IPI Research on Equilibration

Recommendations for storage temperature and relative humidity levels have been periodically re-examined following the latest advancements in preservation science. Research by Garry Thomson, David Erhardt, Marion Mecklenburg, Stefan Michalski, and others has focused on defining a suitable temperature and humidity range for collection materials. Temperature and humidity flat-line “standards" have been reconsidered and the buffering role of microclimates has been studied. In the 1990s, studies detailed the physical response of various organic collection materials to changes in temperature and RH and developed computerized models of the stress behavior for paintings and photographs1. Thermal and moisture equilibration rates for specific materials such as magnetic tapes, photographic film, photographic prints, and other collection materials have been evaluated to various degrees by scientists at the Image Permanence Institute.

These past studies underscore the importance of the dynamic relationship between the storage environment (the macro-environment) and the space immediately surrounding the artifact (the microenvironment). IPI has conducted three NEH-funded studies exploring this relationship, the first dealing primarily with the use of microclimates in film preservation, the second studying the effect of changing environments on library and archive materials, and a third evaluating the impact of intentional short-term temperature and RH changes on collection materials.
 

Environment and Enclosures in Film Preservation

IPI’s NEH-funded research on Environment and Enclosures in Film Preservation2 (1994-96) concluded that enclosures mitigate humidity fluctuations and that the currently recommended range of RH fluctuations for film storage may be unnecessarily narrow. Certain enclosure types slowed the rate of moisture equilibration but no enclosures significantly affect the rate of temperature equilibration.

Temperature Fluctuations

The following table shows the length of time it took for various materials to reach 50% and 90% thermal equilibration. Materials were exposed to a one-time temperature change and the temperature was monitored to 100%, or full, equilibration. The response to the changing temperature of a 1000-ft. roll of motion-picture film was not significantly altered by an enclosure made of metal or polypropylene. On the other hand, the thermal equilibration rate was strongly altered by film mass, as we can see by comparing the results for a stack of film cans vs. a single can.

Conclusion: The object's response to temperature change was relatively unaffected by enclosures. Full equilibration was reached within hours.

Tested Material

Storage Configuration

Time to reach 50% Temp Equilibration

Time to reach 90% Temp Equilibration

35mm Motion Picture Film, Triacetate Base

Single 1000 ft. Roll in Metal Can

45 minutes

3.5 hours

            

Single 1000 ft. Roll in Polypropylene Can

45 minutes

3.25 hours

 

Stack of six 1000 ft. Rolls in Metal Cans

1.5 hours

7.5 hours

 

Single 100 ft. Roll in Metal Can

20 minutes

1.25 hours

 

Single 100 ft. Roll in Cardboard Box and Low Density Polyethylene Bag

20 minutes

1.25 hours

 

Stack of six 100 ft. Rolls in Metal Cans

30 minutes

2.33 hours

4" x 5" Sheet Film on Acetate Butyrate Base

Stack of 500 Sheet Films in Paper Envelopes inside Metal Box

2 hours

6.25 hours

 

Stack of 500 Sheet Films in Polypropylene Sleeves inside Metal Box

2 hours

6 hours

3.5" x 5" Resin-coated Photographic Print

Stack of 1000 Prints enclosed in Drop-front Cardboard Box

1.5 hours

4 hours

 

Relative Humidity Fluctuations

The following table shows the length of time it took for various materials to reach 90% moisture equilibration. With the exception of permeable enclosures such as cardboard boxes, most enclosures slowed the rate of moisture equilibration. Tight enclosures such as metal or plastic containers postponed the attainment of equilibrium with the surrounding air. Such enclosures temper short-term fluctuations in relative humidity, and to a lesser degree mitigate the effect of seasonal RH drift. The rate of RH equilibration or moisture diffusion is dependent on the width of the roll of film or tape—the size and length don’t matter.

Conclusion: Enclosures moderate daily fluctuations in RH and may help postpone response to seasonal drift, depending on their moisture-buffering capacity.
 

Tested Material

Storage Configuration

Time to reach 90% Moisture Equilibration

Hardcover Book

Sitting on a Shelf

One month

35mm Motion Picture Film, Triacetate Base

Enclosed in a Cardboard Box

Two weeks

35mm Motion Picture Film, Triacetate Base

No enclosure

Two weeks

35mm Motion Picture Film, Triacetate Base

Enclosed in a Metal Can

Six months

2" Datatape

Enclosed in a Plastic Container

Six months

 

Effects of Fluctuating Environments on Library and Archives Materials

IPI’s NEH-funded research on the Effects of Fluctuating Environments on Library and Archives Materials3 (1998–2001) investigated three primary topics: the effect of cycling environments on the chemical stability of paper and film, the rate of moisture conditioning for a variety of materials and enclosures, and the impact of cycling conditions on the microenvironment and moisture content of library materials.

To determine whether environmental fluctuations are inherently damaging to collection materials, IPI research scientists studied paper and acetate film—both inherently fast-decaying materials. Would changes in the environment cause chemical decay in these materials beyond what would normally be expected? It was observed that their behavior could be explained by current thermodynamic models and there was no evidence that the cycling between one humidity level or temperature and another caused extra decay.

The findings of this study reinforced IPI’s development of the Preservation Metrics™, particularly the Preservation Index and Time-Weighted Preservation Index (TWPI). Based on dynamic modeling of chemical decay rates, the TWPI algorithm is used to assess the risk of chemical decay on the natural aging of organic materials in a changing environment. Read more about the metrics.

