Summary of Potential Artifact Damage from Low Temperature Pest Control

April 3, 2009

The Textile Specialty Group Postprints: A Joint Session with the Objects Specialty Groups Concerning Composite Objects.  American Institute for Conservation 30th Annual Meeting Miami Florida, June 2002.

 

Summary of Potential Artifact Damage from Low Temperature Pest Control

 

Abstract

Preventive freezing for pest control during the relocation of the ethnographic collection of the Smithsonian Institution’s National Museum of the American Indian, Suitland, MD afforded the opportunity to undertake an observational study of the potential damage to vulnerable categories of materials and to investigate the possible causes.  The observational study revealed no visible damage to any of the materials frozen, although minor changes on a molecular level are likely.  Moisture issues are less of that threat than effects related to low temperature alone, such as shrinkage, embrittlement, and molecular alteration.  While many of these changes are reversible upon warming, the danger of cumulative effects from repeated preventive freezing of objects is questioned.  The conservation tradition of borrowing information from other fields proves difficult to apply to a low-temperature low-moisture content closed system.  This study contributes to an informed approach for the freezing of composite objects, cracked objects, lamellar objects, and waxy or oily objects.  Concepts of condensation, moisture content, concentration effects, glass transition temperature, coefficient of thermal expansion, polymorphism, lipid autoxidation, protein denaturation, ratcheting and shakedown are reviewed.  

 

INTRODUCTION

The Smithsonian Institution’s National Museum of the American Indian (NMAI) is in the process of moving its collections from facilities in the Bronx to the new Cultural Resources Center in Suitland, MD, just outside of Washington DC.  The old facilities had many insect infestations, and the current move protocol includes preventive low temperature treatment of most organic materials before entering the new facility.  Objects are sealed in a close-fitting polyethylene bag with padding and cooled below -20C for at least five days.  This situation afforded the opportunity for an observational study of the potential changes to ethnographic artifacts from low temperature pest management.

 

BACKGROUND

Several categories of artifacts are thought to be cause for concern at low temperatures.  One category is composite objects.  Materials generally not exposed to low temperature treatments, such as glass and metal, may be attached to materials appropriate to treat, such as wool.  Composite objects may also have built-in tension, and one material may restrict the movement of a different material.  A second category is cracked objects.  Concern here lies in possible propagation of the cracks or potential structural weaknesses implied by the presence of cracks.  Delamination of lamellar objects is another area of concern.  Examples include tooth and horn as well as layered constructions such as painted wood or adhesive systems.  The fourth category includes oily or waxy objects which sometimes demonstrate bloom or crystallization.  

 

DISCUSSION

The possible causes of artifact damage divide into those related to water and those not related to water.  Moisture-related issues include freeze-thaw cycling, dehydration, condensation, and swelling.  Conservation scientist Mary-Lou Florian has written extensively about these issues, but moisture remains a persistent concern for many museum professionals.  A well established understanding of damage from fluctuations in relative humidity (RH) leads to the extrapolation that artifacts may suffer from swelling and condensation in cold environments since RH increases as temperature decreases.  Standard operating procedure for pest management at low temperatures involves sealing the object in a close-fitting polyethylene bag with a buffering material such as tissue paper.  The total amount of moisture inside the bag is finite and in fact very low (Florian 1990ab, 1992.)  Buffering materials compete with the object for humidity the air can no longer hold, and porous organic objects have the ability to accommodate small increases in RH.  The bag itself prevents condensation on the object after removal from the freezer.

 

Experience in the kitchen also influences the understanding of organic materials at low temperature.  Water is critical to issues of food preservation.  Ice formation causes the 9% expansion in water volume responsible for freeze-thaw damage.  (Franks 1985.)  An increase in membrane permeability at low temperature causes loss of turgor pressure and wilting of fresh plant materials (Reid 1987.)  Removal of water from a solution via ice formation causes the remaining solutes to increase in concentration.  These so-called “concentration effects” can drastically alter pH, viscosity, oxidation-reduction potential, salt concentration, and enzymatic reactions (Taylor 1987.)

 

The fact of the matter is, most museum objects do not possess sufficient moisture content to form ice.  Most organic artifact materials in a museum environment have between 8% and 12% moisture content (Florian 1986.)  Artifact material with an equilibrium moisture content (EMC) of up to 28% does not form ice at the temperatures used for pest control (Zachariassen 1985.)  It is for this reason that some conservators avoid the term “freezing” and its implication of ice formation when discussing museum pest management.  It is also worth noting that many materials can take weeks or months to reach EMC at room temperature, and cold temperature tends to slow the process even further (Grattan and Barclay 1988, Howell 1996, Adelstein et al. 1997.)  The low temperature moisture content in the closed bag situation at approximately -20C is in fact rather unique and analogies are not easily found in the literature from other fields.  

 

Dr. Dana Elzey, research assistant professor of materials science at the University of Virginia, Charlottesville, VA consulted on the potential problems related to low temperature exclusive of moisture issues.  These areas of concern include shrinkage, embrittlement, thermal shock, polymorphic phase change, and molecular alteration.  Shrinkage may serve to counteract that small amount of swelling mentioned earlier.  Practically all materials shrink as temperature is lowered because of reduced vibration on the molecular level.  The coefficient of thermal expansion (CTE) is a measure of this change and is dependent on the strength of interatomic bonds.  Materials with weaker bonds, such as many organics, shrink more than those with stronger bonds, like metals.  At low temperatures, composite objects may be at risk for damage from “CTE mismatch.”  There are published tabulations for expansion coefficients on some common materials, but there may be no data for many materials in aged or altered condition, no data in the appropriate temperature range, or simply no data at all.  Often materials are simply categorized as high or low relative to each other.  During cooling, the low CTE material goes into tension and risks cracking or delaminating while the high CTE material is in compression and in danger of deformation or crushing.  CTE mismatch can also be seen within a single material, particularly one that demonstrates anisotropy.  The bonds in anisotropic materials are direction dependent and expand differently in different directions.  Examples include materials that tend to crack in a preferential direction, such as wood, bone, tooth and lamellar structures.  Cracking is not the only manifestation of CTE mismatch.  If the high CTE material is sandwiched between two layers of low CTE material, it may be extruded by pressure from the surrounding material.  While many materials have the ability to deform elastically and then recover, at sufficiently high stress some materials may lose the ability to deform elastically, resulting in non-reversible plastic deformation or failure.  

