Temperature and Relative Humidity (RH)
Temperature and Collections
The detrimental effects of incorrect temperatures (too high or too low) result in slow deterioration that may only evident with the passage of time and thus the results may be underestimated. High temperatures accelerate chemical reactions driving up the rate of overall degradation of organic materials. High temperatures can also cause desiccation in organic materials, leading to loss of flexibility and cracking. While most collections could benefit from cooler storage temperatures than those normally used in museums, temperatures that are too cold can cause embrittlement, hazing and cracks in some materials. Cultural institutions often must compromise between the temperatures that are best for the preservation of the collection, what is economical in terms of equipment and energy policies, and what is best for the comfort of staff, researchers, and visitors.
Humidity and Collections
Relative humidity (RH) is the amount of water vapor present in air expressed as a percentage of the greatest amount the air can absorb at that temperature. Many hygroscopic objects benefit from RH levels that are moderate and stable. In these cases, it is generally recommended to maintain museum environments within the range of 40-60% while attempting to minimize dramatic swings of more than 5% within a 24-hour period, even if broad seasonal trends are hard to avoid. More sensitive objects will require specific and tighter RH control, depending on their composition, condition, and history. For example, objects with metal components may benefit from low RH levels that inhibit oxidation.
In addition to temperature and humidity extremes, rapid fluctuation presents risk to collections. Most objects are composed of multiple materials, each with its own rate of thermal expansion and contraction. Rapid changes in humidity will cause minute movement in these materials relative to one another, which can cause physical damage such as cracking or delamination.
A variety of different strategies can be used to control the collection environment. 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 done improperly, can cause structural damage. Relative humidity can sometimes be more effectively and affordably controlled by enclosing sensitive objects inside a micro-climate using one or more of the following:
- Well-sealed storage cabinets or exhibit cases
- Buffering materials such as acid free tissue
- Conditioned silica gel
Even with limited environmental control, it is useful to monitor the collection environment so that problems can be detected, and solutions can be devised. Monitoring can be conducted through heating ventilation and air conditioning (HVAC) and/or building management systems (BMS),or using dataloggers or recording hygrothermographs.
Temperature and Relative Humidity in Collections
Organic Collections: Zoology, Botany, and Material Cultural
Most zoology collections, botanical collections, and collections of material culture constructed from plant and animal materials benefit from a moderate storage environment. For skin, fur, feathers, taxidermy, osteological materials, and dry specimen collections, low humidity levels will cause desiccation, while high humidity can result in mold or fungal attack. Cooler storage temperatures inhibit pest infestation.
Fluid-preserved specimens also benefit from moderate and stable environmental conditions. High temperatures accelerate deterioration in specimens and evaporation of ethanol, while low temperatures may cause fats and lipids to solidify, leaving deposits on specimens. Temperature fluctuations cause screw-on lids to loosen due to differential expansion in glass and plastic.
Inorganic Collections: Paleontology, Geology, and Material Culture
Though inorganic minerals, fossils, and material culture derived from them are less sensitive to temperature and RH than the collections described above, extreme conditions and severe fluctuations still pose risks for these materials. In geology and paleontology collections, incorrect temperatures can lead to changes in the crystalline structure of minerals, and temperatures that are too cold can promote thermal shock, in which a specimen becomes brittle and cracks or shatters.
High RH can promote oxidation and corrosion of certain minerals, such as iron pyrite, leading to “pyrite disease”. In shells (marine and egg), high humidity can accelerate “Byne's disease”, which occurs when acid vapors in the environment attack the shell and form salts. Low RH can cause shrinkage in absorbent minerals such as shale. Fluctuations in RH may result in fractures and crumbling as a specimen alternately absorbs or releases moisture, causing swelling or shrinkage, and can lead to extensive surface and structural damage in porous materials such as stone and ceramic that have absorbed soluble salts from burial or other sources, as the salts crystalize and deliquesce with fluctuations in humidity.
Inappropriate temperature and humidity can also have important impacts on associated materials used in the preparation, repair, or storage of inorganic collections. For example, high temperatures can promote the physical aging and deterioration of adhesives used in repair of collections and fossil preparation, while high relative humidity can lead to mold growth on collections, labels, and storage containers.
The National Park Service Conserve-O-Gram series has several documents that deal with practical topics relating to environmental monitoring of collections including:
- Using a Psychrometer to Measure Relative Humidity
- Calibration of Hygrometers and Hygrothermographs
- Datalogger Applications in Monitoring the Museum Environment, Part I: Comparison of Temperature Relative Humidity Dataloggers
The Canadian Conservation Institute Notes offer practical advice about issues and questions related to the care, handling, and storage of cultural objects, including the CCI Environmental Monitoring Equipment Loans Program.
Appelbaum, Barbara. 1991.Guide to Environmental Protection of Collections. Madison, Connecticut: Sound View Press.
Bachmann, Konstanze . 1992.Conservation Concerns: A Guide for Collectors and Curators, Washington DC: Smithsonian Institution Press.
Thomson, Garry. 1986.The Museum Environment, 2nd ed. London: Butterworth-Heinemann.