Exhibition presented by Carole Jeanneret
from the "Open Book" workshop
24 Rue de Lausanne in Fribourg
"The mission of the conservator/restorer: what if your books could last another 100 years?"
From June 3 to 27, 2026 at L'Atelier, Place Notre-Dame 16 in Fribourg
Environmental factors
Ambient temperature (avoid extreme temperatures and especially sudden variations).
Among the determining factors of preservation conditions, climate plays a central role, as it acts directly on materials while influencing all other degradation mechanisms. Temperature, in particular, has a major impact on the rate of chemical reactions, but also on the physical and biological phenomena affecting books and archival documents.
An increase in temperature accelerates chemical degradation processes, particularly the hydrolysis and oxidation of paper components. It also promotes the growth of microorganisms and insects, whose activity is closely linked to thermal conditions. Furthermore, temperature interacts with relative humidity: warm air can hold more water vapor, which can exacerbate humidity imbalances and their effects on materials.
To limit these risks, it is generally recommended to maintain a stable and moderate temperature. In storage areas, a temperature between 16°C and 18°C is advised to slow down degradation reactions while ensuring good long-term preservation conditions. In dining or consultation rooms, where the presence of people necessitates a compromise with thermal comfort, a slightly higher temperature, between 18°C and 22°C, is acceptable.
Beyond absolute values, thermal stability is essential. Rapid or repeated temperature variations can cause materials to expand and contract, leading to mechanical stresses, deformations, and ultimately, structural damage. These fluctuations can also disrupt the moisture balance, further increasing the risk of degradation.
Relative humidity (too much or too little humidity can damage materials).
Relative humidity (RH) is probably the most crucial environmental factor for the preservation of books and archival documents. It plays a decisive role in the majority of degradation processes, whether chemical, physical, or biological. By directly influencing the water content of materials, it affects their stability, mechanical resistance, and the rate of degradation reactions.
As a general rule, a relative humidity of around 45% is recommended for the preservation of collections. However, this value should be adjusted according to the nature of the materials and their state of preservation. For example, a document that has suffered water damage or shows signs of residual moisture should be temporarily stored in a drier environment to limit the risk of biological growth and stabilize its condition.
When relative humidity exceeds 60 to 65%, it promotes the growth of microorganisms such as mold, which can lead to rapid and often irreversible deterioration of supports and media. This biological degradation is frequently accompanied by stains, odors, weakening of materials, and sometimes a loss of information.
Conversely, a relative humidity below 40% causes the materials used in books and documents to dry out. Paper, parchment, leather, and glues lose their flexibility, becoming rigid and brittle, with an increased risk of cracking, warping, or breaking. In some cases, this damage can be irreversible.
It is also worth noting that, for acidic materials, a lower relative humidity—around 30%—can help slow down chemical degradation reactions, particularly acid hydrolysis, and thus significantly extend their lifespan. However, this type of adjustment must be considered carefully to avoid negative effects on other components of the object.
Light and radiation (which can cause discoloration and weaken the materials).
Light is a form of energy capable of triggering or accelerating the chemical reactions responsible for the degradation of paper and, more broadly, all organic matter. A comparison of different light sources shows that their harmfulness varies considerably (see table below).
Table of the influence of light on preservation
Photograph of the painting page 167 of © "DeTutela Librorum" by Andrea Giovannini
Paper sensitivity varies greatly depending on its composition. Papers containing wood pulp, such as newsprint, are particularly sensitive and react very quickly to light due to the high reactivity of lignin. Conversely, pure cellulose papers (without wood) or those made from textile fibers (rags) exhibit lower sensitivity.
Furthermore, some media undergo fading that can transform colored lines into almost invisible ones. Different writing media react very differently to light: printing inks or pencils are relatively insensitive, while many inks and pigments used for writing, decorating texts, or even for dyeing leather and textiles (book covers, endbands), are altered by visible light and even more so by ultraviolet radiation.
Ultraviolet radiation causes paper to yellow and fibers to degrade rapidly. Infrared radiation, on the other hand, heats up materials, leading to paper fragility. X-rays, although less common in conservation settings, are a very energetic form of radiation that can cause significant and rapid deterioration of materials, also contributing to their fragility.
The effects of light are even more damaging in a humid atmosphere (RH > 60%). Furthermore, excessive exposure to light often causes the object to heat up, which significantly accelerates chemical deterioration. It is estimated that the rate of degradation doubles for every 8°C increase in temperature.
Pollutants (dust, gases, particles that can attack surfaces)
Air pollutants are among the most harmful agents of degradation for books and archival documents. The main culprits are sulfur oxides (SO₂), nitrogen oxides (NOₓ), and ozone (O₃). These compounds, present in ambient air due to industrial activities, road traffic, and certain indoor sources, react chemically with the materials that make up the works. In particular, they promote the acidification of paper, leading to weakened fibers, a loss of mechanical strength, and ultimately, irreversible degradation of the material.
Dust is another major factor in deterioration. It tends to become deeply embedded in fibrous materials such as paper or parchment, making complete removal virtually impossible. This penetration leads to lasting alterations in the appearance and integrity of the artwork, thus irreversibly changing its original state. Furthermore, dust can act as a carrier of chemical and biological pollutants, exacerbating the deterioration process.
Some dusts also pose direct risks to human health, particularly when they contain fine particles, mold spores, or toxic substances. Even in the absence of immediate toxicity, prolonged and repeated exposure can lead to serious respiratory effects, including chronic illnesses or cancers linked to the inhalation of particles that penetrate deep into the lungs.
In this context, the use of appropriate protective equipment is essential for conservation and restoration professionals. Wearing filtering masks, gloves, and, if necessary, other protective devices helps to limit health risks during dusting, cleaning, or treatment of artworks.
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