 

Embrittlement is another area of risk that may be reversible upon warming if elastic deformation is not exceeded.  Embrittlement occurs at low temperatures because molecules are resistant to motion.  The glass transition temperature (Tg) is an indicator of material flexibility.  Below Tg, applied stress may cause brittle fracture; above Tg, elastic deformation is more likely to occur.  Examples of materials that become brittle at temperatures used for pest control include rubber, oil paint, synthetic polymers, acrylic paint and soft vinyl (Michalski 1991.)  Vibration from a faulty freezer or rough handling before the object returns to room temperature are two sources of stress.  Embrittlement is usually reversible upon warming.

 

Any discussion of the risk of damage from shrinking should include an introduction to the terms “ratcheting” and “shakedown.”  Ratcheting describes the accumulation of plastic strain.  A ratcheting crack grows each time it is exposed to the same stress.  Damage evolution due to this kind of cycling is known as fatigue and will eventually lead to macroscopic failure.  The other option, shakedown, involves a reduction of the incremental strain per cycle.  Most of the damage in this process occurs the first time an object is opposed to stress, and each subsequent cycle results in less damage per cycle (Elzey 2001.)

 

Thermal shock is the condition in which rapid temperature change leads to excessive internal stress resulting in damage or failure.  It is the phenomenon that causes a hot ceramic plate to shatter under cold water.  Several factors influence magnitude of stress: overall change in temperature, rate of cooling, size of the object, coefficient of thermal expansion, elastic stiffness, conductivity, and strength.  Objects most at risk for thermal shock have high CTE, high elastic stiffness, low thermal conductivity, and low strength.  A large, rapid change in temperature increases the risk of thermal shock.  Although most organic materials possess high coefficients of thermal expansion, conduct heat poorly, and are held together by low-strength secondary bonds, they have the advantage of very low elastic stiffness and are comparatively resistant to the effects of thermal shock.  It is the inorganic components of certain composite objects that are of concern here.  

 

Polymorphic phase change is another factor to consider in low temperature pest management.  Phase change involves a change in state, such as from solid to liquid or liquid to gas.  Polymorphic phase change involves a solid-to-solid phase change from one crystalline arrangement to another.  In some cases, one polymorphic phase may be more stable than another at low temperature.  Tin disease is one such example.  At room temperature, pure tin is a shiny white metal.  As temperature decreases, a non-metallic crumbly gray powder becomes the more stable form, reaching a maximum stability at -30C.  Tin disease is inhibited by most of the common alloying metals used with tin.  Most of the museum’s tin artifacts, such as cone tinklers on Native American artifacts from the Great Plains, are alloys and therefore safe from polymorphic phase change in the freezer.  However, the textbook example of tin disease involves Napoleon’s attempted 1812 winter invasion of Moscow, which failed in part due to the disintegration of the pure tin buttons on the soldiers’ clothing.  Low temperature is also a factor in structural change because some materials, such as rubber and some fats and waxes, become crystalline at low temperature.  This phenomenon seems to be at least partially reversible upon warming (Baker, 1995.)

 

The final area of concern addressed here is molecular alteration, particularly regarding protein denaturation, lipid autoxidation, and loss of moisture regain in materials demonstrating hysteresis. Conformational stability in protein is dependent on a complex energy balance involving a variety of intermolecular forces.  Cooling weakens some forces, such as hydrophobic interactions, but enhances others, such as hydrogen bonding (Taylor 1987.)  The technology to study proteins at low temperature in the absence of ice formation has only been developed in the past decade.  The formation of ice and the concentration effects that occur when water is removed as ice forms continue to be at the center of scientific research, making the question of permanent denaturation of proteins from low temperature alone difficult to resolve (Taborsky 1979; Fahy 1995; Franks 1995.)

 

The Arrhenius equation states that the rate of chemical reactions tends to slow with decreasing temperature.  The oxidation of lipids is sometimes an exception.  Lipids contain a wide variety of fatty acids that differ in chemical and physical properties as well as their susceptibility to oxidation.  Some follow the Arrhenius equation and oxidize more slowly at room temperature.  However, low temperatures can accelerate autoxidation of unsaturated fatty acids (Karel 1985.) Mechanisms for this are frequently described in the literature as “enzyme-catalyzed.”  Since enzymes are proteins produced by living organisms functioning as biological catalysts in living organisms, it is doubtful that there are any active enzymes remaining in museum artifacts.  

 

“Because enzymes function nearly to perfection in living systems, there is great interest in how they might be harnessed to carry on desired reactions of practical value outside living systems.  The potential value in the use of enzymes (Separate from the organisms that synthesize them) is undeniable, but how to realize this potential is another matter.”    (Roberts and Caseiro 1977.)

 

Furthermore, solute concentration effects that allow enzymes and substrates to come into contact influence some enzymatically-catalyzed oxidation in lipids (Reid 1987.)  Museum objects that cannot form ice are not subject to concentration effects. 

 

The loss of moisture regain ability due to changes on the molecular level is another potential concern.  Many organic materials are able to absorb and desorb moisture to keep in equilibrium with environmental humidity.  Taking up moisture brings them to a more stable energy state and generally occurs faster than desorption, as the material is reluctant to give up that moisture.  This relationship between water activity and moisture content is illustrated by a sigmoidal curve known as the moisture sorption isotherm.  For example, room temperature wool at 55% RH has a lower moisture content if it is in the process of getting wetter than it does under the same conditions if it is getting drier.  At low temperatures, molecules with potential water-holding sites may draw closer together and bond, creating a reduced capacity to hold water in the future.  (Timar-Balazsy and Eastop 1998.)  The conservation literature suggests there may be a distinction between damage from long term cold storage and short term low temperature exposure for pest control (Wolf et al. 1972; Williams et al. 1995; Pool 1997.)  

 

CONCLUSIONS

Exploration of the literature and consideration of materials science issues raise two areas of concern.  One involves the likelihood of repeated freezing cycles for some objects, particularly those actively loaned or exhibited and therefore subjected to low temperature treatments with each re-entry into the museum collection or new venue.  Data involving wood (George et al 1992; Erhardt et al. 1996;) textiles (Holt et al. 1995; Jansson and Shishoo 1998; Peacock 1999), synthetic fishing gear (Toivonen 1992;) paper (Bjordal 1998) and insect collections (Rawlins 2001) suggest no significant structural damage with repeated low temperature treatment for pest control.  Theoretically, however, embrittlement, shrinkage, and thermal shock have the potential to cause damage if the limits of elastic deformation are exceeded, or if ratcheting occurs within the elastic range and leads to fatigue (Elzey 2001.)

 

The second area of concern involves the permanent physical changes that are likely to occur (and perhaps accumulate) on a molecular level but remain invisible to the naked eye, such as loss of strength, loss of elasticity, distortion, crystallization, molecular alteration, protein denaturation, and loss of regain ability.  In some cases there may be synergistic effects in which interrelated damage mechanisms combine to cause further problems.  

