Most objects in museums collections are hygroscopic. This means they
have an inherent moisture content, and consequently can change with
their environment. When humidity rises, the object will take on, or
absorb, moisture causing an expansion and when the humidity lowers, the object will release, or evaporate, moisture causing a contraction of the object. A continual expansion and contraction can cause a strain on the structure of the object. In addition, many objects may be made up of more than one material that may react differently; for example, one may absorb more moisture during high humidity. Long-term fluctuations in the environment can cause structural damage to objects. For these reasons, a stable environment is more important than struggling to maintain a specific humidity (museum optimum range is between 45 to 55%, often difficult in tropical environments).
Museums keep a close eye on their environmental controls. In the home this isn’t as easy, since many of us turn the air down at night and up during the day. This constant change is a fluctuation in the environment. It is important to understand the more barriers placed around an object, the slower the rate of change will be within the object. A framed object will be less affected than one exposed directly to the environment. There are also additional conservation products that can help in the home to create a microclimate. For example, a silica / desiccant pack can be put in a case or cabinet to absorb excess moisture when the humidity increases.