To what extent and how quickly would library and archive collection materials feel changes in RH? To answer this question, the rate of moisture equilibration was measured at various temperatures for film, photograph, book, paper, and magnetic tape samples. To quantify the mitigating effect of common enclosure configurations, samples with and without enclosures were used. The study allowed IPI to characterize various material/enclosure combinations as fast, medium, or slow to reach equilibrium with the surrounding environment. These results are shown in the table below. In general, the “Fast” equilibration configurations are more susceptible to environmental changes. The data indicated that daily changes should not be a concern, however, sustained changes for a week or longer have the potential for altering the moisture content of materials to varying degrees.
 

Time to Reach 50% RH Equilibration

Time to Reach 90% RH Equilibration

Rate Description

Material and Enclosure Combination

1 to 2 Days or Less

Week

Fast

16mm Film Rolls without Enclosure

Sheet Film Stack without Enclosure or in Cardboard Box

1 to 2 Weeks

Weeks to One Month

Medium

Film Roll without Enclosure, in Cardboard Box, or in Vented Plastic Can

Sheet Film Stack inside Metal Cabinet

Stack of Mounted Photographs without Enclosure or in Cardboard Box

Stack of 250 Sheets of 8″ x 10″ Office Paper without Enclosure or in Cardboard Box

Hardcover Book

1″ Tape without Enclosure or in Cardboard Box

More than 2 Weeks

Months to Years

Slow

Film Roll in Non-Vented Plastic Microfilm Box

Stack of Mounted Photographs in Museum Case or in Portfolio Box

Stack of 250 Sheets of 8″ x 10″ Office Paper in Folder inside a Metal Cabinet

2″ Tape in Plastic Shipping Container

 

Methodologies for Sustainable HVAC Operation in Collection Environments

IPI’s NEH-funded research on Methodologies for Sustainable HVAC Operation in Collection Environments4 began in 2010 and will be completed in April 2014. This project was designed to evaluate the effect of intentional temperature and relative humidity changes on collection materials. The feasibility of moving from a static environmental management approach, where maintaining a steady temperature and RH in the space is the goal, to a dynamic approach involving regular nightly, weekend, or seasonal adjustments to HVAC settings was examined. The experimental program included both laboratory testing and field experiment. The objective of the laboratory research was to document the behavior of typical material configurations when exposed to a wide range of short-term and sustained temperature and/or RH changes. Twenty-five different scenarios were tested; each designed to mimic intentional temperature and/or RH adjustment during unoccupied hours involving an 8-hour daily adjustment at night, and a 32-hour adjustment during the weekend. Test samples included:

  • Books on a shelf
  • Manuscripts in an archival document box
  • Maps in a metal flat-file cabinet
  • Prints and photographs in an archival drop-front box
  • Prints and photographs in a museum case
  • Mounted prints and photographs in an archival drop-front box
  • Mounted prints and photographs in a museum case
  • Paper stacks in an office file cabinet

Dataloggers were embedded in the center of the test material (e.g., a hard-cover book,a stack of paper, a stack of mounted prints). Loggers were also placed close to the outer surface of the object inside its enclosure (e.g., box, museum case, flat-file cabinet). The environment in the space was also monitored, providing macro-environment, microenvironment, and core data. During the experimental phase the temperature was adjusted to simulate a mechanical system shutdown—during the summer, the temperature was raised during nights and weekends, and during the winter the temperature was lowered during these periods. At the same time, the RH was adjusted to allow us to conduct the experiment at a constant dew point.

Dataloggers in center of materials

The following graph shows relative humidity data from the storage space (blue) and from an electronic sensor embedded in the middle of a hard-cover book (red).  The data illustrates that fluctuations in RH were barely noticeable at the core of the object. While the macro-RH fluctuates between 50% and 70% daily, the RH measured inside the book did not display any significant change.
 

 

In contrast, the data demonstrated that the core of a book WAS significantly affected by gradual seasonal RH change. The following graph shows a full year of RH data from the storage space (red) and the core of a hard cover book on a shelf (blue). While the book equilibrated quickly to the ambient temperature, moisture equilibration at the core occurred slowly, lagging behind the ambient RH. Most importantly, the data demonstrates that given enough time, the entire book will equilibrate to the long-term high and low extremes of seasonal RH. Although RH "peaks and valleys" are not felt at the core of the book, in a real situation of gradual seasonal RH change, the book has time to “feel” the change.

 

 

Moving Forward

The large amount of data gathered during IPI’s research indicates that the behavior observed for a book on a library shelf is representative of the other common humanities materials tested. These findings underscore the absolute necessity for seasonal moisture control in collection storage areas, and the need to minimize the impact of seasonal changes on humanities collections. The next step for IPI’s research in this area is to develop an effective method for managing the storage environment to avoid periods of sustained high RH, improve the preservation quality of the storage space, and extend the life of your collections.

 



  1. M. F. Mecklenburg, M. McCormick-Goodhart, and C. S. Tumosa, “Investigation into the Deterioration of Paintings and Photographs Using Computerized Modeling of Stress Development," JAIC, Vol. 33 (1994), 153-170.
     
  2. J.-L. Bigourdan and J. M. Reilly, Environment and Enclosures in Film Preservation, National Endowment for the Humanities, Grant #PS 20802-94
     
  3. J.-L. Bigourdan, J. M. Reilly, and K. A. Santoro, Effects of Fluctuating Environments on Library and Archives Materials, Division of Preservation and Access, NEH Grant #PA-23159-98.
     
  4. Methodologies for Sustainable HVAC Operation in Collection Environments, Division of Research and Development, NEH Grant # PR-50087-10.