Northern States Conservation Center

Providing collection care, preservation and conservation treatment services to collectors and collecting institutions.


Collections Caretaker

Vol. 2 No. 2 Fall 1999 Temperature and Relative Humidity
How temperature and relative humidity affect collection deterioration rates

by Helen Alten

Are you working in collection storage in a T-shirt or huddled and shivering with layers of clothes? If you are comfortable, your collection may not be. Worse, it could be deteriorating. Temperature and relative humidity levels or fluctuations can be the biggest cause of environmental damage to a collection.

Temperature is the outward manifestation of the amount of energy contained within an object. At higher temperature atoms and molecules move faster. Because they are moving faster, chemical reactions occur more quickly. Thus, higher temperature increases the rate of decay, a chemical reaction. For most materials, the rate of decay is unacceptably fast at temperatures humans find comfortable.

Relative humidity (RH) governs the amount of moisture contained in materials at equilibrium with the environment. This is almost independent of temperature. As relative humidity changes, the object's water content adjusts to the new relative humidity level, creating a new equilibrium. At higher RH, there is more water in objects. This occurs slowly, depending on the thickness and absorbency of the material. For example, experiments by the Image Permanence Institute (IPI) in Rochester, NY show that a book on a shelf will take four to six weeks to equilibrate in the center to a one-time change. The outside layers of the book equilibrate quickly and the middle is slowest to change.

To predict how an item might deteriorate, one must know the type of collection, its composition, how it decays and how it is stored or exhibited. No single environment is ideal for all objects.

Temperature and relative humidity affect three decay processes: chemical, biological and mechanical.

  • Chemical deterioration, sometimes called "natural aging," is when a chemical reaction occurs, causing damage to an object. Chemical deterioration includes metal corrosion, increased fading and glass decomposition from high relative humidity levels. Plastics and organic materials have inherent and spontaneous deterioration reactions whose rate is determined by temperature and relative humidity. Increases in either temperature or relative humidity speed the deterioration.
  • Biological deterioration is damage caused by living organisms such as insects, bacteria and mold. Relative humidity and temperature levels determine whether these organisms flourish or exist at all.
  • Mechanical deterioration is related to either the amount of water absorbed by organic materials, or thermal expansion in inorganic materials, especially metals. The item changes size and shape, leading to cracking, splitting and warping. Stress or the application of different materials will affect the susceptibility of a piece to mechanical deterioration when the environment fluctuates.

Chemical Deterioration
Chemical reaction rates increase with higher temperature, increased concentration of reactants and increased pressure. Water is a reactant in many decay processes. Increased relative humidity increases the concentration of water.

The Image Permanence Institute has researched the chemical decay of archival materials, especially film and photographs. Not everything lasts the same amount of time. Each material has a unique decay rate. For example, among papers, rag paper is slow and newspaper is fast, but both decay rates increase with higher temperature and relative humidity.

Because of IPI's work, acetate film is the best-studied historical stored material. As the film deteriorates, vinegar, or acetic acid, is formed. A noticeable vinegar odor, known as "vinegar syndrome," begins at different ages depending on temperature and relative humidity levels in storage areas. IPI's experiments showed that lowering storage temperature from 75 degrees Fahrenheit to 30 degrees Fahrenheit, while keeping relative humidity at 50 percent, changed the time before vinegar syndrome started in acetate film from 25 years to 700 years. Lowering the relative humidity from 60 percent to 20 percent, while keeping the temperature constant at 70 degrees Fahrenheit also increased film life, but less drastically, from 30 years to 90 years.

Research on other materials supports the theory that hotter, moister environments cause faster decay. It also reinforces that, although relative humidity levels contribute to chemical decay of materials, temperature is the more potent factor. Cooler temperatures provide substantial increases in material life.

Using the life span of an acetate film as a known standard, conservators have extrapolated the amount of deterioration occurring to an entire collection. However, actual museum conditions are constantly changing. Does a cool period offset a warm period? The Image Permanence Institute found that hot, humid periods have a particularly destructive effect. More damage is caused during these hot, humid periods than an equal amount of cool, dry time can offset.

In order to measure the effect of environmental fluctuations on a collection; IPI developed the Time Weighted Preservation Index (TWPI). The index provides a single value to express decay rate over time. This measurement can be used to adjust environmental conditions to maximize collection life. IPI found that improvements of one to three times greater collection life are possible within human comfort ranges. The TWPI combines the effects of temperature and relative humidity over time, with accurate weighting for particularly hot, humid times. TWPI can be used to evaluate environmental performance and document mechanical system improvements in concrete terms. Using data from a year of monitoring, temperature and relative humidity values over set intervals can be calculated and plotted to give the TWPI.

