Climate can be controlled through various means, including heating, ventilation, and air conditioning (HVAC) and through passive means when HVAC is not available.
One of the most important factors affecting the longevity of library and archival materials is the environment in which they are stored, used, and displayed. Research has demonstrated the damage that can be done by temperature, humidity, light, and pollutants. Controlling these effects is the highest priority of any preservation program.
In general, the higher the temperature the more rapidly materials will deteriorate, and the lower the temperature the longer materials will last. However, because people must have access to library and archival materials, there is a practical limit to how low the temperature can be maintained. Ideally, the temperature should be closely controlled at 20° to 21° Centigrade (68° to 70° Fahrenheit) and consistently maintained in all areas of a storage facility. This temperature range is regarded as tolerable to staff and users and appropriate for most materials. Keep in mind, however, that standards for temperature as well as for relative humidity are based on conditions in temperate climates and may be unattainable in very dry and humid tropical areas. If temperature cannot be controlled because of inadequate mechanical systems, other preservation measures can be taken (see Passive Climate Control).
The term relative humidity (RH) refers to the percentage of water vapor in the air at a given temperature. When the temperature is high, air can contain a great deal of water in the form of vapor, but when the temperature drops, vapor condenses into water. In other words, temperature is closely linked to relative humidity. Because cooler air cannot hold as much vapor as warmer air, a rapid lowering of temperature can result in condensation, causing absorbent materials such as paper to become damp. The ideal RH range for most paper-based library collections is between 30 and 50%, preferably closer to 30% where deterioration occurs more slowly.
Recent research about energy use suggests that it is permissible to heat to cooler temperatures in winter weather. The lower temperatures reduce moisture in paper-based collections, slowing deterioration, while saving money on fuel.
Exposure to intense light causes fading (especially of inks and colors), darkening and yellowing (especially of paper containing wood and lignin), and the weakening of fibers. Both sunlight and artificial light (especially fluorescent) are sources of ultraviolet rays, the most harmful of the light wavelengths. However, all direct light is damaging to some degree. Materials are at their most vulnerable when exposed tto light as a result of long-term display or when stored under strong, constant light, for example, in front of a window.
The amount of light is measured in lux, or footcandles, with one lux (or lumen) equivalent to 0.09 footcandles. A 150-watt incandescent light bulb will produce 50 lux at a distance of one meter. In storage areas, lights should always be turned off when the area is not in use; when turned on, lights should be limited to 100 lux unless materials are covered. Materials on exhibition should generally not be subjected to light stronger than 80 lux. In addition, ultraviolet shields should be employed if the lights are fluorescent. Window light should be filtered through screens or blinds.
Pollutants are generally in the form of gasses and particulates. While most pollution enters a building from outside, pollution can also be caused by construction materials, paints, untreated wood, particleboard, and plastics, which emit gases that are harmful to paper. These include sulfur dioxide, carbon monoxide, formaldehyde, and a wide variety of industrial gases. Pollution also occurs in the form of tiny solids, or particulates, such as grit, dust, and smoke. Small dust particles are especially damaging, and HVAC filters must be changed regularly to ensure that air does not bypass the filter when it is clogged. Pollution can be controlled through heating, ventilating, and air cooling (HVAC) systems. HVAC systems filter incoming air to remove particulates and gasses.
HVAC and the use of dessicants
HVAC systems, the primary weapon for preserving library and archival materials, also regulate humidity and temperature and suppress mold growth, performing these tasks consistently throughout storage areas. Libraries and archives have found that the best conditions are maintained by constant-volume, all-air HVAC systems whose filtration, dehumidification and humidification, maintenance, and monitoring tasks are handled by central (rather than distributed) stations. For those libraries and archives that are having HVAC systems installed or built in, it is important to consider specifications that will produce a continuous conservation environment.
Some library materials require more stringent storage conditions than are required for paper-based materials. Motion picture film, color film, and microfilm, for example, must be kept in a cool, dry environment. In this case, even if HVAC is not practical for a larger storage space, a small vault or cabinet with good controls should be used. Such a cabinet should maintain a low temperature, around 20°C. More important is the level of humidity, which should not exceed 35 percent. This can only be achieved through the use of a desiccant such as silica gel. Silica gel can be conditioned to absorb humidity within a tightly closed cabinet. When the silica gel becomes saturated, the humidity can be heated out and the gel used again.
The most common type of film material that libraries must store is microfilm. When microfilm is produced to archival standards, three generations of film are created, with the original camera negative regarded as the archival copy. It is imperative that the negative be protected by appropriate storage.