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Drawing the Line on Acceptable Relative Humidity Fluctuations

Enclosure exampleThe significance of environmental fluctuations is a frequent topic of discussion among conservators, registrars, curators, collection managers, and other preservation professionals. Traditional environmental guidelines emphasize “straight line” or stable conditions – with narrow bands of “acceptable” fluctuations in temperature and relative humidity – as critical to the long-term preservation of collections. The history of these prescribed environmental recommendations has been well documented in recent publications, primarily with the aim to address the need to re-evaluate the accepted standards and guidelines for temperature and relative humidity.1, 2, 3 The 2010 roundtable discussion, The Plus/Minus Dilemma: A Way Forward in Environmental Guidelines4, organized by International Institute for Conservation (IIC) and American Institute for Conservation (AIC) and the 2010 meeting Rethinking the Museum Climate,5 sponsored by the Boston Museum of Fine Arts and the Getty Institute, are just two of the many recent conferences centered around this topic. While no new, specific recommendations have been accepted or published, the general consensus is that the traditional guidelines need to be reviewed, reconsidered and redefined to reflect the current scientific understanding of material behavior and response to environmental changes. Research suggests that most objects can safely withstand much wider fluctuations, especially in relative humidity, than previously accepted.6, 7, 8 The question is not only where to draw the line, but how? Either way, the line is unlikely to be “straight.” One aspect of material behavior that will help inform the decision is how materials equilibrate to their environmental changes.

At any given moment, an active exchange of energy (heat) and moisture (if the material is hygroscopic) is occurring between an object and its environment. The amount of heat and moisture available for this exchange is determined by the temperature and relative humidity of the environment but, if given enough time, the object will eventually reach a point when it is neither gaining nor losing heat or moisture from this on-going exchange. At this point, the object has reached a state of equilibrium with its environment. For the collection manager, concerned with the care and preservation of the collection objects, this time-dependent relationship of equilibration is of particular interest:

  • If the humidity suddenly changes in the collection storage room, will the materials be adversely affected?
  • How long will it take for the collection object to fully adjust to changing environmental conditions?

To answer these questions, one must understand how materials equilibrate to the environment. Such an understanding will not only give the collection manager greater knowledge about the characteristics of the materials; it will also provide context for interpreting graphs of temperature and relative humidity and enable decisions about environmental management strategies.

The June 2009 issue of Climate Notes focused on thermal and moisture equilibration. To review the basic facts about the process of equilibration, refer to the articles in Equilibration: Rates and Responses to Environmental Changes. Typical equilibration behavior, however, can be summarized as follows:

  • Thermal equilibration (the process of the adjusting to the temperature of the environment) is relatively fast (generally occurring in a matter or hours) and is not significantly altered by enclosures.
  • Moisture equilibration is much slower (generally requiring days, months, or years) and can be dramatically influenced by enclosures.

In the previous articles, the rate of moisture equilibration was discussed only for materials at a constant temperature and without enclosures. This depicts the general trends of moisture equilibration – including the material’s inherent capacity to control diffusion of water vapor into or out of the object - but it does not characterize the process of moisture equilibration as it would occur in real-life storage situations, where objects are stored in enclosures or stacked tightly with other materials. To more accurately answer questions about the length of time required for materials to equilibrate to moisture changes, we must therefore consider the impact of enclosures on the rate of moisture equilibration.

Continue reading on The Moisture-Buffering Capacity of Enclosures...

[1] Erhardt, D, Tumosa, C and Mecklenburg, M. “Applying Science to the Question of Museum Climate.” Museum Microclimates: Contributions to the Conference in Copenhagen, 19-23 November 2007. Copenhagen: National Museum of Denmark, 2007, pp. 11-18.

[2] Hatchfield, Pamela. “Crack Warp Shrink Flake: A New Look at Conservation Standards.”  Museum: American Association of Museums (AAM), January-February, 2011.

[3] Padfield, Tim. “The Role of Standards and Guidelines: Are They a Substitute for Understanding a Problem or a Protection Against the Consequences of Ignorance?” Durability and Change, Krumbein, W. E. et al (editors). Wiley 1994, pp191-99.

[4] The Plus / Minus Dilemma: The Way Forward in Environmental Guidelines. Dialogues for the New Century: Discussions on the conservation of cultural heritage in a changing world.International Institute for Conservation (IIC) and the American Institute for Conservation (AIC) of Historic and Artistic Works. Milwaukee, WI. May 13, 2010.

[5] Rethinking the Museum Climate, Museum of Fine Arts, Boston, and The Getty Conservation Institute. Boston, April 12-13, 2010.

[6] Erhardt, David and Marion Mecklenburg. "Relative Humidity Re-Examined." Preventive Conservation: Practice, Theory, and Research. Preprints of the Contributions to the Ottawa Congress. London: The International Institute for Conservation of Historic and Artistic Works (IIC), 1994, pp. 32-38.

[7] Erhardt, D, Mecklenburg, M., Tumosa, C. and McCormick-Goodhard, M. “The Determination of Allowable RH Fluctuations.” Newsletter: Western Association for Art Conservation. Vol 17, Number 1 (Jan 1995): p.19.

[8] Erhardt, D, Tumosa, C and Mecklenburg, M. “Applying Science to the Question of Museum Climate.” Museum Microclimates: Contributions to the Conference in Copenhagen, 19-23 November 2007. Copenhagen: National Museum of Denmark, 2007, pp. 11-18.



Bigourdan, J.-L., P. Z. Adelstein, and J. M. Reilly, "Moisture and Temperature Equilibration: Behavior and Practical Significance in Photographic Film Preservation," La Conservation: Une Science en Evolution, Bilans et Perspectives, Actes des Troisiemes Journées Internationales d'Etudes de l'ARSAG, Paris, 21 au 25 Avril 1997, (Paris: Association pour la recherche scientifique sur les arts graphiques, 1997) pp. 154-164. (Open PDF file)

Bigourdan, Jean-Louis, and James M. Reilly. “Effects of Fluctuating Environments on Paper Materials- Stability and Practical Significance for Preservation.” 2003.