Standard freezing of room-temperature cultural materials can be accomplished by using a chest-type, upright, walk-in freezer, or freezer truck. Whichever kind is used,  it should be capable of maintaining minus 20 degrees F (minus 29 degrees C). Ultimately the appropriate exposure period for insect eradication will depend on the minimum operating temperature of  the freezer and the type of insect. Review why insect identification is important before proceeding with treatment. For more specifics on freezer types see the “Materials and supplies” section below.

A freezer capable of reaching minus 20 degrees F (minus 29 degrees C) will sufficiently lower the materials to the freezing point within four hours, which kills adult insects as well as their eggs. If the temperature drop takes much longer, some insects are capable of producing an ‘anti-freeze’ that allows them to survive freezing. With wood pests such as powderpost beetles, a second treatment may be necessary to insure complete kill. This is particularly important for large wooden objects, where temperature change may take longer then four hours to penetrate to the core.https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/controlling-insects-low-temperature.html

In order to avoid damage from ice build-up on the object in the freezer or from condensation after removal, artifacts should not be put into freezers directly. They should be sealed in plastic either using re-sealable plastic bags or polyethylene plastic sheet sealed with tape that will not fail in low temperatures. If a number of objects are being treated at once, objects in bags should be put in trays or boxes for easier handling. Items may also be placed in boxes and the boxes wrapped.

Typical low-temperature treatment for infestation involves freezing for a minimum of 72  hours, although many museums leave objects in freezers for longer; some, particularly when freezing wooden objects or tightly-packed paper in boxes, leave them for a week. Several older publications recommend repeated freeze/thaw cycle(s) as a precaution, but recent work indicates that one longer treatment at a low enough temperature should suffice.

After removal from the freezer, objects should be allowed to acclimate to room temperature, still completely wrapped, until they are at room temperature, at least 24 hours. Because many materials become temporarily brittle at low temperatures, post-freezing handling should be minimized until the objects return to room temperature.

There is no harm in leaving the objects in the plastic bags after treatment.  On the contrary, it will prevent future infestation.

While infested items can be placed in a refrigerator to halt activity, most museum pest activity will stop at 5 degrees C; insects will not be killed at this temperature.  It is important that these items be allowed to return to room temperature before beginning the low temperature treatment in your freezer.

An important part of any treatment of collection material is documentation. It should be possible to access the history of any one object related to both infestation and eradication. The IPM policy statements on this site give examples of the ways that various museums carry out documentation. Click here to access the Policy and Procedures templates. It is recommended that all pest incidents and any treatment such as freezing should be recorded, ideally in separate documentation (e.g. collection database, object treatment file), and a note placed in the bag with the object or artifact.

Pros

• Procedure is non-toxic to humans
• Safe for almost all organic and composite materials
• Relatively time-efficient
• Low cost after initial investment for freezer
• Some freezers can be modified to reach appropriate low temperatures
• Does not entail extensive staff training or staff time for maintenance during procedure

Cons

• Requires initial financial investment
• Some smaller freezers are not large enough for oversized items
• Walk-in freezers require space planning and setup
• Some maintenance is often required for large walk-in freezers
• Not appropriate for all materials
• Temperatures that do not fall fast or low enough will not achieve a good kill rate and treatment will not be fully effective

The success of freezing treatments depends on the ability to quickly drop the temperature in the freezer and sustain it through the length of the treatment. As a result, choosing the appropriate freezer is essential. According to Strang (2008) in practice, the ideal freezer system for killing insect pests operate between minus 22 degrees F (minus 30 degrees C) and minus 40 degrees F (minus 40 degrees C). Institutions must choose a freezer setup based on these requirements as well as the costs and practical needs of the collection.

The information below on freezers is drawn primarily from Strang’s “Controlling Insect Pests with Low Temperature” (1997, updated 2008). The full text should be consulted for details in the Resources  Bibliography page of this website.

Household Freezers:
Household chest freezers that reach and maintain minus 20 degrees F (minus 29 degrees C) can be very effective, and are usually less expensive than commercial freezers. Most household freezers are “frost free” which is acceptable. While there has been some anecdotal concern about the defrost ‘warming’ cycles, these are of relatively short duration, and the general response time of bagged materials commonly ‘frozen’ is comparatively longer than the warming phase. The target insects will generally be kept cold though the cycle by the cooling or cold object and will experience the lowest temperatures the freezer can offer. They will have been immobilized early in the cooling cycle.

