Microclimates

Contributors: Noël Siver, Julie Unruh, Alison Whyte, Anna Zastrow
==Microclimates==
A microclimate is an area in which the environmental conditions differ from those of the larger surrounding area. This can be the climate within an object’s housing as compared to the closed cabinet in which it is stored, the climate in the cabinet as compared to the room, or even in the room as 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. For these reasons, creating a beneficial and controlled collections storage environment can be crucial to collections preservation.
A discussion of the current views about environmental control and historical background can be found on the Environmental Guidelines portion of this wiki.
===Active and passive control===
To maintain a specific microclimate, either active or passive control measures may be implemented. Active control of an environment relies on engineered systems to condition the environment, such as HVAC or refrigeration systems that are designed to maintain specific temperature and relative humidity ranges within a room, exhibition case, storage unit, or transport vehicle. Passive control uses buffering agents or scavengers to maintain desired conditions. Passive agents include silica gel to absorb or desorb moisture, oxygen scavengers to remove oxygen, and pollutant scavengers to mitigate harmful airborne volatiles. Once implemented, passive components must be monitored, and occasionally renewed or replaced, to ensure successful continual control of the environment.
==Enclosures for storage==
Enclosures, including storage cabinets, storage boxes and cases, exhibition vitrines, frames, transport crates, and transport vehicles, 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 considering 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. It must also be considered whether placing an object in a ventilated enclosure will negatively affect the neighboring collection items in storage.
===Cold environments===
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===
Anoxic environments are useful for storage of collections that are susceptible to degradation caused by oxidative reactions, particularly organic objects, copper alloys, ferrous materials, and pyrite. Low oxygen environments can be employed for objects on exhibit to prevent fading of colorants. One method of creating an anoxic environment is to flush an enclosure with an inert gas such as argon or nitrogen, replacing the oxygen in the air with a non-oxidizing compound.[Library of Congress. “Anoxic Encasements: Visual Storage.” Accessed January 29, 2025. https://www.loc.gov/preservation/scientists/projects/anoxic_cases.html.] Another method is to place susceptible objects in gas barrier bags with the appropriate amount of oxygen scavenger by volume and then sealing the bag to prevent future gas exchange.Burke, J. 1996. “Anoxic Microenvironments: A Simple Guide.” SPNCH Leaflets 1 (1). Technical Publication Series of the Society for the Preservation of Natural History Collections:1–4. Both methods are only viable solutions for infrequently accessed objects, but can be instrumental in preservation. If oxygen scavengers are used, it is important to check occasionally that the barrier and scavengers are still effective in creating and maintaining the anoxic environment. This can be done via color-coded oxygen indicator cards placed in the enclosure where they can be seen.
Anoxia is generally not used to prevent pigment 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 environments (desiccated storage)===
Humidity control is important in collection storage, but some objects, including most metals, 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.
==Creation and monitoring of microclimates==
===Construction of microclimate enclosures for storage of metal objects using a vapor barrier film===
The corrosion of archaeological metal objects (especially iron) can be halted or slowed down by storing them in microclimate enclosures constructed from water vapor and oxygen-resistant barrier films along with an appropriate desiccant and/or oxygen scavenger. MarvelSeal 360, an opaque barrier film that consists of aluminized polyethylene and polypropylene, and Escal, a heavyweight barrier film that has a polypropylene outer layer with an inner layer composed of a vacuum-deposited ceramic on a PVA substrate, can both be used to create enclosures that can be heat sealed. However, it can be difficult to heat seal Escal[Brown, J.P. 2010. “The Field Museum Archaeological Metals Project: Distributed, in Situ Micro-Environments for the Preservation of Unstable Archaeological Metals Using Escal Barrier Film.” In Objects Specialty Group Postprints, 17:133–46. Washington, D.C.: American Institute for Conservation. https://resources.culturalheritage.org/osg-postprints/v17/brown/.] to itself and an enclosure made completely of opaque MarvelSeal 360 makes it impossible to view the object inside. An alternative is to use the two films together with the opaque MarvelSeal 360 forming the bottom layer and the transparent Escal forming the top layer.
Materials and equipment:
*Support material (optional)
*Exacto knife to cut support material (if rigid insulation foam is used)
*MarvelSeal 360
*Escal
*Tape measure or ruler
*Sharpie or other marking pen
*Scissors
*Bulldog clips
*Hermetic bag seal clips
*Towel
*Piece of wood 2 inches by 1 inch and of an appropriate length (optional)
*Iron or tacking iron or impulse heat sealer
*Perforated clear plastic bags containing self-indicating silica gel (blue gel or orange gel)
*Ageless® Oxygen Absorber (optional)
*Humidity indicator cards
Step 1 Select a smooth, flat surface for constructing the bag. Cover the surface with a heat resistant padding (a large towel can be used).
