A Brief Overview of Temperature and Moisture Equilibration

Moveable StorageThis issue of Climate Notes presents some previously covered ground—temperature and moisture equilibration—that the collections care community continues to grapple with:

  • What really happens to collection materials when short-term changes in temperature or relative humidity occur?
     
  • How long does it take for fluctuations in temperature and relative humidity to impact preservation quality?
     
  • What impact do enclosures have on how soon objects "feel" a change in temperature or RH? 

The December 2011 issue of Climate Notes reported on acceptable relative humidity fluctuations and the moisture-buffering capacity of enclosures. Based on preservation research over the past two decades, that article stated that:

  • Most objects react to changes in temperature quickly—often in a matter of hours, and enclosures don’t make much difference. Adjustment time varies according to the material and the storage configuration (amount of exposed surface, amount of thermal mass).
     
  • The amount of heat (temperature level) in the environment is generally more significant to preservation than fluctuations in temperature.
     
  • Hygroscopic materials react to changes in relative humidity slowly and enclosures can make a difference. Adjustment times vary according to the objects material, size and surface exposure, the materials used for and the construction of enclosures, storage configuration, and the temperature.
     
  • Periods of sustained high or low humidity (seasonal or other slow, gradual changes) are much more significant for preservation than short term fluctuations.

Enclosures:

Sleeves, folders, boxes, cabinets, exhibit cases, etc.

The amount of material inside the enclosure and the storage configuration affect the impact of enclosures.
 

Fluctuations:

May be caused by moving material from one environment to another, mechanical equipment failure, gradual seasonal changes, or intentional daily or weekly T & RH setting changes.

Thermal equilibration (the process of adjusting to the temperature of the environment) is relatively fast—generally occurring in a matter of hours. Although enclosures and housings offer physical protection for objects, they don’t necessarily protect the objects from temperature change. While some enclosures act as moisture barriers (slowing down moisture equilibration), they aren’t a significant barrier to fluctuations in temperature.

Evaluating the impact of relative humidity fluctuations is more difficult. Moisture equilibration is influenced by more variables than thermal equilibration. Enclosures and storage configurations influence the rate at which objects equilibrate to changes in RH. The ambient temperature also influences the rate of moisture equilibration. Despite these variations, moisture equilibration is consistently slower than temperature equilibration. Real-life storage conditions of museum and library materials – books stacked tightly on shelves, objects stored in boxes, cabinets and other protective housings – may act as moisture barriers and slow the rate of moisture equilibration even further.

The information provided in Climate Notes is based primarily on research undertaken by IPI over the last few decades. Studies undertaken in the area of equilibration are highlighted in this issue.