TSG Chapter III. Environmental Concerns for Textiles - Section A. Temperature & Relative Humidity (RH)

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Temperature and Relative Humidity

Introduction

Definitions

Temperature

“A degree of hotness or coldness measured on one of several arbitrary scales based on some observable phenomenon (as the expansion of mercury).” (Webster's 3rd Ed, 2355)

Relative Humidity

Amount of water in a given quantity of air, divided by the maximum amount of water that the air can hold at that temperature, times 100 (Thomson, 1986, 68.)

Recommendations

• Currently textile conservators tend to advise ranges for temperature (60–70º F/15–20º C) and RH (40–60%) rather than single target figures. Slow, seasonal change is recommended to transition between seasonal ranges.
• Textiles are often part of composite artifacts or are stored in mixed collections and thus recommendations must consider various materials. Specific homogeneous subgroups of textiles may benefit from narrow ranges for temperature and RH.

Factors to Consider

Relationship between temperature and RH

• For a fixed amount of moisture in a fixed space, the higher the temperature, the more water the air can hold, so the relative humidity will be reduced as the temperature rises. For a fixed amount of moisture in a fixed space, the lower the temperature, the less moisture the air can hold; the humidity relative to its capacity to hold moisture is increased, so the relative humidity will rise.
• Natural fibers absorb moisture quickly and desorb moisture slowly. The rates towards equilibrium with the relative humidity are functions of the type of fiber; generally absorption to equilibrium occurs within hours and desorption to a lower relative humidity equilibrium is in weeks unless energy is expended. In other words, natural fibers act as their own buffers to changes in relative humidity.
• Natural fibers approaching equilibrium with relative humidity exhibit a hysteresis effect— a retardation of an effect when the forces acting upon a body are changed (Merriam-Webster.com, 2017); the amount of moisture in the fiber at the equilibrium point will be different depending upon whether equilibrium is approached from a point above it or below it. Typically the moisture content of the fiber at the point of equilibrium will be higher when the fiber has previously been wet (100%) and lower when the fiber has been bone dry (0%).

Responsiveness of textile materials to changes in temperature and RH

• Fibers gain or lose moisture to the environment.
• Rapid cycles of high and low humidity cause the most damage over time. The risk of damage is greater with short and extreme cycles. Slow, seasonal change, within recommended ranges, is acceptable.
• Severely degraded textiles are at greater risk of damage from low RH than less degraded textiles.
• Changes in RH can cause changes in the dimension and shape of the textile/fiber and can catalyze chemical reactions, causing damage.
• Higher temperatures cause greater rates of deterioration by adding energy to accelerate chemical reactions.

Responsiveness of associated materials to changes in temperature and RH

Associated materials that are part of a textile (e.g., paint, embroidery, beads) may respond at different rates to changes in temperature and RH.

• Metals are extremely responsive to high RH, as seen in the rusting of metal hangers or the accelerated corrosion of copper alloy buttons.
• Plastic, paper, pigments, binders, etc. may have RH sensitivities that must be considered.

Mounts and stabilization materials associated with textiles may respond at different rates to changes in temperature and RH. For example, a wood strainer will react differently from the textile to which it is attached. When a textile is restrained either externally (by a mount or strainer) or internally (by construction, materials, adhesives, repairs, etc.), wide fluctuations in RH that cause significant shrinkage of the textile are likely to cause irreversible damage.

• Hygroscopic storage materials buffer the textiles stored within them from changes in RH.
  

Role of temperature and RH on other conditions that may affect preservation of textiles (mold growth, insect infestation, pH, etc.)

• Mold growth: At 68º F (20º C), RH above 65% and lack of air circulation are likely to promote mold growth. However, if mold growth is already established, or if a textile has a high equilibrium moisture content, then growth can occur at lower RH. See III.D. Biological Attack.