Commercial Freezers and Chest-type Freezers:
Commercial freezers and “top of the line” household chest-type freezers can usually achieve the necessary temperatures to ensure kill rates. Commercial “ice-cream hardening” freezers in vertical or horizontal configurations are designed to operate at minus 44 degrees F (minus 42 degrees C) and may not be much more expensive than standard commercial freezers. It is recommended that performance be monitored with a separate thermometer. Ensure that the freezer is well insulated, and provides for adequate air circulation inside the freezer around collections materials.

Walk-in Freezers:
Some walk-in laboratory freezers allow users to set the operating temperature. “Air-blast” freezers are preferable because their forced air circulation increases the cooling rate. Check local vendors and manufacturers for “off the shelf” and custom-designed units. Thermometers on the outside of the freezer to monitor interior temperature are recommended.

Freezer Trucks and Frozen Food Warehouses:
Frozen food warehouses and their freezer trucks can be useful for low temperature treatment of large quantities of material or for when other freezing options are not available. Frozen food warehouses and freezer trucks used by frozen food processors/storage firms are typically held at 0 degrees F (minus 18 C) , which is cold enough to preserve food but not to eliminate the various stages of insect pests within a 72 hour period. In order to use a truck or warehouse for pest disinfestation, the material should be left for at least one week.

Low temperatures may be compromised if doors are opened for long periods of time during loading. Large quantities of materials should be palletized and shrink wrapped. Shrink-wrapping helps to allay fears owners of the building or truck may have about insects escaping the load during treatment. Items to be placed in freezer trucks should be palletized to maximize air circulation within the unit. The unit should be loaded as quickly as possible.

Frozen Food Shipping Containers

Insulated frozen food shipping containers, which are capable of maintaining the desired 20 degrees F (minus 29 degrees C) temperature, may be available for rent from dockside firms. This option may be suitable for institutions located near seaports. Rates are usually charged on a monthly basis. The firm will transport and drop the shipping container in your parking lot and pick it up when the treatment has been completed. A steel shipping container is usually 40′ long x 8′ wide x 9.5′ tall (roughly 3,000 cubic feet of space.) Items should be shrink-wrapped and  palletized to maximize air circulation within the unit. The unit should be loaded as quickly as possible.

Selected bibliography

Bergh, Jan-Erik. et. all. “A Contribution to Standards for Freezing as a Pest Control Method for Museums” Collection Forum 21 (1-2) (Fall 2006): 117-125.

Carrlee, Ellen. “Does Low-Temperature Pest Management Cause Damage? Literature Review and Observational Study of Ethnographic Artifacts” Journal of the American Institute for Conservation 42 (2003): 141-166.

Florian, Mary-Lou. Heritage Eaters: Insects and Fungi in Heritage Collections. James & James Publishers; 1997. Chapter 12.

Mibach, Lisa. Modifications to Home Freezers for Pest Control. WAAC Newsletter. 1994 Jan; pp. 26-27. Article included in page 11.

Strang, Tom and Kigawa, Rika.  “Combating Pests of Cultural Property.” Canadian Conservation Institute, Technical Bulletin 29, 2009.

Strang, Tom. “Controlling Insect Pests with Low Temperature” Canadian Conservation Institute Note 3/3, 1997, updated 2008.

Strang Tom. A Review of Published Temperatures for the Control of Pest Insects in Museums. Collection Forum 8(2) (Fall 1992): 41-67.

Case Studies & Other Resources on MuseumPests

The Smithsonian Institutions’ National Museum of the American Indian (NMAI) relies on “freezing” as their preferred treatment method for most collections. Read the case study Low Temperature Treatments at the National Museum of the American Indian for details.

The American Museum of Natural History used freezing to treat infested items found in the ethnographic collections. Read the poster presented at the 2017 SPNHC Annual Meeting.

The Freezer Fact Sheet provides freezer specs from various institutions.  It is unlikely that institutions looking now to purchase a freezer for low-temperature treatments will be able to find the exact brands and models listed in this document.  The information is provided to show the range of products that can adequately meet the technical specifications for this type of treatment.

Mechanical Behavior of Animal Hides at Low Temperatures summarizes results from a 2009 Smithsonian Institution Museum Conservation Institute / National Museum of the American Indian research project investigating the effects of low temperature treatment on stretched/constrained animal hides.  The project was undertaken to inform treatment decisions for drums and other composite items with stretched animal skins.