 

In summary, it might be helpful to state this information plainly.  Based on this investigation which involved freezing several hundred artifacts, reviewing the literature, and discussing the topic with many museum and scientific professionals, a list of factors has been prioritized from highest-to-lowest concern.  On the whole, low temperature pest control appears to be safer for artifacts than might have been suspected.  

 

1. Freeze-thaw and dehydration should not occur because there is not enough moisture in museum artifacts.  

2. Condensation should not happen if artifacts are bagged properly.  

3. Swelling probably happens a little bit, but not much because there is so little moisture inside the sealed bag.

4. Polymorphic phase change does happen with some materials, usually fats and waxes, but this is usually reversible upon warming except in rare cases such as tin disease.

5. Thermal shock is not an issue for most organics because the temperature change is not drastic or sudden enough.  Inorganics are at greater risk, but no reports of this kind of damage were found.

6. Shrinkage undoubtedly occurs, but at this temperature it’s fairly minor and perhaps counteracted by the small amount of swelling.  The reason it is placed higher on the list is because of CTE mismatch danger.  Drums are one of the few objects not frozen at the NMAI.  

7. Embrittlement is also very likely to happen, but is usually reversible upon warming and objects are mainly at risk from vibration or rough handling until they warm up.

8. Molecular alteration is a bit of a wild card.  Protein denaturation may occur, but it may be reversible upon warming.  As far as lipid autoxidation goes, this may not happen at all without enzymes and sufficient moisture content.  Loss of moisture regain ability appears to be more of a danger with long term cold storage.

 

ACKNOWLEDGEMENTS

Thanks to Dr. Dana Elzey, department of Materials Science and Engineering at the University of Virginia, and the staff at NMAI, including Marian Kaminitz, Emily Kaplan, Jessie Johnson, and Leslie Williamson for their support and feedback.  I would especially like to acknowledge Mary-Lou Florian for her excellent work on this topic.  Thank you to the Andrew W. Mellon Foundation for making this research possible.

 

REFERENCES

Adelstein, P.Z., J.L Bigourdan, and J,M, Reilly.  1007.  Moisture relationships of photographic film.  Journal of the American Institute for Conservation 36(3):193-206.

 

Baker, M.T. 1995. Ancient Mexican rubber artifacts and modern American spacesuits: studies in crystallization and oxidation.  In Materials issues in art and archaeology.  vol. 4, ed. P.B. Vandiver et al.  Pittsburgh: Materials Research Society. 223-232.

 

Baker, M.T. 1995. Thermal studies on ancient and modern rubber: environmental information contained in crystallized rubber.  In: Resins: ancient and modern.  ed. M.M. Wright and J.H. Townsend. Edinburgh: Scottish Society for Conservation and Restoration. 53-56.

 

Bjordal, L. 1998.  Effects of repeated freezing on paper strength.  Proceedings of the third Nordic symposium on insect pest control in museums.  Stockholm, Sweden: Naturhistoriska Rikmuseet. 54-56.

 

Elzey, D.M. 2001. The effects of thermal cycling on the structure and properties of solids.  Lectures given at the National Museum of the American Indian Research Branch, Bronx, NY and Cultural Resource Center, Suitland, MD.   Department of Materials Science and Engineering.  University of Virginia, Charlottesville, VA.

 

Erhardt, D. M.F. Mecklenburg, C.S. Tumosa, and T.M. Olstad.  1996.  New versus old wood: differences and similarities in physical, mechanical, and chemical properties.  In ICOM Committee for Conservation preprints.  ed. J. Bridgland.  11th Triennial Meeting, Edinburgh, Scotland. Paris: ICOM. 903-910.

 

Fahy, G.M. 1995 Cryobiology: the study of life and death at low temperatues.  21st century medicine.

 

Florian, M.L. 1986.  The freezing process: effects on insects and artifact materials.  Leather Conservation News 3(1): 1-4.

 

Florian, M.L. 1990a.  Freezing for museum pest eradication.  Collection Forum 6(1):1-7.

 

Florian, M.L. 1990b.  The effects of freezing and freeze drying on natural history specimens.  Collections Forum 6(2):45-52.

 

Florian, M.L. 1992. Saga of the saggy bag.  Leather Conservation News. 8:1-11.

 

Franks, F. 1985.  Biophysics and biochemistry at low temperatures.  Cambridge: Cambridge University Press.

 

Franks, F. 1995.  Protein destabilization at low temperatures.  In Protein Stability. ed. D.S. Eisenberg and F.M. Richards.  Advances in Protein Chemistry 48.  New York: Academic Press.  105-139.

 

George, M.F., B.C. Cutter and P.P.S. Chin. 1992. Freezing of water in hardboard: absence of changes in mechanical properties.  Wood and Fiber Science.  24(3):252-259.

 

Grattan, D.W., and R.L. Barclay.  1988.  A study of gap-fillers for wooden objects.  Studies in Conservation.  33(2):71-86.

 

Holt, L., Y. Chen and W. Dodd.  1995.  The effect on wool fabrics of multiple freeze/thaw treatments for insect control.  Textile Conservation Newsletter 29:28-35.

 

Howell, D. 1996.  Some mechanical effects of inappropriate humidity on textiles.  In ICOM Committee for Conservation Preprints, ed 1.  J. Bridgland. 11th Triennial Meeting.  Edinburgh, Scotland.  Paris: ICOM. II:692-698.

 

Jansson, P., and R. Shishoo. 1998.  Effect of repeated freezing treatment on the mechanical properties of new wool.  Proceedings of the third Nordic symposium on insect pest control in museums.  Stockholm, Sweden: Naturhistoriska  Rikesmuseet.  57-60.

 

Karel, M. 1985.  Lipid oxidation, secondary reactions, and water activity of foods.  Autoxidation in food and biological systems.  New York: Plenum Press.

 

Michalski, S. 1991.  Paintings: their response to temperature, relative humidity, shock, and vibration.  In: art in transit: studies in the transport of paintings.  ed. M.F. Mecklenburg.  Washington, DC: National Gallery of Art.  223-248.

 

Peacock, E.E. 1999.  A note on the effect of multiple freeze-thaw treatment on natural fiber fabrics.  Studies in Conservation  44(1): 12-18.

 

Pool, M.A., 1997.  Preliminary analysis of the effects of cold storage on fur garments and mammal skins.  Collection Forum  13(1):25-39.

 

Rawlins, J. 2001.  Personal communication.  Section of Invertebrate Zoology, Carnegie Museum of Natural History.  Pittsburgh, PA.

