Solutions – Heat Treatment

Brief description of treatment

Heat treatment for infested collection items is an effective method for assuring 100% efficacy in killing insect pests.  In addition, it is a method that can be carried out in situ and has been applied at scales ranging from small individual objects to entire multi-story buildings. It is particularly useful for treating large immovable items with minimal impact on surrounding operations. Heat treatment can also be a good option where resources are limited.

A short exposure of 55oC (130oF) is sufficient for eradication of all life stages of insects (Strang, 1992). The limiting factor for treatment is the time needed to heat through the affected object. Thus, it will take longer for the effective temperature to reach the center of thick or dense items that are effective thermal insulators, than for thin and more thermally conductive items. Temperature probes may be necessary to insure that the required temperature has been reached. A useful thermal treatment guide (for both low temperature treatment and heat treatment) including estimated treatment times can be found in Strang and Kigawa, 2009:

Note that microwaving as a heat treatment is not recommended for any collection material due to the risk of excess heating around metallic inclusions.

Thermo-lignum® treatment is a proprietary method of eliminating insects with heat using a climate controlled heating chamber which controls moisture content during heating and cooling. See Ackery et al. 2005 for more information.

What collections materials cannot be treated this way?

There are some contra-indications for heat treatment:

Changes in temperature can drive changes in relative humidity and moisture content. Some heat treatment procedures allow for control of object moisture content by containment (bagged in moisture barrier film) or active control (humidity adjusted during treatment to maintain moisture content). Other heat treatment procedures do not allow for this humidity control. Therefore other treatment options might be preferable for items that are extremely sensitive to changes in relative humidity treatments if controlling the object moisture content cannot be achieved.

Do not use heat treatment for low melting point waxes, certain adhesives which may slump (if in doubt, ask a conservator), flammables, explosives and plastic materials that melt or deform from heat in the treatment range (usually less than 60°C).

DNA extraction and PCR amplification have been shown to be relatively insensitive to heat treatments for pests (again, in the 52°C to 55°C range).   Rarity, high scientific value,  and other concerns should be considered before applying any  treatment to  collections, but  there is strong evidence that heat is far less harmful to DNA and protein structure than many of the fumigants applied in the past  (Ackery et al. 2004, Kigawa et al 2003, Kigawa 2012, Kigawa et al. 2011.)

Items with low activation energies, e.g. magnetic media and urethane binders, may be particularly susceptible to damage from accelerated aging caused by heat treatment as heat increases the rate of hydrolysis.  However, this factor should be weighed against the relatively small cost given the short duration of heat treatments. Many widely-accepted routine conservation treatments for paintings, paper, textiles and magnetic media (for example relieving “sticky shed” syndrome) involve temperatures as high, or even higher, than those required for heat treatment for pest eradication.  If in doubt, consult a conservator, and refer to published estimates of the severity of this factor (Strang 1995, 2001, 2012; Michalski).

General procedures       

Heat treatments can be carried out in ovens, environmental chambers with properly engineered room heaters and circulating fans, or in enclosures or buildings with heat supplied by outside heat generators through temporary ducts.

Heat treatment is now a common strategy for killing bed bugs, so exterminators offering this service use industrial machines that can heat whole rooms to the desired temperature, with monitoring equipment to assure that the proper levels are achieved. Note that if entire building structures are being treated attention should be paid to possible loss of heat via heat sinks such as ground floor building pads and openings. Risks to building components should also be considered and mitigated.

Solar bagging is a variation of the heat treatment. With this method, objects are bagged in black polyethylene sheeting before heat treatment to stabilize the moisture content of the objects and thus minimize any damage which would otherwise be induced by excessive shrinkage. A solar bagging method, developed by Tom Strang at the Canadian Conservation Institute, uses sunlight to produce the temperatures lethal to insects.  uArtifacts are wrapped in cotton to buffer moisture fluctuation and sealed in black plastic bags which prevent moisture loss and damage from ultraviolet and visible light. This bag is placed within a clear plastic enclosure outdoors that maximizes interior temperatures, delivers heat to the shade side, and screens out rain and contaminants. The wrapped object packets are then exposed to the sun, and the maximum heat rise is controlled by the angle of incidence. Inside the packets, temperatures may rise as much as 40°F to 70° F above the outdoor environment. This causes adult insects, eggs, and the stages in between to be killed in a matter of hours, due to increased rate of dehydration and enzymatic dysfunction.