Freezer Modifications for Museum Pest Control contains an excerpt the Society for the Preservation of Natural History Collections (SPNHC) Newsletter and the Western Association for Art Conservation Newsletter (WAAC) on how to modify home and chest freezers for museum pest control.

Bed Bugs in University Library Collections – 3 Case Studies

There are several factors to consider, the type of object and pest type but a general guide from the literature suggests 3 days (72 hours) in a -30°C (-22॰F) or 14 days in a domestic freezer at -20°C (-4॰F) will kill most insect pests. (Strang 1992, Strang and Kigawa 2009).

Figure on the left (Strang 1997) shows the interpretation of the low temperature data presented on the right (Strang 1992). Across a number of common pest species and their pre-treatment acclimation there is a range of vulnerability to cold. There are, however, limits to the survival of species which have been studied and beyond this is a region where successful control by low temperature is expected.

Insects are somewhat variable in their tolerance for cold temperatures. The goal for cultural property is to eliminate as many insects that might be hiding or tunnelling inside an object. We take a  cautious approach by specifying treatment conditions that ensure there is the least chance of survival. If we don’t know the species of insects that are present, treating at lower temperatures and longer duration (see the right-hand side of the data cloud in the charts below) is a general recommendation.   

Figure from Strang (2014) showing the mortality data for all stages of coleopteran (left) and lepidopteran (right) pests of cultural property in black dots against the full (grey) dataset. 

• To reduce the chance of moisture on objects, low temperature treatments are carried out in a bag or box. This allows a considerable amount of moisture buffering  during treatment of organic objects. It also physically protects objects during treatment. Exceptions and best practices are listed below.
• Moisture gradients establish with a thermal gradient and moisture can accumulate in the  coldest part  of a system. During cool-down this potential for accumulation will be in the chilled headspace of the vapour resistant enclosure. During warm-up the object will be cooler than the headspace until the object surface warms so the accumulation could be on the object. However, very little moisture is present in the bag when starting at 50% RH (8.7 grams per cubic metre of air at 20°C (68॰F). To minimise condensation issues, wrap the object with tissue paper to avoid condensation on the object. The tissue will act as a primary buffer for vapour exchange and intercept condensation. 
• Another gradient hazard is avoided when one ensures the bagged object does not contact the walls or floors of a freezer. Contact will bridge the insulation property of the object with the freezer and prevent effective cooling at that junction. This results in a temperature gradient within the object/bag enabling moisture movement from the warm portion against the chamber wall. Also it prevents that portion of the object achieving the desired setpoint for killing insects.
• If you don’t have plastic films or bags it can be fine to use cardboard boxes that have all gaps sealed with tape provided the treatment is monitored for freezer failure. Should failure happen it can lead to damp conditions in the freezer chamber which then compromises box strength and given time can  dampen the box contents. 
• For metals the main concern is surface corrosion from condensation rather than pooling and staining. Slide paper under metal attachments on fabrics. Wrap in tissue paper to avoid condensation.

• Not allowing the object to reach the required temperature for long enough to kill all stages of pests. 
• Allowing your object to contact walls or floors of the freezer which prevents a portion of the object from reaching the desired temperature. 

• Promote even warming by having the object on a pallet, but preferably a poor insulator such as metal wire shelving. Allow space between the bagged object and walls and floor to allow  air movement. While a gradient in temperature must form between the object’s enclosure and the surroundings, prevent undue insulation of the object that might slow warming in that area. 
• Waiting 24 hours for smaller objects should be sufficient. More time may be required for very thick objects.  Warming is dependent on the self-insulation property and dimension of the object as well as air circulation.  The goal is to have the temperature of the object equal to the temperature in the room before unbagging.  The temperature of the bag, if even a bit lower than the room air can be an indicator that the object in the bag is even colder. Surface temperature measurements are easily done with an IR thermometer. Sometimes, the spot which a warming object sits on a table can be observed to condense room air (still quite cold), or feel cool to the hand. 
• To estimate cooling and warming time consult … (I did make the warming/cooling estimation graph that is in my 1995 bangkok conference paper and reworked in the TB appendix – TS)
• A sensor wrapped in the object, or in a surrogate object that is at least as thick as the objects being treated, can be instructive. For example, a sensor in the centre of a rolled or folded textile or a hollowed-out log or book can act as a guide for completion of a treatment. When that sensor reads room temperature, less dense and thinner materials should be at room temperature as well.