 

Reid, D.S. 1987.  The freezing of food tissues.  In The effects of low temperatures on biological systems.  ed. B.W.W. Grout and G.J. Morris.  London: Edward and Arnold Publishers. 478-487.

 

Roberts, J.D. and M.C. Caserio.  1977.  Basic principles of organic chemistry.  Reading, Massachusetts: W.A. Benjamin Inc. 1270.

 

Taborsky, G. 1979.  Protein alterations at low temperatures: an overview.  In Proteins at low temperatures.  Ed: O. Fennema.  Advances in chemistry series 180.  Washington DC: American Chemical Society: 1-26.

 

Taylor, M.J. 1987.  Physio-chemical principles in low temperature biology.  In The effects of low temperatures on  biological systems.  ed. B.W.W. Grout and G.J. Morris.  London: Edward Arnold Publishers. 17-23.

 

Timar-Balazsy, A. and D. Eastop.  1998.  Chemical principles of textile conservation.  London: Butterworth-Heinemann.  15-25.

 

Toivonen, A.L. 1992.  Investigation of the effects of cold winter conditions on fishing gear materials.  Journal of the Textile Institute 83(1):163-177.

 

Williams, S.L., S.R. Beyer, and S. Kahn.  1995.  Effect of “freezing” treatments on the hydrothermal stability of collagen.  Journal of the American Institute for Conservation.  34:107-112.

 

Wolf, M., J.E. Walker, and J.G. Kapsalis.  1972.  Water vapor sorption hysteresis in dehydrated food.  Journal of Agricultural Food Chemistry 20 (5):1073-1077.  

 

Zachariassen, K.E., 1985. Physiology of cold tolerance in insects.  Physiological Review 64(Oct):799-832.

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Integrated Pest Management Made Easy

March 19, 2009

http://www.museums.state.ak.us/documents/bulletin_docs/bulletin_29.pdf

Bulletin No 29, Winter 2007

Your building has pests.  Yes, it really does.  Ours does, too.  But are they a threat to your collection?  With an Integrated Pest Management (IPM) system, you can be active in your prevention of infestation and effective in your response if one occurs.  In the past, museums would respond to evidence of an infestation with poisons.  Many of those substances are now illegal, some contaminated or damaged the artifacts, and most were dangerous to museum staff as well.  Museums took a cue from the agriculture industry, which needed to control bugs on stored grains without contaminating the food with toxins.  An IPM system uses good housekeeping to keep pests out, traps to monitor the presence of bugs, and low temperature to treat infestations.

1. Good housekeeping aims to keep the pests out in the first place.  If you can avoid carrying in new pests, prevent them from entering the building from outdoors, and reduce things that attract them, you’re preventing the problem in the first place.  Here are some of our policies at the Alaska State Museum:
*       Eating is only allowed in the kitchen and conference room.
*       Eating during receptions is kept in a limited area.  The carpet is vacuumed immediately afterwards and trash is disposed of outside the building right after the event.
*       Beverages are not permitted at staff desks with the exception of water, coffee or tea in a closed container.
*       Collections spaces are kept free of non-collections materials and clutter is not allowed.  The cleaner your space, the quicker you will notice something is not right.
*       Packing materials are disposed of in the dumpster outside the building.
*       No plants or flowers are allowed in the building.  None.  They are a proven source of bugs as well as food for the bugs.
*       Structural gaps in the building are closed with silicone caulk, weather stripping or door sweeps.  For rodents, brassy steel wool can plug holes (and doesn’t rust.)  Mice can get through spaces the size of a quarter.
*       ¼” steel hardware cloth is used to cover floor drains.  Rats swim!
*       Keeping water drains on the roof clear eliminates many gnats. Usually a hose works fine.

2. Monitoring your populations with sticky traps gives you an early warning of trouble afoot.  We order our traps through Insects Limited: (317) 896-9300 www.insectslimited.com  The cost is approximately $50 for a box of 100 traps that can be torn into thirds.  That makes 300 traps at about 17 cents each.  For our three floors and approximately 24,000 square feet, we set about 50 traps.  They are also called “blunder” traps, so place them where a bug is likely to stroll in.  This includes along the wall, near sources of water like drains, and next to doorways.  Number each location on a map, and label each trap with its number, location and date.  Change the traps every three months, and keep a chart that describes what you found in each trap.  This task usually takes about 3 hours at the ASM.  If you take a flashlight, checking those dark corners for rodent droppings or other debris is also useful.  Our traps at the Alaska State Museum usually contain lots of spiders and sowbugs (also called pillbugs) as well as ants, large black click beetles, and centipedes.  Google images is helpful, and so are www.bugguide.net and www.museumpests.net.  When we find an insect that looks like a “heritage eater,” but we aren’t sure, we put out extra traps in that location for next time and send the trap to the Forest Service for positive identification.  We also ask staff to catch any bugs they see on a piece of scotch tape.  Anything that was originally a plant or animal has potential for insect infestation.  At the top of the list for tasty bug treats are fur, feathers, leather, and wool.

3. Treatment involves a freezer.  Research indicates that our “heritage eaters” can be killed in all phases of their life cycle by one week below -20°C.  However, many museums only have access to a frost-free freezer, with temperatures that cycle well above -20°C.  Many insects are “frost tolerant” and can make a substance like antifreeze to survive a dose of cold.  But our brains are bigger!  The artifact can be placed in the freezer for a week, then removed and allowed to reach room temperature for 24 hours, and put back in the freezer for another week to deliver a deadly second round of cold.  It is very important to package the artifact properly for low temperature treatment.  You must wrap the artifact in a soft absorbent material such as plain tissue paper, white paper towels, or a soft cloth.  This helps protect it against both the increase in relative humidity at lowered temperature and the slight increase in brittleness when things are cold.  Then, the artifact needs to be placed in a plastic bag that is well sealed.  Squeeze as much air from the bag as you can and seal the Ziplock or use a heat sealer if possible.  Lucky for us, most museum artifacts don’t have enough water in them to create ice.  However, upon removal from the freezer, condensation will form, and it is much better for that moisture to form on the plastic bag than on your artifact!  After a day of adjusting to room temperature, you can safely remove your artifact from the package.  Removing all the old bug debris is a good idea, so any future bug debris will be a clue to a new infestation.  Brushing the debris into the nozzle of a vacuum cleaner with a soft paintbrush usually does the trick.

