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Contributors: Anna Zastrow

A microclimate is an area where the environmental conditions differ from those of the larger surrounding area. This can be the climate within an object’s housing compared to the closed cabinet it is in, the climate in the cabinet compared to the room it is in, or even the room compared to the rest of the building. In more specific terms, microclimates are the result of differing temperature, relative humidity, and environmental pollutants. Often microclimates are intentionally employed to benefit collection objects. However, microclimates can be unintentional and problematic as well. Areas that are susceptible to large changes in environment such as basements, attics, or areas along an exterior wall can create unwanted microclimates that may wreak havoc on collections. Minimal air flow combined with off-gassing from an object can create a self-destructive environment. Relative humidity fluctuations lead to the expansion and contraction of materials which in turn can cause distortion, strain, and long-term damage. Damp environments accelerate deterioration of many collection types. They can cause the emulsion layers of film or photographic materials to adhere to housings or each other, metals to corrode, and colorants to bleed. Dampness also leads to the development of condensation, mold, and mildew. It attracts insects which are drawn to edible materials such as papers, glues, and textiles. Rodents may also be attracted by these edible materials and the presence of insects. This is why creating a beneficial and controlled collections storage environment is so crucial.

General Control: Temperature and Relative Humidity[edit | edit source]

When looking at maintaining a specific microclimate, active or passive control measures may be implemented. Active control of an environment relies on engineered systems to condition the environment, such as HVAC systems that are designed to maintain specific temperature and relative humidity ranges. Passive control uses a buffering agent to maintain desired relative humidity conditions. Once implemented, passive components must be monitored, and occasionally replaced, to ensure successful continual control of the environment. These components include silica gel to absorb or desorb moisture and pollutant scavengers to mitigate harmful volatiles. The idea of creating optimal conditions for object storage is not new and a discussion of the current views and historical background can be found on the Environmental Guidelines portion of this wiki.

Considering Enclosures for Storage[edit | edit source]

Enclosures, whether a box or case, can help mitigate the effect of short-term fluctuations in the macro-environment and help slow the change of microenvironment conditions when the macro-environment changes. This can be particularly advantageous when it comes to moving collection objects from one environment to another. One thing to keep in mind is items stacked in a box will experience a range of moisture variation. The outer portions of the stack are more susceptible to changes in moisture than the central portions of the stack. This is similar to the way a book cover responds to moisture before the central portion of the text block.

When thinking about enclosing items, especially in well-sealed enclosures, it is important to take into account an object’s own off-gassing. It may be necessary to include barriers between certain items or scavengers to absorb the pollutants. Another option when off-gassing and pollutants are a concern is a ventilated enclosure. However, it is important to consider that a ventilated enclosure will not protect the object from fluctuations in relative humidity and exposure to dust. You must also consider whether placing an object in a ventilated enclosure will negatively affect the neighboring collection items in storage.

Cold environments- Creation and Monitoring[edit | edit source]

Cold environments are used for a variety of reasons. Most commonly cold storage is used for photograph collections or natural history specimens. When considering cold storage it is important to understand the materials in your collection and what conditions are optimal for them. ISO guidelines establish specific temperature set points that help define terminology and give guidance for the proper storage of photographs; they are: cool storage (54°F), cold storage (40°F), and frozen storage (below 32°F). These temperature set points indicate the starting point of a range of temperatures that fall into the defined category. For example, collections stored in frozen conditions are generally kept at a temperature similar to what one would have in their freezer at home. Also included in this category however, is cryogenic storage (-238°F and below) which is necessary for some specimen collections. When removing an object from cold and especially frozen storage it is important to plan ahead as time and care must be taken. The flexibility of some objects may be affected by cold or frozen storage and handling them before they have been allowed to equilibrate may lead to damage. Condensation can form on objects as they acclimatize to room temperature. To prevent condensation from affecting your collections, a good quality, vapor barrier material should be used to protect the object, and insulated containers can be used to slow equilibration when objects are removed cold or frozen storage.

Freezing can also be used for pest control and to stop the growth of mold.

Anoxic and Low Oxygen environments- Creation and Monitoring[edit | edit source]

Anoxic and low oxygen environments are used for a number of reasons. Anoxic environments are incredibly useful for storage of archaeological collections that are susceptible to degradation caused by oxidative reactions, particularly objects made of copper alloys or ferrous materials. In this case objects are placed in gas barrier bags with the appropriate amount of oxygen scavenger by volume and then sealed to prevent future gas exchange. This is only a viable solution for very infrequently accessed objects, but can be instrumental in preserving items that are already suffering from degradation. It is also important to check occasionally that the barrier and scavengers are still effective in creating and maintaining the anoxic environment. This is done via indicators placed in the bag where they can be seen prior to sealing. Low oxygen environments can be employed for objects on exhibit to prevent fading of colorants. This is generally not used to prevent micro fading in storage because restricting light is an easier, more cost effective solution and some colorants react adversely to low oxygen environments.

Anoxic environments can be used in a shorter term capacity for pest control.