Unbagging an object before it reaches the same temperature as the room too quickly can cause a rapid temperature change (thermal shock) at the surface of the object, which can cause condensation and with higher vulnerability assemblages – mechanical damage.

Rapid temperature change to an object increases the potential for mechanical damage and is material and assemblage dependent. For instance, stretched canvases like paintings are more likely to experience mechanical issues than woven textiles. Glass and ceramics can crack if it experiences thermal shock and this is enhanced by rough edges that raise stresses. Great care should be taken to gradually increase temperature to room temperature for glass objects. Normally glass is triaged out of cold treatment but some glass component objects have been treated. Materials that transition from pliable at room temperature to stiff or brittle at low temperature can be protected by careful packaging and handling. If unsure – consider using a controlled atmosphere fumigation approach (see examples and case studies on the Solutions page).

Objects that have been stored in cold environments may have pests that have adapted to the cold and can be less susceptible to freezing treatments. This can be counteracted by quarantining at warmer temperatures (ideally, at or above room temperature for 1 or more weeks before carrying out low temperature treatments. Evidence of the mortality increasing effect of holding Attagenus unicolor (Brahm) at 15॰C (59॰F) for a week after being acquired from cold winter conditions is presented in  Hou et al. (2001).

• Quarantine is a period of isolation of objects or specimens out of a certain area to prevent the spread of disease or pests. Putting an item into quarantine will not alter its state in any way, it just separates it from everything around it. https://museumpests.net/prevention-introduction/prevention-examination-and-quarantine/
• Quarantine is often used to establish whether pests are active on an object to decide if treatment is warranted. Quarantine allows enough time for the pest population’s activity to become visible. This has the advantage of minimal handling and limited exposure to extreme temperatures; however, it often requires several months sealed in a container.
• Freezing is a treatment. Putting an object in the freezer for a certain amount of time can kill any insect pests that are contained within or on the object.
• Freezing is sometimes chosen to reduce the need or time for quarantine.

Low temperature treatments can be done as a preventive measure and requires much less time than quarantining. If low temperature treatments become the standard for ensuring pests are not travelling on an object, it is important to consider the risk to the object being sent on loan. There may be a greater likelihood of adverse impact to the object or the safety conferred by its mount/cushioning environment when being exposed to low temperatures multiple times if safe handling protocols are not adhered to for each treatment.

Yes, but it depends on the materials. Look at the materials, and combinations of materials – look to see if the structures combined can withstand the thermal cycle. If you can, try it out with similar material with no value. Self education by treating different materials and constructions (trinkets, jumble sale items) can be a constructive way to explore the use of cold in your institution.

There is no accepted definitive list of materials that can and can’t be frozen. This page includes the list based on recommendations from the MPWG participants. There are some suggested lists and case studies in the literature reporting successes and failures when using low temperature treatments. Some institutions have put tens of thousands of items from general mixed collections through low temperature control.

No, freezing twice was postulated as necessary, however,  it has been shown from review of a larger pool of efficacy data that a single low temperature cycle is sufficient (Strang 2012).

As long as strong polyethylene bags that are not easily torn are used or well-sealed cardboard boxes, a single vapour barrier layer is sufficient to avoid excessive water incursion and condensation. If necessary, thin poly bags can be used with careful handling to prevent tears.  

First you should consider the space available to house a freezer, the size and weight of objects you will need to put in the freezer, and the financial resources available to you. Beyond this, there are a few things to consider.