When infestations occur, not only do the artifacts go into the freezer, but the infested space must be vacuumed, carpet steam-cleaned, and the perimeter of the area dusted with boric acid.  Occasionally, it is necessary to turn to bait.  Ant traps and D-Con are examples of bait, which are not pesticides but kill the pest through mechanisms like thinning the blood to induce internal bleeding.  Bait typically kills much more efficiently than traps.  A recent infestation of picnic ants at the Alaska State Museum was controlled with ant bait that was carried back to the nest.

Many museums do preventive treatment of incoming artifacts with the freezer.  A donation of a fur parka, for example, would definitely go in our freezer before it went into our clean collections room.  What if you don’t have a freezer, or the incoming artifact is too big?  Careful visual inspection in dark crevices can help set your mind at ease.  Look for holes, loose hair, bald patches, live bugs, bug parts, cocoons, webbing, bug nests, and tiny bug droppings known as “frass.”  Frass is round, so suspicious looking dirt can be sprinkled on a piece of paper and the paper tilted…if it rolls easily, it might be frass.  If you don’t see this evidence, the next step is to lay the artifact on a pristine white surface and place some sticky traps around it.  Wait two months or so to allow any eggs to hatch and get active.  If you see no debris on the white surface and nobody in the sticky traps, you’re probably safe.  Preventive treatment is also done with items for sale in the Alaska State Museum gift shop.

An Integrated Pest Management system is part of professional museum practice, just like monitoring your temperature and relative humidity, and keeping your light levels appropriate.  Dealing with an infestation after it happens is upsetting, time consuming, difficult, and often means irreversible damage to museum collections.  An ounce of prevention is truly worth a pound of cure.  Have questions?  Call us!  Scott Carrlee 465-4806 or Ellen Carrlee 465-2396.

Heritage-eating bugs

Common Heritage Eaters
1:00 Cigarette beetle
2:00 Drugstore beetle
3:00 Confused flour beetle
4:00 Saw-toothed grain beetle
5:00 Carpet beetle (black, white, and orange)
6:00 Common carpet beetle larvae
7:00 Varied carpet beetle  (black, white and gray)
8:00 Common dermestid beetle
9:00 Larder beetle
10:00 Webbing clothes moth (ragged wings)
11:00American spider beetle
12:00 Hide beetle (has white tummy)

insect debris

Insect debris from L to R: light brown frass and wood bits from a powder post beetle infestation, #2 pencil, larva and striped shed larval casings, soft white cocoons from the casemaking clothes moth.
Harmless bugs

 
A dime gives scale to these “harmless” bugs as well as the generally smaller-sized “heritage eaters.”

1:00 and 2:00 spiders are very common and may make webs and nests but eat other insects, not collections.  A spider population out of control can be reduced by setting out a large number of sticky traps.
3:00 Minute scavenger beetles eat mostly molds and fungi.  These were living in damp plastic bags used to stuff out a mukluk.
4:00 Common weevil, a grain eater.
5:00 Carpenter ants do not eat artifacts, but if you see one, your building itself could be in trouble.
6:00 Common housefly, mostly a nuisance for leaving droppings called “flyspecks” on artifacts.
7:00 Picnic ants are looking for sugar.  This one was attracted to a puddle of punch spilled under a printer during a reception.
8:00 and 9:00 Sowbugs or pillbugs come in many shapes and are found in damp areas.
10:00, 11:00 and 12:00 Carabids, click beetles and other large beetles are generally harmless and die soon after coming indoors


Does Low-Temperature Pest Management Cause Damage?

March 19, 2009

http://cool.conservation-us.org/coolaic/jaic/articles/jaic42-02-002_indx.html

ABSTRACT—Preventive low-temperature treatment as a means of pest control during the relocation of the ethnographic collections of the National Museum of the American Indian afforded the opportunity to undertake an observational study of potential damage from this treatment. Does low temperature harm materials thought to be vulnerable? What are the possible causes of damage from this treatment? The study revealed no visible damage to any of the materials treated, although the literature suggests that minor changes on a molecular level are likely. The literature also indicates that the effects of changing relative humidity and water relationships are less of a threat than effects related to low temperature alone, such as shrinkage, embrittlement, and molecular alteration. The first part of the article discusses the threat (real or imagined) of various damage mechanisms, and the second part addresses several categories of vulnerable artifacts in relation to these damage mechanisms. The observational study and literature review indicate that low-temperature pest control may be appropriate for a wider range of materials than was previously assumed.

1 INTRODUCTION

The National Museum of the American Indian (NMAI) is in the process of moving approximately 168,000 ethnographic objects and 632,000 archaeo-logical artifacts from outdated facilities in New York City (the Research Branch) to new facilities near Washington, D.C. (the Cultural Resources Center). An integral step in this move is to carry out a preventive pest management protocol to minimize the possible transfer of live insect pests within the collections to the new facility. For much of the collection, the preventive method is to treat objects with low temperature prior to relocating. Included in the vast number of objects treated are certain “borderline” types of materials for which low temperature was feared to be detrimental in theory, but to which experience and anecdotal evidence showed no damage.

Low-temperature (in the range of –20°C to –30°C) pest eradication in conjunction with an integrated pest management system has been the preferred method for museum pest control since the mid-1980s (Jessup and Ballard 1997; Strang 1997), in part as a reaction to federally mandated legislation limiting the use of pesticides. Subsequent development of both anoxic environments and the use of thermal methods has produced important applications in museum pest control. However, the ease, nontoxicity, speed, cost effectiveness, and definitive kill provided by low-temperature pest control continue to make it the favored pest control process whenever possible.

The existing conservation literature emphasizes the aspects of proper temperature, freeze duration, insect resistance, condensation, and practical low-temperature procedures. Little has been published about damage to artifact materials, although several references give lists of materials that are not recommended for this treatment (Florian 1986b for lists and procedures; Berkouwer 1994; Raphael 1994; Michalski 1996; Strang 1997; Baughman 1999). The types of potential damage feared include cracking, delaminating, fatty bloom, staining, corrosion, and fungal growth. Some NMAI objects considered vulnerable to low temperatures are not treated in the freezer but instead are managed with Vikane (sulfuryl fluoride), anoxic treatments, or careful inspection and isolation. The expense and time required for these alternatives, however, make low temperature the preferred option for the thousands of objects involved in the NMAI project. Some potentially vulnerable objects are being frozen at NMAI because they are part of a composite object where the risk of infestation seems greater than the risk of damage or because the object shows evidence of prior infestation.

Within the limitations of the move process, an observational study was undertaken to test the hypothesis that these materials are not damaged by low-temperature pest control. Four categories were investigated: materials in composite objects, cracked objects, lamellar objects, and waxy or oily objects. The results of this study, together with information about the mechanisms of damage due to low temperature and the characteristics of artifact materials at low temperature, contribute to an informed approach for the treatment of potentially vulnerable materials.