Step 2 Decide if the object for which you are constructing a bag needs to be supported from underneath. Then decide if the object along with its support will both be inserted inside the bag or if the object in its bag will be resting on top of the support. External supports can be made from a variety of materials including rigid insulation foam or wood; however, if the support material is going to be placed inside the bag, it is imperative that it is inert.
Step 3 Measure the object, determine the size of the bag to be made, and cut pieces of the two films. Use bulldog clips to keep the two films in alignment with one another. If not using a hermetic bag seal clip, leave an excess of both films at one end to allow for cutting open and resealing the bag to change out silica gel or if access to the object is needed at some point. The MarvelSeal 360 is placed dull side up and the Escal is placed on top of it, dull side down (i.e., dull sides facing each other). It is important to ensure that the surfaces of the films are clean and free of dust. Otherwise, it will be difficult to adhere them to one another.
Step 4 Seal the edges. Either use a heat sealer or an iron. If using an iron, set it to just below the number 2 setting. If your iron does not have numerical settings, choose a low temperature one. Iron the edges of three sides of the bag leaving one end open so you can insert the object. Apply pressure as you iron. It is possible to overheat the Escal so be careful. If it has been overheated, the Escal starts to bubble and is no longer useable. The seal should be roughly 1.5 inches wide. Avoid air bubbles and puckering. Make sure the dull sides of each film do not come in contact with the iron (these are the heat sensitive sides and will stick to the iron).
Step 5 Once the seal has been made from the Escal side, heat can also be applied to the MarvelSeal 360 side to ensure a good seal. Flip the bag over and iron the edge from the MarvelSeal 360 side.
Step 6 Insert object, its label, perforated bags of self-indicating silica gel, Ageless® Oxygen Absorber (optional), and humidity indicator card. Be sure that humidity indicator card is not in danger of flipping over inside bag after sealing. A bulldog clip can be attached to the outside to keep the card in place face side up.
Step 7 Seal the fourth side. Either use a hermetic bag seal clip or an iron. If at this point it is not possible to flip the bag (because of the object) the MarvelSeal side can still be ironed by folding the end so the MarvelSeal side is up. Place a towel between the end and the rest of the bag. Alternatively you can fold the end of the bag over a piece of wood approximately 2 inches by 1 inch and of an appropriate length. With the MarvelSeal side facing up, iron it on that surface.
Suppliers
MarvelSeal 360, supplied as 48” x 10’ roll, 48” x 50’ roll, or 48" x 200 yards roll
Talas
https://talasonline.com
Escal Barrier Film sheet, supplied as a 1 x 5 meter roll or 1 x 100 meter roll
Keepsafe Microclimate Systems
https://keepsafe.ca
Hermetic Bag Seal Clips, reusable small, large and extra-large hermetic bag seal clips made of LDPE plastic
Talas, see above
Ageless® Oxygen Absorber: Ageless® Z100 sachets, Ageless® Z1000 sachets
Keepsafe Microclimate Systems, see above
Orange indicating silica gel
Conservation Resources International, LLC
https://conservationresources.com
Humidity indicator cards, supplied as pack of 5, or 10, or pack of 25 depending on supplier
Conservation Resources International, LLC, see above
Museum Services Corporation
https://museumservicescorporation.com
University Products Inc.
https://universityproducts.com
===Construction of desiccated microclimate boxes for archaeological metals storage===
The metals corrosion process requires two things: water and oxygen. If either of those things are removed, metals corrosion will not occur. This procedure describes a way to desiccate the air inside a storage box to a low enough RH that corrosion will be slowed or prevented, using silica gel as the desiccant. The same technique can be adapted for use with an oxygen scavenger or with a combination of a desiccant and an oxygen scavenger.
The target RH to prevent metals corrosion has been a matter of debate. One study recommends an RH of 12% or below.Watkinson, D., and M. Lewis. 2005. “Desiccated Storage of Chloride-Contaminated Archaeological Iron Objects.” Studies in Conservation 50 (December 1). doi:10.1179/sic.2005.50.4.241. It is generally agreed that the goal should be to attain the lowest RH achievable.
Step 1 Identify a vapor-impermeable (usually polypropylene or polyethylene) container with a tight lid. It is widely believed that constructing a microclimate in a container with a silicone gasket and a locking lid will improve the microclimate seal. Research demonstrates that several factors determine the robustness of the seal: however, a locking lid "may produce a more consistent performance."Thunberg, J.C., D.E. Watkinson, and N.J. Emmerson. 2021. “Desiccated Microclimates for Heritage Metals: Creation and Management.” Studies in Conservation (April 3). https://www.tandfonline.com/doi/abs/10.1080/00393630.2020.1799599. Locking lids do provide a degree of artifact safety in boxes that are frequently moved. Note that though gaskets and locking lids are preferred, they are not absolutely necessary.Senge, D.K. 2014. “Testing and Implementation of Microclimate Storage Containers.” In Objects Specialty Group Postprints, 21:123–40. Washington, D.C.: American Institute for Conservation of Historic and Artistic Works.