More information on solar bagging is available:  Baskin 2001 provides a case study, Strang and Kigawa 2009, and Brokerhof 2001 provide various solar treatment designs, and papers in Strang 2012 discuss heat risks.

For small items of lesser value: place a pan of water on the bottom rack of a kitchen oven in order to maintain high humidity in the oven. Place the object onto an upper rack. Turn the oven to its “warm” setting, which is usually 140 degrees F/60 degrees C. (170 degrees F or 77 degrees C is too high.). Leave the object for three hours, and then turn the oven off, leaving the object inside. Remove the object when the oven is cool.

Pros and Cons of this treatment
Pros
  • Inexpensive and low-tech solutions available
  • Easy to conduct
  • Effective over a short period of time
  • Low equipment maintenance
Cons
  • Not all materials may be treated this way.
Bibliography

Ackery, P. R., J. M. Testa, P. D. Ready, A. M. Doyle, and D. B. Pinniger. 2004. “Effects of High Temperature Pest Eradication on DNA in Entomological Collections.” Studies in Conservation 49 (1): 35–40.

Ackery, P. R., David Pinniger, Adrian Doyle, and Karen Roux. 2005. “Heat Treatment of Entomological Drawers Using the Thermo Lignum Heat Process.” Collections Forum 19 (1-2): 15–22.

Baskin, Bonnie. 2001. “Solar Bagging: Putting Sunlight to Work to Eliminate Insect Infestations in Mere Hours.” Newsletter – Western Association for Art Conservation 23 (2) (May): 20–21.

Brokerhof, Agnes W. 2004. “The Solar Tent: Cheap and Effective Pest Control in Museums.” AICCM Bulletin 28: 93–97.

Chalfoun, David J., and Noreen Tuross. 1999. “Botanical Remains: Utility in Protein and DNA Research.” Ancient Biomolecules 3: 67–79.

Kigawa, Rika, Tom Strang, Noriko Hayakawa, Naoto Yoshida, Hiroshi Kimura, Gregory Young. 2011. “Investigation of Effects of Fumigants on Proteinaceous Components of Museum Objects (Muscle, Animal Glue and Silk) in Comparison with Other Non-Chemical Pest Eradicating Measures.” Studies in Conservation 56 (3): 191–215.

Michalski, Stefan. 2000. Technical Bulletin 23: Guidelines for Humidity and Temperature in Canadian Archives. Ottawa: Canadian Conservation Institute, Canadian Heritage.http://www.cci-icc.gc.ca/publications/downloads/technicalbulletins/eng/TB23-GuidelinesforHumidityandTemperatureforCanadianArc.pdf

Pinniger, David. 1996. “Insect Control with the Thermo Lignum Treatment.” Conservation News (United Kingdom Institute for Conservation of Historic and Artistic Works) 59: 27–28. http://www.bcin.ca/Interface/openbcin.cgi?submit=submit&Chinkey=166154.

Strang, T .J. K. 1994. “Reducing the Risk to Collections from Pests.” CCI Newsletter 14. Canadian Conservation Institute Newsletter: 8–10.

Strang, T .J. K. 1995. “A Brief Guide to Thermal and Controlled Atmosphere Treatments for Insect Eradication.” ICOM-CC Preventive Conservation Working Group Newsletter: 4–5.

Strang, T. J. K.  1995. “The Effect of Thermal Methods of Pest Control on Museum Collections.” In Proceedings of the 3rd International Conference on Biodeterioration of Cultural Property, 334–353. Thammasat University, Bangkok, Thailand: Thammasat University Press.

