How to Prevent Foxing in Your Rare Book Collection

Foxing — the scattered brown spots that appear on the pages of books over time — is one of the most common and most frustrating forms of paper deterioration that collectors face. While treatment of existing foxing is limited and risky, prevention is highly effective. With proper environmental controls and storage practices, you can dramatically reduce the likelihood that your books will develop foxing during your lifetime of ownership.

Understanding What You Are Preventing

Foxing is caused by two overlapping mechanisms: fungal growth on or within the paper, and chemical oxidation of iron particles embedded in the paper during manufacturing. Both mechanisms are accelerated by the same environmental conditions: high humidity, elevated temperatures, and poor air circulation.

The good news: the same storage conditions that prevent foxing also prevent almost every other form of book deterioration. A properly stored collection is resistant to foxing, mould, insect damage, and accelerated paper degradation simultaneously.

The Environmental Targets

Humidity

Target: 30–50% relative humidity, with 40% as the ideal midpoint.

Humidity is the single most important factor in foxing prevention. The fungal organisms that cause biological foxing require moisture to grow and reproduce. Below 50% relative humidity, most paper-dwelling fungi cannot establish active colonies. Below 40%, fungal growth is essentially impossible.

Conversely, humidity above 65% creates a hospitable environment for fungi and accelerates the chemical oxidation reactions that cause iron foxing. Extended exposure above 70% humidity virtually guarantees foxing development over time.

Key practices:

• Use a digital hygrometer to monitor humidity in your book storage area
• Deploy a dehumidifier if your baseline humidity exceeds 55%
• In dry climates, use a humidifier to prevent humidity from dropping below 25% (extremely low humidity makes paper brittle)
• Avoid storing books in basements, bathrooms, or rooms with exterior-facing walls (all humidity-prone locations)

Temperature

Target: 60–70°F (15–21°C).

Higher temperatures accelerate both fungal growth and chemical degradation. The rate of chemical deterioration roughly doubles for every 18°F (10°C) increase in temperature. This means a book stored at 80°F degrades at roughly double the rate of one stored at 62°F.

Key practices:

• Avoid storing books in attics (extreme heat in summer) or near heating vents
• Maintain consistent temperature — fluctuations cause condensation, which introduces moisture
• In hot climates, air conditioning is a preservation necessity, not a luxury

Air Circulation

Target: gentle, continuous air movement around and between shelved books.

Stagnant air creates microenvironments of elevated humidity and temperature, particularly in tightly packed bookshelves where books touch each other and the shelf backing. These microenvironments are where foxing most often begins.

Key practices:

• Do not pack books tightly on shelves — leave space for air to circulate
• Position bookshelves a few inches away from walls, particularly exterior walls
• Ensure that the room’s HVAC system provides air movement to the book storage area
• Avoid enclosing bookshelves behind glass doors unless the enclosed space has its own ventilation

Shelving and Storage Practices

Shelf Materials

Avoid shelving that releases harmful gases (off-gassing):

• Metal shelving is ideal — steel or aluminium shelves with baked enamel finish do not off-gas
• Sealed wood shelving is acceptable if the wood has been sealed with polyurethane and has had time to cure fully (at least two weeks)
• Raw wood shelving should be avoided — especially oak, which releases acetic acid that accelerates paper deterioration
• Particle board and MDF should be avoided — they off-gas formaldehyde

Acid-Free Materials

Any material that touches your books should be acid-free:

• Acid-free tissue for wrapping fragile books or interleaving between plates and text
• Acid-free boxes (Solander boxes, clamshell boxes) for the most valuable items
• Mylar dust jacket covers to protect dust jackets from handling and environmental exposure

Arrangement

• Shelve books upright with enough support to prevent leaning (use bookends)
• Large, heavy books (folios, atlases) should be stored flat, not upright, to prevent spine strain
• Do not stack more than two or three books on top of each other when storing flat
• Keep books away from direct sunlight and fluorescent lighting (UV accelerates all forms of deterioration)

Quarantine Practices

When you acquire a new book — from a dealer, auction, estate sale, or used bookshop — quarantine it before adding it to your collection:

Visual inspection. Examine the book under strong light for any signs of active foxing, mould, or insect damage. Pay particular attention to the endpapers, the edges of the text block, and the pages facing any tipped-in plates.

Smell test. A musty or mouldy smell indicates active or recent fungal activity. A book that smells musty should be isolated until it has been thoroughly aired and confirmed free of active growth.

Isolation period. New acquisitions should be stored separately from your main collection for one to two weeks. This allows any dormant fungal spores to manifest and any insect stowaways to be detected before they can spread.

Environmental acclimation. If the book was stored in conditions very different from your own (a damp British bookshop, a hot attic), allow it to acclimate gradually to your storage environment. Rapid changes in temperature or humidity can cause condensation and actually trigger the deterioration you are trying to prevent.

Special Cases

Books with Existing Foxing

If you acquire a book that already shows foxing, the foxing itself is generally stable — it will not spread to other books on your shelf. However, if the foxing was caused by active fungi, those fungi could potentially colonise adjacent books in humid conditions. The safest practice is to store foxed books in acid-free enclosures and maintain the environmental conditions described above.

Tropical and Subtropical Climates

Collectors in humid climates face a particular challenge. In regions where outdoor humidity routinely exceeds 70%, climate control is not optional — it is the single most important investment you can make for your collection. A dedicated air-conditioned room with a dehumidifier, maintained at 40–50% RH and 65–70°F, is the minimum standard.

Seasonal Considerations

In temperate climates, the transition seasons (spring and autumn) can produce humidity spikes as weather patterns shift. Monitor your hygrometer closely during these periods and adjust your dehumidifier settings as needed.

The Long View

Foxing prevention is a long-term commitment, not a one-time intervention. The books in your collection may outlive you by centuries — many already have. Maintaining proper environmental conditions year after year, decade after decade, is the single most effective action you can take to preserve your collection’s value and physical integrity.

The investment is modest: a hygrometer ($20–$50), a dehumidifier ($150–$300), acid-free materials ($50–$200 per year), and the discipline to monitor and maintain conditions. Compared to the value of the books themselves, this is a trivial cost with an enormous return.