Step 2 Pack the objects in the driest part of the day if possible.
Step 3 As needed, pack artifacts within individual bags or on supportive mounts. For dry air to surround the artifacts, it needs to reach the artifacts. For that reason, prior to housing artifacts inside bags or other enclosures, use a dental pick or other sharp tool to poke a few holes in the artifact bags or enclosures.
Step 4 New silica gel should be at 0% RH. If the silica gel is not at 0%, it can be reconstituted to 0% by baking it in an oven at low temperatures. For instructions, consult the manufacturer of the silica gel or search online.
Step 5 The quantity of silica gel needed for a given volume can be calculated,Weintraub, Steven. 2002. “Demystifying Silica Gel.” In Object Specialty Group Postprints, 9:1–24. Washington, D.C.: The American Institute for Conservation of Historic & Artistic Works. https://www.talasonline.com/images/PDF/ProductInformation/silica_gel_info.pdf.National Park Service. 1999. Using Silica Gel in Microenvironments. Conserve O Gram. Washington, DC. https://www.nps.gov/museum/publications/conserveogram/01-08.pdf. but if the silica gel cannot be monitored for an extended period of time – as is the case in a sealed depot accessed only during an excavation season – it is prudent to use larger amounts of gel. Thunberg et al. have published helpful guidelines for determining the necessary amount of gel for various situations, including long-term seals. Silica gel may be purchased loose or as pre-manufactured packets, and loose gel may be put into perforated polyethylene bags, unbleached muslin bags, or other air-permeable bags for ease of use. Place the silica gel or silica gel packets in the bottom of the microclimate container. Immediately re-seal the container of dry silica gel tightly so that the stock of dry gel doesn’t absorb any more moisture than necessary.
Step 6 Put an RH indicator card into the box to monitor the RH. If using a transparent box, put it against the side of the box so that it can be read without opening the box. It can be taped in position to hold it in place if necessary.
Step 7 Carefully pack all objects into the box.
Step 8 Seal the box:
*OPTIONAL: Lay a sheet of aluminum foil or plastic sheet over the top to serve as an additional vapor barrier layer. The addition of an extra vapor barrier layer can be particularly useful if the box does not have a tight-fitting lid. Inspect for pinholes or tears. Smooth the sheet down well over the edge of the box to make sure there is good contact between the sheet and the top of the box. Fold the corners somewhat neatly (like wrapping a gift), so that a wrinkly channel is not created that would allow air ingress into the box. Tape the lower edge of the vapor barrier sheet to the box using packing tape (a semi-vapor barrier). The goal is to seal the edge of the vapor barrier to the sides of the box so that no air can get into the box via the lower edge of the vapor barrier.
*Put the lid on. If there are locking handles, lock the handles.
*Use packing tape to tape around the lid (and handles, if there are handles), to seal the lid to the upper edge of the box (or to the vapor barrier layer over the upper edge of the box). The goal is to prevent air ingress at the interface between the lid and the top edge of the box. If there are handles, taping them down securely also prevents the handles from popping up accidentally. Packing tape can be stretched as the tape is attached; doing so creates a tight, form-fit seal. It is recommended to go around the whole box lid at least twice with the tape. Really make an effort to get a good seal between the lid and the box, so that there can be no possibility of air exchange. Mash the tape down afterwards to make sure it’s well-adhered and a tight fit.
After a few hours, the RH indicator card should read ~10% RH or less. Monitor it for as many days as possible to make sure the box is adequately holding a low RH.
In archaeological fieldwork, it can be prudent to wait to seal microclimate boxes until the final week of the dig, in the hopes that all artifacts needing desiccated storage are identified and all specialists have finished studying the artifacts for the season. It never fails that the microclimate will need to be re-opened anyway. If so, cut the seal, complete work inside the box as quickly as possible, and if there will be a delay before the box can be re-sealed, keep the lid on until re-sealing. If the lid is only open for a few minutes at a time, the silica gel inside the box should not take up significant moisture and it should be possible to re-seal the box without having destroyed the low RH.
Periodically, and if possible at least once per year, assess the RH in every box to see how the sealed boxes have fared. If the RH is holding, no action needs to be taken. If the RH is higher than optimum, investigate what went wrong with the seal, switch out the silica gel for dry gel (or recondition the gel to a low RH), and fix the seal.
==References cited==
==Further reading==
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.
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