For those with minimal math skills, Jim Reilly at IPI is developing "Climate Notebook" software to provide TWPI using data from a variety of loggers. For the adventurous, he can provide the formula for your own calculations.

Biological Deterioration
Fewer models have been developed for biological deterioration.

The Image Permanence Institute and the Canadian Conservation Institute (CCI) in Ottawa have created graphs to model the likelihood of mold growth. Again, mold is a function of temperature, relative humidity and time. In a warm, moist environment mold appears rapidly. At 100 percent RH, 80 degrees Fahrenheit, mold starts within two days. However, lower the relative humidity 25 percent and it will take 90 days for mold to appear. Mold growth is optimum, appearing within 10 days, between 50 to 104 degrees Fahrenheit and 85 to 100 percent RH. According to IPI's research, mold will not grow below 65 percent RH, below 30 degrees Fahrenheit or above 110 degrees Fahrenheit.

The likelihood of insect attack is affected by too many factors, such as maintenance and cleanliness, to create an effective model. Some insects require mold to be present at the start of an infestation. Most museum insects are less active in dry environments. Conversely, they flourish in warm, moist environments. High (150 degrees Fahrenheit) or sub-zero (-10 degrees Fahrenheit) temperatures kill most insects over time. (See Collections Caretaker Vol. 1 No. 3.) Tom Strang of CCI has used research on specific insects' kill temperatures to develop non-chemical eradication techniques.

All biological decay is deterred by lower relative humidities.

Mechanical Deterioration
Fluctuating relative humidity and temperature stress materials. A change in temperature can expand or contract metal, stone and other inorganic materials. All objects come to temperature equilibrium quickly, rarely taking more than 24 hours. In a mixed collection, temperature change does not damage as many museum objects as significantly as relative humidity fluctuation. A change in RH may cause warping, splitting, delamination and other dimensional changes in moisture-absorbing, or hygroscopic, materials (wood, ivory, skin and other organic materials). Seasonal slow drifts, allowing slow equilibrium, are less harmful than abrupt changes. Of course, the extent of damage depends on the material and sometimes which part is exposed. For example, each end of wood equilibrates at a different rate. CCI's experiments showed that the end grains react 10 times faster than the sides.

In general, RH equilibrium occurs slowly for the entire mass of an object. Therefore, seasonal changes have the greatest effect. However, the surfaces may feel daily changes, causing stress within the structure. Moisture Equilibrium Curves have been charted for many materials showing how rapidly the object's moisture content equilibrates with the relative humidity. Few museum objects respond significantly to fluctuations under an hour in duration. Many objects take days, weeks or even months to respond. Because of the long time required for many organic materials to reach moisture equilibrium with their environment, seasonal cycles may be the most important operative cycles according to Jim Reilly of IPI. Daily or diurnal RH cycles are important for objects with short equilibration times, such as thin or absorbent materials. For example, on movie film hanging on a clothesline, gelatin equilibrates in minutes while the thin plastic equilibrates in hours. This difference could cause the gelatin to separate from the plastic substrate when RH fluctuates.

For each collection material, the time scale of RH equilibration is important. Placing the object in a cabinet, drawer or box can slow equilibration lessening the likelihood of mechanical decay. Within a confined space, objects act as a buffer, dampening the RH oscillation. Conservators call this enclosed space a microclimate. Enclosure and cabinet arrangements are important. A closed drawer will not feel a 10 percent RH change over 24 hours. A closed drawer filled with textiles will take much longer. Thus, Stefan Michalski of CCI recommends filling a chest of drawers with textiles in an unregulated, but heated, historic house. This changes the response time of the wood cabinet and could prevent cracking in dry winter months. When artifact equilibration times run in weeks and months, it may be more important for the museum to examine and worry about longer RH cycles instead of minute changes.

This is not to say that small RH fluctuations should not be considered. Small RH fluctuations can lead to fatigue in materials. Usually small fluctuations are not important when doing a risk assessment and cost-benefit curve. But for some materials, each cycle causes tiny fractures to grow. Deterioration increases as cracks are opened further. Research on painting deterioration by Stefan Michalski suggests that this may be a cause of surface cracks.

The amount of mechanical damage is directly related to the type of material, the constraint on the material and amount of RH change. Small changes cause small effects. Big changes cause big effects.

If tension climbs in a constrained material it eventually reaches a breaking point. For example, wood needs room to expand and contract when RH fluctuates. If a wood panel is nailed into a frame it will split. If it is placed in a groove, without glue, with space for expansion and contraction, it probably will not crack. The Canadian Conservation Institute, looking at wood artifacts and painted panels, divided materials into four levels of vulnerability to RH changes. The chart on page seven lists these vulnerability definitions.