2 CONCERNS FOR MATERIAL CHANGES FROM FREEZING

Several categories of physical change were considered during this study to determine what mechanisms might be responsible for potential damage to artifacts during low-temperature treatment. References for low-temperature damage found in several nonconservation fields proved tantalizing but difficult to apply to the museum situation. The fields of cryogenics (the study of living systems at low temperatures) and low-temperature physics utilize a temperature range much colder than proposed for museum pest control. In general, Arctic studies, refrigeration engineering, and the food preservation industry deal with considerably more free water than pertains to the museum treatment situation. Freeze-thaw, dehydration, condensation, swelling, embrittlement, shrinkage, thermal shock, polymorphic phase change, and molecular alteration have all been mentioned by conservators as possible areas of concern. Some of these are relevant to museum pest control, and others are not. The field of materials science suggests factors of greater relevance.

2.1 MOISTURE AND HUMIDITY ISSUES

Little information is available to conservators about potential damage to museum objects from low temperatures. Most concerns about the treatment are theoretical, extrapolated from the extensive information available about the reaction of museum objects to changes in relative humidity and from empirical evidence from daily life (such as one’s home freezer). Some of these assumptions are incorrect. In a museum treatment situation using proper packaging, most objects lack sufficient moisture for freeze-thaw or dehydration mechanisms to occur. Inclusion of adsorbent buffering materials and the counteraction of shrinkage mitigates swelling as a concern, while proper bagging eliminates the danger of condensation on the object and creates a closed system. Conservation scientist Mary Lou Florian of the Royal British Columbia Museum has written extensively about moisture relationships and proper low-temperature protocol and included lengthy bibliographic references outside the field of conservation in her articles (Florian 1986a, 1986b, 1987, 1990a, 1990b).

2.1.1 Freeze-Thaw

Use of the term “freezing and thawing” to describe the museum pest control process should perhaps be replaced with the more accurate “warming and cooling.” The term “freezing” is loosely used in the conservation literature to imply temperatures below 0°C, and in this context does not necessarily imply a phase change from liquid (water) to solid (ice). Review of the scientific literature indicates ice does not tend to form in museum objects. A brief review of water dynamics is helpful in understanding several issues surrounding potential damage to artifacts.

Moisture content (MC) is the percent weight of water in relation to the dry weight of the material. Water activity (Aw) reflects the portion of water within the moisture content that can be used for “activity” such as chemical reactions, availability to microorganisms, or exchange of humidity between the material and its environment. Water activity is given as the ratio of vapor pressure of water in a material compared to pure water under identical conditions. Sometimes this number is multiplied by 100 and called the equilibrium relative humidity (%ERH). Equilibrium moisture content (EMC) is a measure of the amount of water in an object after it has reached equilibrium with its surroundings over time. Water can exist in many physical states on a continuum reflecting how strongly the water is bound to another material. On one extreme, it can be tightly bound in a single layer to the polar sites of molecules. This water does not freeze (Nomura et al. 1977). At the opposite extreme is free water. If sufficient free water were present in museum objects, the phase transitions between liquid water and solid ice implied by the terms “freeze” and “thaw” could cause damage from the expansion of ice. However, ethnographic objects in a museum setting generally do not have sufficient equilibrium moisture content to undergo the formation of ice. Most organic artifact materials in museum environments have 8–12% moisture content (Florian 1986b). Artifact material with equilibrium moisture content of up to 28% does not form ice at –20°C (Zachariassen 1985). For example, in beef dried to 22.5% moisture content, no ice formed regardless of temperature (Fennema 1981). The water present in most museum objects is physically adsorbed or chemically combined water and is therefore not available for ice formation. (Adsorption is the adherence of water molecules as a monomolecular layer, as distinguished from absorption, in which the penetration is deeper into the molecular structure.) Furthermore, free water in capillaries smaller than 30 micrometers (as in collagen) is physically altered and does not solidify above –40°C (Horne 1969). Even in fresh collagen (45–60% water), the water of hydration is still in a state of mobility at temperatures well below the freezing point of ordinary water, remaining unfrozen at temperatures as low as –50°C (Dehl 1970). In frozen muscle, approximately 20% of the water was considered unfreezable in nuclear magnetic resonance (NMR) studies (Taylor 1987).

When it does occur, ice formation is commonly known to cause freeze-thaw damage from the 9% expansion in volume that takes place as water changes from the liquid to the solid phase (Franks 1985). Other phenomena, however, are also related to the formation of ice. Living plants hold their shape in part due to “turgor pressure” or the pressure of water inside the cells of the plant tissues. At low temperatures, an increase in the permeability of the cellular plasma membrane (due to pressure from the 9% increase in volume as ice crystals grow) causes a loss of cellular water, thus turgor pressure and the subsequent “wilted” appearance of some frozen plant materials (Reid 1987). However, the most damaging problems associated with freezing are the “concentration effects.” As an aqueous solution freezes, water separates out of the mixture as ice, and the concentration of the remaining solutes increases (Taylor 1987). This result affects a variety of factors within the cell, including ionic strength, viscosity, oxidation-reduction potential, pH, salt concentration, and enzyme reactions. The majority of the literature associated with freezing damage focuses on these effects (Hawthorn 1968; Poulsen and Lindelov 1981; Kobs 1997). At low-moisture-content levels such as those present in most museum objects, the possibility of ice formation, loss of turgor pressure, and damage related to concentration effects is eliminated.

2.1.2 Dehydration

Dehydration does not occur in objects inside the freezer if there is insufficient free water to be lost (Florian 1986b; Strang 1997). The ability of air to hold moisture is temperature-dependent. The word “relative” in “relative humidity” refers to the relationship of moisture present to the maximum of it that air can hold at a given temperature. Wintertime air, for example, is drier than summertime air because cool air cannot hold as much humidity as warm air. When dry winter air is warmed inside a building, it is able to hold more humidity, and inhabitants of that building (including food, plants, and people) help provide the moisture to bring the air into equilibrium at a higher RH. Complaints of dry, itchy skin in the winter are often related to the ability of human skin to provide water for warmed, dry air “hungry” for moisture. The cold air inside the freezer is not “hungry” for moisture. The equilibrium moisture content in an adsorptive object actually tends to increase due to the decreased ability of the air to hold moisture. The relative humidity measured in freezer-bagged enclosures does not correlate to the experience of the object at the same RH in a standard room temperature situation. A reading of the RH measured in the enclosure must take into account the moisture the air has already given up to the object and buffering materials. In this case, the lower RH does not signify a situation where the object is being desiccated by a dry environment. On the contrary, reducing the temperature allows the object to slowly increase in EMC.

