Understanding Deterioration

Appropriate and effective environmental specifications should be based on an understanding of the role of temperature and RH on material decay. Deterioration occurs through three basic modalities: chemical change, biological decay, and mechanical or physical damage.

Chemical change in collection objects arises in response to heat energy (temperature) and available moisture. Because the change occurs from spontaneous chemical reactions, this type of deterioration is also known as Natural Aging. One form of chemical change is metal corrosion, which is primarily driven by high RH. Other forms of spontaneous chemical change include discoloration, embrittlement, and fading of organic materials (paper, plastics, leather, dyes, textiles, etc.). Temperature and RH combine to control the rate of undesirable chemical reactions. Temperature matters more than RH in the sense that great benefit can result from very cool temperatures and great harm can result from very warm temperatures. The warmer it is and the higher the moisture content of collections, the faster they deteriorate. Chemical change is a slow process—what matters most are the long-term average temperature and RH. Short-term fluctuations are not usually significant nor does any harm result from transitioning from one rate of change to another (in other words, there is no penalty for variations per se.)

Biological decay is caused by living creatures, most notably mold and insects. Of course, living organisms depend on biochemical processes, so there are some similarities in the kinds of environments that promote chemical change and those that drive biological decay. Temperature does help determine the likelihood and severity of biological attack, but the major environmental factor is RH. Both molds and insects thrive at moderate temperatures and elevated RH. Biological decay is an opportunistic threat, so it is best to avoid the environmental conditions that would allow the organisms to thrive.

Mechanical or physical damage is a form of decay that affects hygroscopic organic materials (those that absorb significant amounts of moisture). Relative humidity is the environmental factor that governs the moisture content of collection objects. At low moisture contents, objects shrink and become brittle. At high moisture contents, they swell and soften. Both dryness and dampness can lead to stresses and deformations, cracks, tears, and delamination. Another type of mechanical damage risk is excessive excursions back and forth from dryness to dampness, causing microcracks to widen with each cycle. For fine and decorative arts collections, mechanical decay is often considered the most important risk, but because many objects and finishes are made of organic materials, chemical change should also be considered a major risk.