• Insect infestation: RH above 65% provides an environment hospitable to most insect pests.
• pH: High temperatures and/or high RH can accelerate chemical reactions. If textiles are in contact with materials that are incompatible in terms of their respective pH values (e.g., cotton fabric in contact with acidic wood products, dyed silk fabric in contact with potash glass beads), undesirable reactions may occur, especially in the presence of high temperature or RH.

Monitoring Temperature and RH

• Inspect conditions in storage and display areas at regular intervals.
• Measure and record temperature and RH in collections spaces either continuously (using a hygrothermograph or data loggers) or at regular intervals (using a hygrometer or psychrometer).
• Choose a measuring or measuring/recording device that is accurate, use it properly, check it regularly — ideally against a psychrometer, the industry standard — and calibrate it regularly.
• Analyze recorded conditions to check that climate management systems are working properly or to make recommendations for improvements.

Environmental Control Methods

HVAC (Heating, ventilating, and air conditioning) systems

• HVAC system capacities, controls, and zones should be designed to accommodate the recommended conditions for collections, building type and age, and building use.

• If the building’s HVAC system has a logging capacity it should periodically be checked with an independent monitoring device (See 3.b) above).

• If the HVAC system has a humidification component, the method of introducing humidity should be carefully designed to avoid mold/bacteria build-up or potential damage from boiler treatment chemicals.

• It is worthwhile to engage the services of a building engineer, preferably one with conservation environment experience, to find cost-effective, energy-saving methods to achieve the best environment possible for collections and buildings.

Heating

Using only heat to control the environment for human comfort usually results in unacceptably low RH for collections.

Air conditioning

Air conditioning will reduce heat and high humidity but may not necessarily achieve the recommended RH range for museum collections.

• The climate, building envelope, and air conditioning equipment influence how effective air conditioning will be in moderating temperature and RH.

• Conditions must be monitored to know if air conditioning is achieving desired goals.

Humidifiers and dehumidifiers

Local humidifiers and/or dehumidifiers can be used to moderate the RH. Note that local humidification or dehumidification is not very effective if the building environment is controlled by a centralized HVAC system.

Humidifiers used for individual rooms add moisture to the air using mechanical methods such as the evaporative method or ultrasonic method. Evaporative systems are thought to be simplest and safest and are somewhat self-regulating. Follow manufacturer’s recommendations for periodic maintenance, which is necessary to prevent buildup of impurities or microorganisms.

Types of dehumidifiers (Note that dehumidifiers both remove water from the air and require constant monitoring. It is best if dehumidifiers can be plumbed into the building’s sewer system.)

Desiccant-type dehumidifiers use a desiccant material (i.e., silica gel) to remove (adsorb) moisture from the air, thereby lowering the RH. The system then regenerates (dries) the desiccant to prepare it for the next cycle. Water is collected and drained.

Refrigerant-type dehumidifiers employ a cooling system (chiller) to reduce the temperature of the air thus causing moisture to condense out. Water is collected and drained.

In temperate climates, electric fans can improve the environment by circulating the air and thereby minimizing microclimates that may promote growth of microorganisms.

Microclimates can be created to slow or buffer changes in temperature and RH.

• Microclimates can be created with active (requiring electricity) or passive (using buffering media) systems.

• Enclosures (cases or containers) can be made of buffering media, such as paper.

References

"Hysteresis." Merriam-Webster.com. Merriam-Webster, n.d. Web. 21 Feb. 2017.

Thomson, G. 1986. The museum environment. 2d ed. London: Butterworths.

Webster’s Third New International Dictionary, Unabridged, 3d edition. 1981. Springfield, Mass.: G.C. Merriam.

Further Reading

Appelbaum, B. 1991. Guide to environmental protection of collections. Madison, Conn.: SoundView Press.