• Low temperature:  Ideally -30°C (-22॰F), though warmer domestic freezers can work with longer exposure times, -40°C (-40॰F) and is a practical solution. Consider lower minimum temperature,  technical limitations and costs as well as estimations that the sum of thermal contraction effects between different materials will start to be important modes of failure.
• The lower the temperature the greater the risk for mechanical damage from thermal contraction and shock during warming. Some materials have a ‘glass transition temperature’ where they move from rubbery through stiff and into brittle behaviour – a change that can increase hazard if improperly handled in the cold state. This risk is minimised by careful handling.
• Frostless freezers help to maintain the ability of a freezer to reach and maintain its lowest temperature. The brief warming cycle that keeps them frost free does not impact treatment success as many objects are not capable of rewarming significantly during the short warm cycle due to their size and the insulating effect of air in their moisture barrier packaging.
• Remote alarm to notify of power or setpoint  failure. Some are capable of connecting to Building Management Systems (BMS).
• For a walk-in unit, a smooth floor for carts and, if possible, have the unit recessed into the floor so there is very little lip or provide a ramp to facilitate entry.
• Ensure the freezer floor is level and that the floor can handle the weight of objects you plan to put in the freezer, as the floor is insulated and requires reinforcement if very heavy objects will be treated.
• The floor can become wet and slippery with condensation of room air coming in when the door is opened, so choose a non-skid floor. While raised bumps can cause significant vibration when rolling carts are used to load collections, wheel type selection (pneumatic over hard rubber) for the carts can mitigate this.
• Low profile LED lights allow tall objects to be more easily installed.  The alternatives are pendant, caged bulbs or plastic sleeved fluorescents. Modern LED fluorescent tubes are already plastic.
• Consider a service and maintenance contract as the compressor and other elements can break down.
• For chest freezers, top loading formats are a bit more space efficient than front loading and can often more easily handle larger objects, though front loading freezers with shelves are useful for freezing multiple small objects and may be less strain when  sorting through or lifting items.

Yes, if the freezer can reach a low temperature of -18°C (-0.4°F) to -20°C (-4°F).  It is important to make sure the objects are kept at this low temperature for a sufficient amount of time to kill all stages of the insect pests. Objects must typically stay in a domestic freezer for  longer than would be required in a  freezer that reaches a temperatures of  -30॰(-22॰F) or below, to make sure that all phases of the pest’s development are effectively killed. Be sure to measure the temperature inside the freezer for the duration of the treatment;  reference the sections above for guidance about appropriate equipment for monitoring temperature.

Not necessarily, you might be faced with a mix of species or have trouble identifying them. The general approach discussed above was designed to help with this so if your object reaches the required temperature for long enough all common insect pests will be killed. If you know the species you are trying to eradicate, you can tailor your treatments to a more specific time and temperature which may save time or add assurance. For example two serious collection pests, the webbing clothes moth Tineola bisselliella and the drugstore beetle Stegobium paniceum have been characterised in this way and the results published (Brokerhof 1992 and 1993;  Gilberg and Brokerhof 1992).

Allow good air circulation around objects. Use spacers to create gaps through units. Minimise the overall thickness of the object if there is some flexibility to do so, for example:

• For textiles, rolling around a large tube is better than rolling around a small tube.
• Raise textile tubes with end spacers to keep the textile from resting on a solid shelf or freezer floor.
• Allow spaces between stacked boxes that air can flow through, gaps of several centimetres will ensure convection flow. 
• Use wire shelving rather than solid shelving, it is temperature conductive and allows air flow to the contained object.

1993. W. Brokerhof, R. Morton and H. Jonathan Banks, 1993. Time-mortality relationships for different species and developmental stages of clothes moths (Lepidoptera: Tineidae) exposed to cold. Journal of Stored Products Research 29(3):277-282.
1994. W. Brokerhof and H. Jonathan Banks, 1993. Cold tolerance of Tineola bisselliella (Lepidoptera: Tineidae) eggs at slow cooling rate. Journal of Stored Products Research, 29(4):305-310.
1995. Gilberg and A. Brokerhof, 1991. The control of insect pests in museum collections: the effects of low temperature on Stegobium paniceum (Linneaus), the drugstore beetle. Journal of the American Institute for Conservation, 30(2):197-201.

Hou, X., Fields, P. and Galloway, T. 2001.  Cold hardiness of black carpet beetle larvae.  Canadian Entomologist, 133:325-327. 

1992. J. K. Strang, 1992. A review of published temperatures for the control of pest insects in museums. Collection Forum 8(2):41-67. URL?
1993. Strang, 1997. CCI Notes 3/3 Controlling insect pests with low temperature. Canadian Conservation Institute, Canadian Heritage, 4 pp.  URL?
1994. J. K. Strang, 2012. Studies in pest control for cultural property. Acta Universitatis Gothoburgensis PhD Thesis, 412 pp. DOI: http://hdl.handle.net/2077/31500
1995. Strang, 2014. Use of thermal control against pests of cultural property. 11th International Working Conference on Stored Product Protection Chiang Mai, Thailand. URL?
1996. Strang and R. Kigawa, 2009. Technical Bulletin 29. Combating pests of cultural property. Canadian Conservation Institute, Department of Canadian Heritage, Minister of Public Works and Government Services, Canada, 44pp.