Mould on Books — Prevention, Identification, and Treatment

Mould can destroy a book faster than almost any other agent of deterioration. Given the right conditions — warmth, humidity, and an organic food source (paper, leather, cloth, adhesives) — mould colonises a book within days and can render it unsalvageable within weeks. The good news is that mould is almost entirely preventable through environmental control.

What Causes Mould on Books

Mould spores are omnipresent in the air. They require three conditions to germinate and grow:

Moisture. Relative humidity above 65% enables mould growth. Above 70%, growth accelerates dramatically. Water-damaged books are at acute risk.

Warmth. Most book-damaging moulds thrive between 70–90°F (21–32°C). Growth slows below 60°F and stops below freezing.

Food. Paper, leather, parchment, cloth, paste, and sizing are all organic materials that provide nutrients for mould. Books are, in effect, food.

Control any one of these factors and mould cannot grow. The most practical factor to control is humidity.

Identifying Mould

Mould on books may appear as:

• White or grey fuzzy patches on covers, endpapers, or page edges
• Black, green, or orange spots or patches
• Powdery or dusty coating on leather bindings
• A musty smell — the characteristic “old book smell” is partly mould metabolic byproducts
• Staining that does not wipe away with a dry cloth

Active vs. dormant. Active mould is soft, fuzzy, and may smear when touched. Dormant mould (from a past infestation that has dried out) is dry, powdery, and does not spread. The distinction matters for treatment.

Prevention

Prevention is straightforward and far easier than treatment:

Maintain relative humidity below 55%. Below 50% is better. Below 45% is excellent. Use a dehumidifier in the collection room and monitor with a hygrometer.

Ensure air circulation. Stagnant air pockets create microclimates of higher humidity. Leave space behind bookshelves for air movement. Do not pack books tightly against exterior walls.

Avoid problem locations. Basements, attics, bathrooms, kitchens, and unventilated closets are high-risk environments. If you must store books in a basement, use a commercial dehumidifier running continuously.

Inspect new acquisitions. Books from estate sales, storage units, garage sales, and damp environments may carry mould. Inspect them before shelving alongside your collection. Quarantine suspect books.

Clean periodically. Dust promotes mould growth by trapping moisture. Brush the tops of books and shelves regularly.

Emergency Response — Active Mould

If you discover active mould on a book:

Isolate immediately

Remove the affected book from the shelf. Place it in a sealed plastic bag or a clean plastic container. This prevents spore dispersal to neighbouring books.

Inspect neighbours

Check books on either side and above/below the affected book. Mould spreads by contact and airborne spores. Isolate any suspect volumes.

Identify the moisture source

Mould does not appear without elevated moisture. Find the cause: a leak, a failed dehumidifier, a humid season, or a book that was already damp when acquired. Fix the source before treating the symptom.

Dry the book

If the book is damp, air-dry it with pages fanned open in a well-ventilated area. A fan circulating air across the book accelerates drying. Direct sunlight kills mould but also fades dust jackets and bleaches paper — use it only briefly if at all.

Remove surface mould

Once the book is completely dry:

1. Work outdoors or in a well-ventilated area (wear a dust mask — mould spores are a respiratory hazard)
2. Use a soft brush to gently sweep mould from covers and page edges
3. Brush away from the spine and into a waste container
4. For interior pages, interleave with clean paper towels and brush gently
5. Wipe cloth and leather covers with a dry, clean cloth

Do not use water, bleach, Lysol, or any liquid cleaner. These either feed the mould or damage the book.

Sunlight treatment

Brief exposure to direct sunlight (15–30 minutes) can kill remaining active mould. Do not leave books in sunlight longer — UV damage is cumulative and permanent. This is a crisis measure, not a routine practice.

Treating Mould Staining

Mould often leaves permanent staining even after the active organism is removed. The stains (purple, brown, black, or grey) are metabolic byproducts that have penetrated the paper fibers.

Home treatment: Limited. Once the mould is dead and removed, the stains are cosmetic. They do not spread and do not endanger other books.

Professional treatment: A paper conservator can sometimes reduce mould staining through aqueous washing and selective bleaching. This is expensive and only justified for valuable books.

The Musty Smell

The characteristic musty smell of mouldy books is caused by volatile organic compounds (VOCs) produced by mould metabolism. Removing the smell:

• Activated charcoal: Place the book in a sealed container with an open container of activated charcoal for several days. The charcoal absorbs odour compounds.
• Baking soda: Similarly, place the book in a sealed container with an open box of baking soda.
• Fresh air: Prolonged airing in a dry, well-ventilated space helps.
• Cat litter (unscented): Some collectors report success with clean, unscented clay cat litter in a sealed container.

These methods reduce but may not eliminate a persistent odour. Books with severe mould damage may smell permanently.

When to Consult a Conservator

Seek professional help when:

• The book is valuable and the damage is significant
• Mould has penetrated deep into the textblock (not just surface)
• Structural damage (weakened paper, detached bindings) has occurred
• You are dealing with a large-scale outbreak affecting many books

The American Institute for Conservation (AIC) maintains a searchable directory of qualified book and paper conservators.

Mould vs. Foxing

Mould and foxing are often confused but are different phenomena:

Mould is a living organism that grows actively in humid conditions. It appears as fuzzy or powdery growths and can spread rapidly.

Foxing is brown spotting caused by a combination of fungal deposits and iron impurities in paper. It is a stable condition — the spots do not grow or spread. Foxing typically affects paper from specific manufacturing periods and mills.

Both reduce a book’s condition grade, but mould is an active threat that requires intervention, while foxing is a stable cosmetic issue.