A review of the process called “freeze-drying” is helpful in understanding why museum objects do not dry out during low-temperature pest control. Freeze-drying is a process involving the removal of frozen water from an object by sublimation. In contrast to evaporation, where liquid water is turned to water vapor and carried off, sublimation is the removal of water vapor directly from ice without its becoming liquid. “Freezer burn” is the surface dehydration of poorly packaged foods caused by sublimation in the freezer. Freeze-drying is a more sophisticated process involving a vacuum in order to take advantage of the properties of water under low pressure. Under low pressure, water will vaporize at a lower temperature. That is, water does not need to be as hot to become water vapor. (This circumstance is related to the phenomenon of water boiling at a lower temperature at a high elevation due to the lower atmospheric pressure of thin mountain air.) With the use of a cold condenser and a gentle heating element, the air in the freeze-drying chamber is kept at a temperature slightly above the temperature of the frozen object, allowing water molecules on the object’s surface to break free and gather on the condenser as frost (Schmidt 1985). This treatment is used only when museum objects are wet and have sufficient free water to form ice and allow sublimation to occur.

2.1.3 Condensation

Condensation is a result of the reduced capacity of air to hold moisture as its temperature is lowered. It is this phenomenon that is observed on a car’s windshield in the winter. When the interior of a car is warmed, the warm air encounters the cold wind-shield, and a microclimate of cold air is created near the surface. This small cushion of cold air cannot hold the same level of humidity held by the warm air, and moisture condenses on the interior of the windshield. Turning on the air vent next to the windshield will alleviate the problem, moving the warm air away from the surface before it has the opportunity to cool.

Some artifact materials, such as wood and hide, are able to adsorb the humidity released by the air at low temperature and release it again when brought slowly to room temperature. Nonadsorbent materials, such as metals and stone, do not have that capacity and are vulnerable to condensation on their surfaces when cooled.(Frost is simply frozen condensation.) Condensation could potentially cause staining, migration of colorants, corrosion, or fungal growth. Addition of adsorbent packing materials such as crumpled tissue reduces the likelihood of condensation on nonporous objects, as adsorbent materials act as a buffer and adsorb available water vapor. Placing the objects in sealed plastic bags with most of the air removed reduces the amount of moisture available in the air for possible condensation. The sealed plastic bag also serves to prevent condensation on the cooled object as it returns to room temperature. Any condensation during warming would form on the outside of the plastic bag, following the model that condensation forms on the warm side of the warm/cool interface. Although polymer films, such as polyethylene bags, are slowly permeable to moisture, it occurs over a longer period of time (several weeks) than the bags would be in use for cooling (several days) (Strang 1997; Florian 1992).

2.1.4 Swelling

Swelling of materials is another consequence of the reduced ability of cold air to hold moisture. The excess moisture can be adsorbed by porous materials, and a small amount of swelling may take place. Again, this result can be mitigated by the inclusion of buffering materials to sacrificially adsorb and release the excess moisture. Wood, for example, swells at low temperature if it has exposure to open air. However, in the bagged situation, the equilibrium moisture content (EMC) change is not significant because the amount of water available for adsorption in the bag is small in relation to the amount that can be adsorbed (Florian 1990a, 1992). Furthermore, materials at low temperatures take longer to reach moisture equilibrium with the environment than the same materials at room temperature. Photographic films have been reported to take between 10 and 30 times longer to reach equilibrium at low temperature (Adelstein et al. 1997). At room temperature, white oak was found to take up to 80 days to completely adsorb or desorb moisture (Grattan and Barclay 1988). Experiments with textiles indicate wool required 14 days and silk more than 3 weeks to reach equilibrium at room temperature (Howell 1996).

2.2 PROPERTIES OF MATERIALS

The biggest potential risks faced by museum objects during freezing stem not from moisture-related issues but from the properties of materials at low temperatures and the mechanisms of cold-induced damage. Important factors include the coefficient of thermal expansion, stiffness, thermal conductivity, and strength of the material. Thermal cycling and the magnitude of temperature change can pose potential dangers. Geometry, aging, residual stresses resulting from manufacture, and the history of each unique object can also play significant roles. An understanding of the damage mechanisms will contribute to a more informed consideration of the risks and a logical approach to decision making.

2.2.1 Embrittlement

Embrittlement occurs at temperatures used for pest control because the molecules are resistant to motion. Increased tendency to fracture is related to this reduced ability to deform. So-called “glassy” behavior occurs when the molecules do not vibrate enough to bump past each other during the application of stress. The “glass transition temperature” (Tg) is the range of temperatures at which molecular motions become slower than the rate at which temperature is changed and the material no longer has sufficient time during cooling to remain in equilibrium. The material then changes from soft and rubbery to solid and glassy with a decrease in specific volume (shrinkage). The Tg is sometimes given as a range because it can be affected by factors such as the age of the material (aging increases the Tg) or rate of cooling (slower cooling gives a lower Tg). Below Tg, brittle fracture can occur by crack propagation. Elastic deformation tends to occur above Tg.

Conservators are perhaps most familiar with the concept of Tg in relation to adhesives. AYAA Poly (vinyl acetate) (PVA) adhesive, for example, has a lower Tg than Paraloid B-72, and is therefore more flexible at room temperature and preferred for use in situations where more flexibility in the join is desired, such as repairing feather quills. However, in an archaeological field setting, the low Tg of PVA may cause slumping of reconstructed ceramics. Thus the higher Tg of Paraloid B-72 is preferred.

Stiffness describes the amount of elastic deformation resulting from a given applied stress. “Elastic” behavior describes the ability of a material to deform under stress and still return to its original conformation. Objects with a high elastic stiffness tend to be brittle. Stiffening of most elastomers occurs below –20°C, while the brittle point does not begin to occur until –50°C (Sehgal and Lindberg 1973). Examples of materials that become brittle in this temperature range include rubber, resin varnishes, linseed oil films (oil paint), synthetic polymers, acrylic paint, and soft vinyl (Mecklenburg and Tumosa 1991; Michalski 1991). Linseed oil, for example, becomes fully glassy at –30°C (Michalski 1991). Although elastic stiffness induced by cooling is usually reversible with warming, materials are potentially vulnerable to structural damage until sufficiently warmed. Dangers could include vibrational stresses from motors inside faulty freezers, rough handling when moved while cold, and even the weight of the object itself (as in the case of a load-bearing adhesive).

