This is one of the most effective treatments but can only be performed on papers that are stable enough to withstand immersion.

Chemical bleaching — controlled application of bleaching agents (hydrogen peroxide, calcium hypochlorite, chloramine-T, sodium borohydride) can lighten or remove foxing stains. Bleaching must be carefully controlled to avoid weakening the paper or causing color change. Over-bleaching is irreversible and can make paper brittle.

Enzyme treatment — enzymatic solutions can break down organic components of foxing stains without the risks of chemical bleaching. This is a newer and less widely available treatment.

Light bleaching — exposing the paper to controlled sunlight or UV light can lighten foxing. This method is slower and less aggressive than chemical bleaching but also carries less risk of paper damage.

What Not to Do

Do not attempt home treatment on valuable items. Common home remedies — bleach, lemon juice, sunlight, erasers — are more likely to damage the paper than to improve its appearance.

Do not accept “minor foxing” in high-value books without considering the market impact. While foxing does not usually worsen dramatically, its presence affects desirability and value.

Foxing and Book Value

Market Impact

Foxing affects the value of rare books and prints to varying degrees:

Light foxing (a few small spots, often confined to the margins or blank pages) — reduces value modestly, typically 10–20% below a comparable copy without foxing. Many collectors accept light foxing as a normal feature of old paper.

Moderate foxing (widespread spots affecting text pages or illustrations) — reduces value more significantly, perhaps 30–50%. Collectors of earlier material (pre-1850) are generally more tolerant than collectors of modern first editions.

Heavy foxing (dense spotting that obscures text or images) — severely affects value and desirability.

Tolerance Varies by Period

• Incunabula and early printed books (pre-1600) — foxing is expected and tolerated; these books are collected for their historical significance regardless of moderate foxing
• Eighteenth- and nineteenth-century books — foxing is common and moderately tolerated
• Modern first editions (twentieth century onward) — foxing is considered a more serious defect because collectors expect cleaner paper

In Catalog Descriptions

Standard terminology:

• “Foxed” — spots are present
• “Light foxing” or “occasional foxing” — mild, limited foxing
• “Some foxing to prelims” — foxing confined to the preliminary pages (endpapers, half-title, title page area)
• “Heavily foxed” — extensive foxing throughout
• “Foxing to plates” — foxing affecting the illustrations (particularly problematic)

Good catalog practice requires honest and specific disclosure of foxing — its location, severity, and whether it affects text or illustrations.

Foxing is one of those inevitable realities of collecting material made from organic fibers. It cannot always be prevented and cannot always be removed. But with proper storage conditions, it can be minimized — and with a realistic understanding of its causes and treatment options, it can be managed effectively.