Question: A hardcover book is stacked between other books on a shelf in a library. The relative humidity of the library has been steady at 30% for the last few months. Suddenly, the relative humidity jumps to 50% and remains there. Approximately how long does it take for the book to fully adjust (equilibrate) to the new relative humidity of the environment?
- One hour
- One day
- One week
- One month
- One Year
Answer: To find the answer, read on...
The importance of climate conditions in the preservation of museum and library collections is well recognized. Researchers at IPI have stated that, “for a large variety of museum objects and archival records, heat and humidity are the primary factors that govern the occurrence and rate of biological decay, chemical instability and mechanical damage.” To this effect, recommendations for environmental management often appear as low and high limits, specifying appropriate average levels of temperature and RH as well as the acceptable range of fluctuations. These recommendations generally give preference to “flat lines” of temperature and relative humidity in the name of avoiding damage from fluctuating environments. However, even with the best environmental controls, periodic climate changes (daily or seasonal) are part of collection reality. In order to begin to understand what these fluctuations in temperature and relative humidity might mean to the collections, we must first consider how collection materials respond to changes in the environment and what changes the objects actually “feel”. In other words, we must understand how the objects equilibrate with the conditions of the environment.
First, let’s consider how an object interacts with the temperature and relative humidity of its environment. At any given moment, an active exchange of energy (heat) and moisture (if the material is hygroscopic) is occurring between the object and its environment. The temperature of the environment determines how much energy (heat) is available to be exchanged, while the relative humidity of the environment determines how much moisture is available for this exchange. Eventually, the object will reach a point in this exchange when it is neither gaining nor losing heat (because it has adjusted to environment’s temperature) and it is neither gaining nor losing moisture (because it has adjusted to the environment’s relative humidity). At this point, the object has reached a state of equilibrium with the climate conditions of its environment.
Given enough time, any object will equilibrate to any environmental condition. However, as we all know from our experience using modern refrigerators, the object will not adjust (equilibrate) to new conditions instantaneously. For example, if we place a bottle of water in the refrigerator, it will not become cold right away. It could take minutes, or even hours, for the water inside the bottle to cool down to the temperature inside the fridge. The same is true for museum objects. When placed in a new environment, or when the environmental conditions change, it takes time for the materials to reach the new temperature and humidity conditions.
How long does it take for an object to adjust to a change in climate conditions, to equilibrate to a new environment? What factors are involved in determining this rate of equilibration? To explore the processes of equilibration, we will consider equilibration with temperature (thermal equilibration) and equilibration with relative humidity (moisture equilibration) separately.