Conserve an antique book: humidity, light, temperature (professional guide)

The preservation of an old book depends on three measurable parameters that you can assess at home with €30 worth of equipment: relative humidity, light, and temperature. When controlled, these three factors determine whether your red and gold Hetzel will survive another two centuries or join the ranks of warped bindings and foxed pages. Below are the thresholds used by the BnF (Bibliothèque nationale de France) and IFLA, adapted for a private library without climate-controlled display cases or a museum budget.

Why old books degrade — and exactly how

Rag paper, parchment, leather bindings, and animal glues are hygroscopic. They absorb and release moisture in response to ambient air. With each cycle, fibres swell and then contract, joints weaken, and gilt edges crack. Conservators call this mechanical fatigue. Two chemical reactions finish the job: acid hydrolysis breaks down cellulose chains (accelerated by humidity) and oxidation yellows and then browns the paper (accelerated by heat and light).

Sven Arrhenius’s rule, adopted by conservation laboratories: for every 5 °C increase, the rate of chemical degradation roughly doubles. Lowering the temperature in your library from 26 °C to 16 °C increases a book’s life expectancy by a factor of four — without touching the binding. It’s the single most cost-effective step you can take to preserve your volumes.

Relative humidity: aim for 45–55 %, monitor stability

This is the trickiest parameter because it’s invisible. The BnF recommends 50 % RH ± 5 % for paper, leather, and parchment in its stores. IFLA allows a wider range, 30–50 % RH depending on the material, with a maximum fluctuation of 5 % over 24 hours. For a private library, aiming for 45–55 % RH is a practical compromise year-round.

What happens outside these limits:

• RH above 65 %: active mould within weeks, brown stains, warped boards, and attacks by booklice and silverfish.
• RH below 35 %: brittle, flaking leather, shrunken parchment, animal glues failing at the headbands, paper breaking when turned.
• Sudden swings: the worst scenario. A shift from 45 % to 70 % RH overnight (summer storm, open window) can permanently deform a binding, sometimes irreversibly.

A useful rule of thumb: a 1°C variation causes approximately a 3 to 4% change in relative humidity in an enclosed space. Regulating the thermometer is already regulating the hygrometer.

Temperature: 16 to 20 °C, stability above all

IFLA standards converge around 16 to 18 °C for storage areas. In a lived-in library, aiming for 16 to 20 °C is manageable without turning your living room into a cold storage room. The key is not the absolute value, but stability. A room that swings from 18 °C at night to 26 °C during a summer day causes more damage than a constant 14 °C cellar.

The classic trap in metropolitan France: the boiler kicking in during October heats the living room to 22 °C, warm air absorbs more moisture, RH drops to 30%, and leathers crack. Simple solution: a small humidifier and a thermostat capped at 19 °C in the library room during the heating season.

Light and UV: 50 lux for reading, zero UV when stored

The BnF sets an exposure limit of around 50 lux for sensitive graphic materials, equivalent to low, indirect light. Beyond this, pigments in illustrations, inks, and paper yellow irreversibly. Field rule: never direct light on a stored book, and a maximum of 50 to 100 lux for reading.

Ultraviolet rays remain the number one enemy. They degrade cellulose, yellow paper within months, and fade Romantic-era bindings like those by Hetzel for Jules Verne. UV comes not only from sunlight but also from fluorescent lights and some cheap LEDs.

Three steps account for 80% of the work:

• UV-filtering adhesive film on the library windows (UV filtration above 99%, lifespan of about 8 to 10 years).
• Warm LEDs with a colour rendering index above 90 and zero UV emission, used only as indirect light.
• Blackout curtains drawn during the day if the room faces south or west.

Target values for a private library

Essential equipment for a home library

No need to turn a cabinet into a heritage storage space. With €150 to €400, you can cover the essentials:

• Digital hygrometer-thermometer (€15 to €40): place it at the centre of the library, never near a radiator. Models with min/max memory are recommended to detect overnight peaks.
• Silica gel sachets (€5 to €15 per ten): neutral, to be placed at the back of shelves. Absorption capacity of about 30% of their weight, regenerable in an oven at 120°C. Useful when relative humidity exceeds 60% regularly.
• Adhesive UV-filtering films for glazing (€15 to €30/m²): self-adhesive, blocking over 99% of UV rays.
• Portable humidifier or dehumidifier (€80 to €250): essential in humid coastal areas or with aggressive central heating.
• pH-neutral archival boxes for fragile items (manuscripts, loose-leaf booklets, Japanese or Chinese papers).

Mistakes that silently destroy books

Over the course of estate appraisals (see our guide to libraries after death and succession), the same pitfalls recur:

1. The cellar — damp, poorly ventilated, cool but unpredictable. Relative humidity often above 70% year-round: a breeding ground for mould.
2. The attic — extreme temperature fluctuations (near 0°C in winter, over 35°C in summer under the roof), condensation, and direct insect access through the framework.
3. A south-facing library behind a large window — UV rays, radiant heat, and bleaching of spines and bindings within a single summer.
4. A radiator against the shelf — chronic dryness for the bottom row, leather shrinking and cracking at the headbands.
5. Books packed tightly together — no air circulation, an ideal environment for foxing and brown spotting. Leave a centimetre between each spine and never tilt a volume.
6. Rainwater leaking through a forgotten window — a single storm can turn two centuries of care into a disaster. Never store books beneath a window.
7. The plastic bag "for protection" — it traps moisture and suffocates leather. Opt for acid-free paper or a neutral cardboard box instead.

Annual routine for a serious collector

1. Weekly checks of the hygrometer (recording min/max readings) in a notebook.
2. Gentle dusting every six months with a soft sable brush. Never use a vacuum cleaner directly on headbands.
3. Seasonal inspection of low and high shelves (most likely spots for mould or insect infestations).
4. Regeneration of silica gel every 3 to 6 months depending on climate.
5. Replacement of UV-filtering films every 8 to 10 years (due to adhesive polymer degradation).
6. Annual binding audit: sagging headbands, cracked hinges, undulating parchment — all signs to consult a professional conservator before further damage.

Special cases: parchment, leather, photographs

The parchment is the most temperamental material. It warps as soon as relative humidity exceeds 55% and shrinks below 40%. For parchment manuscripts, aim for a strict 50% RH and store flat in a conservation box. Leather (morocco, calf, basan) requires a slightly higher RH (50–55%) to remain supple. Dressing with leather balm should be occasional and light: excess grease will stain the boards and darken the leather. Old photographs and albums need a lower RH (30–40%) — incompatible with bound books. Do not store them side by side.