Damage from physical stress. High relative humidity can cause the growth of mold and fungi.
The most significant aspect of temperature is its effect on relative humidity (RH). Relative Humidity is a measure of the amount of moisture in the air relative to the amount the air is capable of holding, expressed as a percentage. If the air at a particular temperature contains half the water vapor it can hold at that temperature, the relative humidity is 50%. Acute changes in temperature and humidity will cause swelling and contraction as the materials in an object or artifact attempt to adjust to the environment. Objects are often composed of more than one type of material. Each material responds differently to water vapor in the air and adjusts to its particular EMC (equilibrium moisture content) at different relative humidities. Of particular concern are the internal stresses created by expansion and contraction of the different materials as moisture diffuses into or out of the surrounding air.
Three different forms of deterioration caused by humidity:
|Dimensional Change||warping, dislocation of joints, splitting, breaking of fibers, delamination, loss of surface material, cracking|
|Chemical Reaction||corrosion of metals, fading of dyes, weeping or crizzling glass (clouding), cystallization and movement of salts, disintegration and yellowing of paper|
|Biodeterioration||mold growth (RH 70% or above), bacteria|
At the Museum, objects are stored and displayed in stable relative humidity and temperature to minimize the risk of damage. When prepared for transport, objects are packed carefully using insulating foams and barrier films (to control variations in temperature and humidity) in custom constructed crates.
What is being done in the Museum to control temperature and humidity levels?
Sophisticated air handling units are the best protection against sharp fluctuations in temperature and humidity. The Museum utilizes a complex network of air handling systems that regulate temperature and humidity levels throughout the entire building. A computer controlled HVAC system maintains 70° temperature/50% humidity environmental condition year round in the galleries and storage areas. Each area within the Museum contains a sensor which will trip an alert if the levels go out of the normal range (±2°). In the case of an alert, building maintenance crews can react quickly to resolve any problems. This provides invaluable environmental protection for the Museum’s collections.
Humidity buffering materials such as silica gel can be used to control relative humidity within a closed microenvironment like a display or storage case.