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Humidity in Museums
By Betty Stephens
The majority of artifacts in historic houses are made of natural organic materials: wood, leather, traditional adhesives (rendered down animal or vegetable matter), natural textiles, paper products, fur, bone, ivory. Observation of the reaction of such artifacts to variations in relative humidity over the last eighty or so years has become progressively codified into recommendations for ‘optimum’ humidity levels for the preservation of these artifacts. The concern is that humidity should not fall to a level which causes brittle failure of organic artifacts, nor rise to a point where mold growth can flourish.
Environmental conditions affect objects in many ways. Some objects are vulnerable in conditions that may not affect other objects at all. Some attempts to improve conditions for an object might actually affect it adversely. For example, moving an object from a poor environment to a theoretically better one might cause severe mechanical damage. There is no one-size-fits-all pattern for good environmental control strategies, nor is there likely to be, no matter how good HVAC engineering becomes. Understanding how environmental factors affect collections helps conservators make consistently good choices.
Damage
The biological threat directly related to humidity is mould growth. Mold is always undesirable in a collection. It causes irreversible and often devastating damage. Mold spores are naturally present in the air around us, and it is impossible to eliminate them. However, it is only when the mold spores have sufficient nutrients, time, and moisture that they grow into destructive mould. As suitable nutrients are readily available in almost any environment, the route to controlling mold is to control moisture. Without moisture, mold spores cannot grow.
Relative Humidity
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 humidity.
Safe Humidity
The safe humidity boundary given to prevent mould growth is 65%. Below the safe humidity boundary, mould will not grow at any temperature. In contrast, mould is very likely to grow at high humidity. For example, at 85% humidity, mould will probably appear in less than a week.
Control of room humidity to a lower limit of 40%RH, it may be good for artifacts, is a potential source of damage to buildings exposed to cold winters. In most of the northern hemisphere above 35° N latitude, heating to the lower range of modern human comfort levels (18°C/65°F) reduces internal humidity to below 20%RH for at least part of the winter. The humidity can be restored by using machines to add moisture to the indoor air (humidification), but this procedure may raise the dew point of the internal air above the external air temperature. In this case, when the warm humid indoor air meets building elements cooled by contact with the outdoor climate (walls, roofs, windows), water will condense out of the air leading to rotting of wooden elements, mold growth on interior finishes, corrosion of metal elements, and masonry damage which can quite rapidly reduce building elements to the point where renovation must be performed.
Ideal Relative Humidity Levels in Museums
Some objects require controlled microenvironments. For example some metal objects are displayed at low relative humidity to slow the rate of corrosion. Other materials, such as high-fired ceramics, are less sensitive to relative humidity.
Museums Control Temperature and Humidity
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 trips 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.