Recommended Levels
Until recently, based on Thomson's Museum Environment , all museums strove for 50 percent RH + five percent and 70 degrees Fahrenheit, regardless of their building envelope, collection type or world location. Because of a lack of vapor barriers, insulation, or climate extremes, many museum buildings cannot maintain a constant relative humidity level. Some cannot maintain uniform temperature levels. When we ask engineers for this completely stable environment, at a modest price, often the engineer can only provide a slightly fluctuating environment at a modest price. In historic buildings, even this may be impossible for an engineer. When a building cannot contain a perfect environment, it makes no sense recommending one. Ernest Conrad, a structural engineer, has developed a building rating system to match mechanical systems with a structure's capabilities. He and Stefan Michalski are part of a task force to improve museum specifications for engineers. The task force has just added a significant chapter to the American Society of Heating, Refrigerating and Air-Conditioning Engineers Applications Handbook. "Chapter 20: Museums, Libraries and Archives" first appeared in the May 1999 handbook. Ernest Conrad's building rating system is included in the chapter with suggested control levels appropriate for each building type. Museum staff and their engineering consultants should read and use this most current edition of the ASHRAE Applications Handbook for referral during museum environmental improvement projects.

To date, there is no model to compare the amount of damage from chemical, biological and mechanical decay on a particular collection. For chemical decay, temperature is the largest influence on deterioration. For biological and mechanical decay, relative humidity plays the larger role. Museums should strive to keep low, non-fluctuating temperatures that are above freezing and a non-fluctuating relative humidity below 65 percent. If you are unsure, a conservator could assist you in determining the best temperature and relative humidity ranges for your museum.

In the Upper Midwest, with its climate extremes, a mixed collection is ideally stored at 45 percent relative humidity and between 65 and 68 degrees Fahrenheit. Year-round temperatures in storage areas may be lower if human comfort and air-conditioning costs are not factors. Lower temperatures in the winter will result in higher relative humidity without adding additional moisture. Adding humidity to a poorly insulated, heated building in the winter may cause condensation on windows and ice damage in walls. Because of this, staff may need to ramp relative humidity between 35 percent (or lower) in the winter and 45 or 55 percent (depending on dehumidification capabilities) in the summer.

Special thanks to Jim Reilly, Image Permanence Institute, 70 Lomb Memorial Drive, Rochester, NY 14623-5604 and Stefan Michalski, Canadian Conservation Institute, 1030 Innes Road, Ottawa, ONT, Canada, K1A 0M5 for their input and review of this article.

Wooden Artifacts and Humidity Fluctuations
Very High Vulnerability
(coating over right angled grain joint or crack)
  • 5% RH change, gradual fatigue fracture
  • 10% RH change, fracture possible each cycle
  • 20% RH change, fracture definite first cycle
High Vulnerability
(veneer over right angled grain joint, lacquer over knot-free wood)
  • 5% RH change, zero fatigue fracture
  • 10% RH change, gradual fatigue fracture or plastic deformation
  • 20% RH change, fracture possible each cycle
  • 40% RH change, fracture definite first cycle
Medium Vulnerability
(wood with little or no coating)
  • 10% RH change, zero fatigue fracture
  • 20% RH change, gradual fatigue fracture or plastic deformation
  • 40% RH change, fracture possible each cycle
Low Vulnerability
(loose wood panels, single component tool handles)
  • 40% RH change, possible accumulation of fatigue fracture or plastic deformation if the freedom to move or the coatings or the slowness of the fluctuation are less than perfect.
(Abridged from Reference number 5 below.)


1. Appelbaum, B. (1991). Guide to Environmental Protection of Collections.

2. Cassar, M. (1994). Museums Environment Energy.

3. Child, R.E. (1993). Electronic Environmental Monitoring in Museums.

4. Michalski, S., (1993). Relative Humidity in Museums, Galleries, and Archives: Specifications and Control. Bugs, Mold & Rot II Workshop. 51-62.

5. Michalski, S. (1998). Climate Control Priorities and Solutions For Collections In Historic Buildings. Historic Preservation Forum, v12, n4. 8-14.

6. Michalski, S. (1994). Humidity Response Times of Wooden Objects. Canadian Conservation Institute.1-4.

7. Michalski, S. (1990). Time's Effects on Paintings. Shared responsibility: A Seminar for Curators and Conservators. 39-53.

8. Reilly, J. (1999). Overview of Temperature and Relative Humidity. Unpublished workshop handouts.

9. Strang, T. (1992). A Review of Published Temperatures for the Control of Pest Insects in Museums. Collection Forum, v8, n2. 41-67.

10. Thomson, Garry (1981). The Museum Environment

11. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (1999). Chapter 20: Museums, Libraries and Archives in 1999 ASHRAE Applications Handbook, SI or I-P edition.

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Updated 11 May 2002