Arenstein, R.P. and S. Alderson. 2011. Datalogger applications in monitoringthe museum environment, part I: Comparison of temperature and relative humidity dataloggers. Conserve O Gram 3/3 (June). Washington, D.C.: National Park Service. https://www.nps.gov/museum/publications/conserveogram/03-03.pdf (accessed Feb 21, 2017).

Ballard, M. W. 1992. Climate and conservation. Textile Specialty Group postprints. American Institute forConservation 20th Annual Meeting, Buffalo. Washington, D.C.: AIC. 31–45.

Ballard, M. W. 1995. Mechanical properties: Preview and review. Textile Conservation Newsletter 28 (Spring): 14–28.

Ballard, M. W. 1997. More on moisture: Cohesive, temporary, or permanent set and hygral expansion. Textile Conservation Newsletter 32 (Spring): 5–20. Canadian Conservation Institute. 1992. Textiles and the environment. CCI Notes 13/1. Ottawa, Ontario, Canada: Canadian Heritage.

Ballard, M. W. 1993: Environmental monitoring kit. CCI Notes 2/4. Ottawa, Ontario, Canada: Canadian Heritage.

Cassar, M. 1995. Environmental management: Guidelines for museums and galleries. London: Routledge.

Craddock, A. B. 1992. Control of temperature and humidity in small collections. In Conservation concerns: A guide for collectors and curators. Ed. K. Bachmann. Washington, D.C.: Smithsonian Institution Press.

Cumberland, D. R., Jr. 1993. Calibration of hygrometers and hygrothermographs. Conserve O Gram 3/2 (July). Washington, D.C.: National Park Service. https://www.nps.gov/museum/publications/conserveogram/03-02.pdf (accessed February 21, 2017).

Erhardt, D., M. F. Mecklenberg, C. S. Tumosa, and M. McCormick-Goodhart. 1995. The determination of allowable RH fluctuations. WAAC Newsletter 17 (1) (January): 19–23.

Finch, K., and G. Putnam. 1977. Caring for textiles.New York: Watson-Guptill Publications.

Fuzek, J. F. 1985. Absorption and desorption of water by some common fibers. Industrial & engineering chemistry product research and development 24 (1):140–144.

Giuntini, C. 1992. Storage of historic fabrics and costumes.In Conservation concerns: A guide for collectors and curators. Ed. K. Bachmann.Washington, D.C.: Smithsonian Institution Press.

Guldbeck, P. E. 1985. The care of antiques and historical collections. 2d ed. Revised and expanded by A. B. MacLeish. Nashville: American Association for State and Local History.

Keene, S. 2002. Managing conservation in museums. Oxford: Butterworth-Heinemann.

Kilby, V. 1993. Using a psychrometer to measure relative humidity. Conserve O Gram 3/1 (July). Washington, D.C.: National Park Service. https://www.nps.gov/museum/publications/conserveogram/03-01.pdf (accessed February 21, 2017).

Landi, S. 1992. The textile conservator’s manual. 2d ed.Oxford: Butterworth-Heinemann.

Mailand, H. F., and D. S. Alig. 1999. Preserving textiles: A guide for the nonspecialist. Indianapolis:Indianapolis Museum of Art.

Meredith, R. 1960. Effect of moisture on mechanical properties. In Moisture in textiles, eds. J. W. S. Hearle and R. H. Peters. New York: Textile Book Publishers. 160–177.

Ogden, S. 2000. Temperature, relative humidity, light, and air quality: Basic guidelines for preservation. Andover, Mass.: Northeast Document Conservation Center.

Orlofsky, P. 1992. Textile conservation. In Conservation concerns: A guide for collectors and curators. Ed. K. Bachmann. Washington, D.C.: Smithsonian Institution Press.

Sandwith, H., and S. Stainton, comps. 2006. The National Trust manual of housekeeping: The care of collections in historic houses open to the public. London: Butterworth-Heinemann.

Stolow, N. 1987. Conservation and exhibitions: Packing, transport, storage and environmental considerations. London: Butterworths.

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