Web Resources

Michalski, Stefan. 2000. Technical Bulletin 23: Guidelines for Humidity and Temperature in Canadian Archives. Ottawa: Canadian Conservation Institute, Canadian Heritage.

National Park Service Conserv O Gram 3/8

National Park Service Integrated Pest Management Manual

Nicholson, Mark and von Rotberg, Werner. 1996. Controlled environment heat treatment as a safe and efficient method of pest control. The 2nd International Conference on Insect Pests in the Urban Environment. Edinburgh, Scotland.

Strang, Tom, and Rika Kigawa, 2009. Technical Bulletin 29: Combatting Pests of Cultural Property. Ottawa: Canadian Conservation Institute, Canadian Heritage.

Strang, T. J. K.  2012. “Studies in Pest Control for Cultural Property.” Doctoral thesis, Gothenburg, Sweden: University of Gothenburg. Faculty of Science.

Strang, T.J. K. 2001. “Principles of Heat Disinfestation.” In Integrated Pest Management for Collections: Proceedings of 2001: A Pest Odyssey, 114–129.

Case Studies & Other Resources on MuseumPests

Bed Bugs in University Library Collections – Three Case Studies

Download the poster Materials Testing: The Use of Heat and Humidity Chambers for Pest Eradication by Dr. Marieanne Davy Ball ACR, Department of Conservation, Cultural History Museum, University of Oslo, Oslo, Norway; and Christina Bisulca and Dr. Nancy Odegaard, Preservation Division, Arizona State Museum, University of Arizona presented at the American Institute for Conservation’s 39th Annual Meeting in 2011 for information on what materials may or may not be considered appropriate for the Thermo-Lignum heat treatment.  See abstract below.

Dr. Marieanne Davy Ball ACR, Department of Conservation, Cultural History Museum, University of Oslo, Oslo, Norway; and Christina Bisulca and Dr. Nancy Odegaard, Preservation Division, Arizona State Museum, University of Arizona

Heat has long been used in conservation for the artificial aging of materials, but now it is also being used as a pest eradication treatment. The heat chamber heats objects to a temperature where insects cannot sustain life (~54–60°C) at a chosen, set relative humidity. The chamber is being marketed to museums as a less time-consuming pest eradication treatment because the process can be completed in approximately 16 hours, as opposed to days or weeks needed for freezing or carbon dioxide treatments.

Museum objects are often complex composites with adhesives, coatings, or natural oils and resins within the materials themselves. Many of these materials have low melting or glass transition (Tg) points, or are prone to thermally induced dimensional or chemical changes. Mechanical properties of certain materials are dependent on relative humdity: of particular concern is Tg, which decreases with increasing relative humdity for many polymers. The migration of waxes and distortion of some objects after treatment in the heating chamber has already been observed. Due to such unknowns, it was deemed important to identify potential problems that can arise from heat treatment.

To assess the potential effects of the heat treatment, samples of 21 common adhesives, resins, and waxes were tested. For each material, four separate sample preparations were assessed using the material as an adhesive on wood and glass joins, as a film on filter paper, and as a film on glass. These samples were weighed, measured, and photographed prior to and after treatment. Glass film sample preparations were additionally analyzed with Fourier transform infrared spectroscopy (FTIR) to assess any chemical change before and after aging. Due to the probability of an object passing through this eradication process several times within its life, cumulative effects were also recorded from a series of five test runs, recording the core and ambient temperature and relative humidity within the chamber.

Results show that certain classes of materials were susceptible to specific types of deterioration from heat treatment. Even with fresh materials, the epoxies yellowed and the hide glues and natural resins were prone to yellowing, crazing, slippage in joints, and/or weight loss. Many waxes and some synthetic adhesives melted or slumped during heat treatment. Several natural resins and oils showed chemical change (oxidation, loss of water) based on FTIR results. With further testing on other materials (horn, skin/leather, bone, etc.), we should be able to make informed decisions about which materials can be treated safely using heat for pest eradication.

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Integrated Pest Management Working Group
Treatment Subgroup March 2014, Updated 2017

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