Low humidity (Desiccated Storage) - Creation and Monitoring[edit | edit source]

Humidity control is important in collection storage, but some objects, like archaeological iron, are particularly sensitive to moisture and must be stored at a very low relative humidity. In some cases, objects that would benefit from anoxic storage, but cannot be stored this way due to budgetary restrictions, will be stored in gas barrier storage bags that are flushed with nitrogen to create a low oxygen environment and then kept at low humidity with silica gel. As with anoxic storage, this is only a solution for seldom accessed collection items and must be monitored to ensure that the gas barrier is still sealed effectively and that the silica gel is maintaining a low relative humidity. This is done in the same way as it would be for anoxic storage, via indicators placed in the bag where they can be seen prior to sealing.

References[edit | edit source]

Beltran, Vincent L., James Druzik, and Maekawa Shin. 2012. "Large- Scale Assessment of Light- Induced Color Change in Air and Anoxic Environements." Studies in Conservation 75 (1): 42- 57.
Researchers at the Getty Conservation Institute tested a wide range of samples for light fastness in air and near anoxic environments; these included: dry pigments, organic dyes, natural history specimens, aniline based dyes, dyed textiles, gouache, watercolors, and fluorescent inks. The vast majority of samples saw less fading in the near anoxic environment, however certain colorants were adversely affected by the lack of oxygen.

Padfield, Tim, and Karen Borchersen, . 2007. "Contributions to the Copenhagen Conference: Museum Microclimates." Copenhagen: The National Museum of Denmark. https://www.conservationphysics.org/mm/musmic/musmic150.pdf.
The conference hosted in Copenhagen November 19- 23, 2007 covered much of the contemporary research involving Museum Microclimates. While they did not cover the more specialized environments, such as anoxic and cold storage, they touched on all manner of environmental control in various types of institutions and for various object types.

Image Permanence Institute. 2020. Storage Suitability. https://filmcare.org/storage_categories.
The Image Permanence Institute at the Rochester Institute of Technology provides information about deterioration of film based materials. This webpage lays out temperature storage guidelines in an easy to read table format.

Mariano, Melissa. 2017. "Oxygen Absorption for the Protection of Archaeological Iron: Improving Maintenance." Metal 2016 Proceeding of the Interim Meeting of the ICOM- CC Metals Working Group September 26- 30, 2016 New Delhi India (ICOM- CC) 203- 210 .
The conservation department at the Japanese Institute of Anatolian Archaeology undertook a study to determine the success of Revolutionary Preservation oxygen absorbers in preserving freshly excavated archaeological materials in short term storage and previously excavated archaeological materials in long term storage.

Michalski, Stefan. 2019. Agent of Deterioration: Incorrect Relative Humidity . https://www.canada.ca/en/conservation-institute/services/agents-deterioration/humidity.html.
The government of Canada’s conservation institute has published a series of webpages on the various agents of deterioration. This page discusses relative humidity, what collection types are particularly vulnerable, the sources of poor relative humidity, and how to correct it. It is also worth looking at the pollutants and temperature page to get a more holistic view of microclimates.

National Park Service. n.d. Cold Storage. https://www.nps.gov/museum/coldstorage/html/conclusion8_3.html.
The National Park Service provides a website with introductory information about cold storage, including material consideration and packaging.

Senge, Dana k. 2014. "Testing and Implementation of Microclimate Storage Containers." Objects Specialty Group Postprints, (The American Institute for the Conservation of Historic and Artistic Works) 21: 123-140. http://resources.culturalheritage.org/wp-content/uploads/sites/8/2015/03/osg021-06.pdf.
Conservators at The National Park Service Intermountain Range conducted research into the use of microclimates in storage for preservation of objects. They took into account ease of maintenance, creation, and access to the object. Their collection spans a wide variety of materials and a summary of their findings along with their methodology is discussed in this article.

Shiner, Jerry. n.d. "Trends in Microclimate Control of Museum Display Cases." Conservation Physics. https://www.conservationphysics.org/mm/shiner/shiner.pdf.
A brief overview of microenvironment control techniques and technologies over the past hundred or so years. There is some focus placed on the more recent developments and theories for active control systems.

Simone, Frank P. 2019. "Low Temperature Storage." Chap. 22 in Preventative Conservation: Collection Storage, edited by Lisa Elkin and Christopher A. Norris, 473- 489. Society for the Preservation of Natural History Collections.
This chapter discusses low temperature storage, from cold to cryogenic, for a variety of objects. It includes a discussion of equipment and cost, as well as safety and handling.

Thurn, James. 2018. Instructions for Constructing Protective Enclosures for Two Foam Rubber D-Day Maps. Internal publication, Collections Conservation Section, Conservation Division, Library of Congress.
In 2018 the Library of Congress rehoused two rubber D-day maps from the Geography and Maps Division. After analysis of the off gassing was completed by the research and testing division it was decided the maps should be stored in ventilated housing rather than anoxic storage. James Thurn, a preservation specialist at the Library of Congress created a protective housing for each map with polyester screening on all four sides which allowed the potentially damaging acetic and formic acid to safely escape the enclosure while still protecting the maps.

Weintraub, Steven. 2019. "Specialized Microclimates and Macroclimates: Options for the Control of Temperature, Relative Humidity, and Pollutants." In Preventative Conservation: Collection Storage, 449- 471.
This chapter gives an overview of the macroclimate vs. microclimate in relation to the control of temperature, relative humidity, and pollutants. It provides guidance for cost effective solutions to building and controlling proper microclimate in collection storage spaces.