Incorrect Relative Humidity

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This is part of a Preventive Care series about the Ten Agents of Deterioration.

Definition[edit | edit source]

Relative Humidity is the amount of moisture in the air compared to how much moisture that air can hold at the same temperature. Relative humidity is calculated by the following equation:

[math]\displaystyle{ %RH = Absolute Humidity of Sampled Air X 100 \lt br\gt Absolute Humidity of Saturated Air at Same Temperature }[/math]

Therefore, if the absolute humidity of sampled air = 8.5g
Absolute humidity of saturated temperature at 68 degrees F = 17.0g
[math]\displaystyle{ %RH = 8.5 X 100 = 50% 17.0 }[/math]

Organic materials all contain moisture; they absorb and give off moisture and try to find a balance between their moisture content and that in the air around them. If the relative humidity (moisture content) in the air goes up, they will absorb moisture and swell, and if it goes down, they will give off moisture and shrink. If this occurs slowly and moderately then no damage will be caused. However, sudden, large and frequent relative humidity fluctuations can cause shrinkage, warping, splitting, and general aging of objects made of organic materials. A sudden increase in relative humidity can cause condensation on metal artifacts, which will promote corrosion.

While different types of collections have substantially different relative humidity requirements and so it is hard to give specific set points as they will be highly dependent on the type of collections, some general recommendations for museum environments are as follows:

  • The optimum relative humidity range for general museum objects is 45-55% ±5% daily even if seasonal drifts in trends cannot be avoided.
  • Fluctuations that may result in physical damage should be avoided.
  • High relative humidity can accelerate chemical deterioration and promote mould growth (for example, above 60-65%).
  • Low relative humidity may cause shrinking, warping, cracking, embrittlement, and desiccation (for example, below 40%).
  • Metals generally benefit from RH levels that are as low as possible.
  • Organic artifacts require more moderate RH levels to prevent desiccation or embrittlement.
  • For composite artifacts (those made out of two or more materials) or artifacts like furniture, it is most essential to keep levels moderate and stable.

Artifacts most at risk are composite objects (those made of two or more different materials), and thin organic objects. Different materials in composite artifacts will react to relative humidity changes at different rates and to different degrees. Since these different materials are attached to each other, they will either push or pull at each other, which may cause damage. Thin organic objects respond very quickly to relative humidity changes. An example is a paper artifact taped around the edges to a card support. The paper will react much more rapidly to changes than the material it is attached to. Therefore, when the paper wants to shrink, it can only do so by splitting.

A variety of different environmental control strategies can be used depending on the situation – at the macro environment level this includes control of air temperature within a building (with or without passive relative humidity control), room level temperature control (e.g. radiators with window mounted a/c units), or entirely passive control based on the buffering effects of the building envelope (e.g. door seals).

Controlling RH at the building or room level can be very expensive, and if not done properly may actually end up causing structural damage. However, RH can sometimes be very effectively, and relatively cheaply controlled by creating a micro-environment around the specimen by a combination of well-sealed storage cabinets or exhibit cases and by using buffering materials, such as acid free tissue, wooden drawers, and passive environmental measures such as silica gel.

Even if there is limited ability to control the environment in buildings or rooms, it is a good idea to know what conditions your collection is being subjected to, so problems can be anticipated or micro-climate level solutions can be devised. Monitoring environmental conditions can be done using equipment such as Heating Ventilation and Air Conditioning (HVAC) and/or Building Management Systems (BMS), reasonably priced electronic data-loggers or recording hygrothermographs.

Resources and Further Reading[edit | edit source]