Temperature and Humidity
Besides light, there are additional environmental concerns, such as temperature and humidity, that can adversely affect a collection. For every 18 degree F (10 C) increase in temperature, it is estimated that chemical reactions in paper double. Relative humidity is a measure of the capacity of air to hold water. This amount varies as temperatures increase or decrease. Paper and other porous materials either absorb or lose moisture as temperature and humidity levels vary. This action causes shrinking, stretching, and the eventual breakdown of structural fibers, while contributing to formation of acids. The effect is similar to the cracking, splitting, and weaknesses that result when an outdoor wooden deck is left unprotected, though on a microscopic level.
While the ideal temperature and relative humidity levels for proper storage of paper are yet to be agreed upon, consistency seems to be the key factor. The best advice is to treat your collection like one of the family. Hot attics and damp basements make poor living quarters; they also make poor storage facilities. Even a closet that abuts an outside wall may be exposed to a large range of temperature and RH fluctuations over the course of a year.
When properly monitored, the combination of heating and air conditioning equipment, as well as humidifiers and dehumidifiers, allows the maintenance of a stable climate. There are also a variety of tools (with a variety of price levels and degrees of sophistication) that can assist in the monitoring. When fluctuations can be controlled, acid formation and mechanical degradation can be slowed significantly.
In the absence of expensive equipment, RH levels can be controlled and stabilized with the help of desiccants such as Silica Gel. Silica Gel is a porous granular, chemically inert amorphous silica that can absorb 40% to 50 % of its own weight in water. The material comes in several forms including reusable canisters, beads, sheets, and packets. Because it can become fully saturated, Silica Gel must be monitored and reconditioned when saturation occurs. One form of Silica Gel changes from orange to a pale pink to indicate it has reached the saturation level. Again, a proper schedule of monitoring your collection should be maintained to achieve a stable environment.
Having addressed the problems associated with fluctuations in temperature and humidity, it is equally important to address the more obvious problems which occur with constant extremes. The combination of high temperature and relative humidity promotes mold growth and encourages insect infestation, whereas a cold and dry environment leads to embrittlement. As you may have guessed, neither scenario is going to improve the condition of your collection.
Acceptable temperatures in your storage area should remain lower than 68 degrees F, with relative humidity between 30 and 50%. Fluctuations should not exceed +/- 5 degrees F in temperature, and +/- 3% relative humidity within a 24 hour period.
Most museums and libraries strive to achieve the ideal conditions described above, but they may fall under the same restraints that you and I are likely to come across. Lack of time, expertise, and money can prevent us from achieving ultimate conditions. In addition, each of our individual geographic locations can present problems unique to that area. All we can do is strive for improvement.