Glass is an example of a material with a high elastic stiffness. In addition, glass is a poor conductor of heat and has a low resistance to crack growth. Generation of small surface cracks is likely to occur from cooling during manufacture as the surface goes into tension. Repeated exposure to low temperatures could result in one of two outcomes for these cracks: “ratcheting” or “shakedown.” Ratcheting describes the accumulation of plastic strain. Each time the crack is opened and closed, the crack grows. Damage evolution due to thermal cycling is known as “thermal fatigue” and will eventually lead to macroscopic failure. The other option, shakedown, involves a reduction of the incremental strain per cycle. Most of the damage in this process happens the first time the object is exposed to low temperature, and each subsequent freezing cycle results in less damage per cycle (Elzey 2001). It is also important to include aging as a factor in considering crack formation and growth. An object repeatedly exposed to freezing temperatures during its use in the Arctic may not tolerate thermal cycling after many years in controlled storage.

2.2.2 Shrinkage

Practically all materials shrink as temperature is lowered because of the reduced vibration on the atomic or molecular scale. (Think of how gas expands during heating due to the more active motion of its molecules.) The decrease in vibration causes the molecules to have a smaller range of motion and thus to take up less space. The “coefficient of thermal expansion” (CTE) is a measure of this change, expressed as a ratio of change in length per degree Celsius compared with the base length at some reference temperature (cm/cm/°C). How much a material might shrink is dependent on the strength of the interatomic bonding. Objects with strong chemical bonds, such as metals and ceramics, expand and contract less with changes in temperature than objects with weaker bonds. Rabbit skin glue, for example, with a CTE of .000025 cm per degree Celsius, will shrink only 0.1% when cooled from 20°C to –20°C (Mecklenburg and Tumosa 1991). In his discussion of paintings, Michalski cites 3% elongation as the elastic limit beyond which a polymer cracks (Michalski 1991).

Low-temperature treatment of composite objects gives rise to a risk of damage due to CTE mismatch if the two materials have different coefficients of thermal expansion. Internal stress, deformation, and damage could occur as the composite object is heated or cooled. There are published tabulations for expansion coefficients of some common materials, but there may be no data for many materials in aged or altered condition, or no data in the appropriate temperature range, or simply no data at all. Often materials are simply categorized as high or low relative to each other. During cooling, the low CTE material goes into tension while the high CTE material is in compression and in danger of cracking or delaminating.

CTE differences or mismatch can also be seen within a single material, particularly those that demonstrate anisotropy. The bonds in anisotropic materials are direction-dependent and expand to different degrees in different directions. Examples include materials with a complex structure that tend to crack in a preferential direction, such as wood, bone, tooth, and lamellar structures. Since the rate of freezing does not affect the CTE mismatch, it is not possible to mitigate damage by controlling the rate of freezing.

Cracking is not the only manifestation of CTE mismatch. If the high CTE material is more vulnerable, deformation or crushing may occur as the material goes into compression. A high CTE material sandwiched between two layers of low CTE material may be extruded by pressure from the surrounding material.

At sufficiently high stress, materials lose their ability to deform elastically, resulting in either failure or plastic deformation. Unlike elastic deformation, which is fully recovered when the applied stress is removed, plastic deformation is permanent. Plastic deformation of capillary structure and loss of water bonding sites is thought to contribute to the loss of moisture-regain ability in skins and furs exposed to cold storage (Pool 1997). Plastic deformation of cell structure and subsequent depletion of gas is thought to contribute to observed Ethafoam shrinkage in the freezer. The inability of air to replace the lower molecular weight gas that may have been squeezed out of the individual Ethafoam cells may also be a factor (Elzey 2001). Low RH is a far more common cause of shrinkage in artifacts than the effects of low temperature alone (Michalski 1991).

2.2.3 Thermal Shock

Thermal shock is the condition resulting when rapid temperature change leads to excessive internal stress resulting in damage or failure. It is the phenomenon that occurs when cold water is poured over a hot ceramic plate, causing it to shatter. The magnitude of the stress is determined by the overall change in temperature, the rate of cooling, the size of the object, and the material’s CTE, elastic stiffness, conductivity, and strength. Materials with high CTE, high elastic stiffness, low thermal conductivity, and low strength and that are exposed to a large overall rapid change in temperature are most at risk for thermal shock. Although most organic materials possess high coefficients of thermal expansion, conduct heat poorly, and are held together by low-strength secondary bonds, they have the advantage of very low elastic stiffness and are comparatively resistant to the effects of thermal shock. Inorganic materials found in composite objects may be more at risk. Ceramics, for example, combine high strength, high elastic stiffness, and poor conductivity. Although they have a low CTE, they are at higher risk for thermal shock. Table 1 is a synthesis of information mentioned in the literature and attempts to describe several key properties influencing the probability of thermal shock. “High,” “medium,” and “low” describe how these materials compare to one another for each property.

Table of Factors Involved in Thermal Shock

For table, see link

http://aic.stanford.edu/jaic/articles/jaic42-02-002.html

2.2.4 Polymorphic Phase Change

Phase change refers to a change in state, such as from a solid to a liquid, or liquid to gas. “Polymorphic” phase change implies a change from one solid state to another, often seen in metals at elevated temperatures. Polymorphism implies an ordered, crystalline structure. Many polymers and organic materials do not undergo polymorphic phase changes because they lack the tightly packed, regular 3-D arrangement of atoms that facilitates the change from one formal crystalline arrangement to another. If organic materials do become crystalline, as some polymeric materials may, then polymorphic phase changes do become possible. Temperature change may cause noncrystalline materials to assume a crystalline formation. Such a phenomenon can be observed in the crystallization of olive oil in one’s home refrigerator. In reference to elevated temperatures, Ellen Pearlstein describes the fatty bloom mechanism as follows:

Polymorphism, the condition in which the same substance can assume different crystal forms, is shared by triglycerides, long chain acids, esters, alcohols, and paraffins. … Temperature conditions influence which polymorphic form is most stable. Fats and waxes, which are semi-solid at room temperature, will continue to respond to subtle temperature changes with phase transitions, reaching a new equilibrium at a new temperature … a varied temperature history and the inclusion of impurities in a sample would make predictions of polymorphic behavior almost impossible. (Pearlstein 1986)

Observation of fatty bloom on a dressed leather saddle treated for pest control suggests that polymorphic phase changes might occur in museum objects at low temperature (Baughman 1999). Leather dressing often includes Neat’s foot oil from which the solid triglyceride portion has been removed through chilling, causing the solids to rise to the surface of the oil. Solid triglycerides remaining in Neat’s foot oil dressing may cause spew at low temperature (Fogle 1985).

Rubber is another material reported to undergo changes at low temperature. The rate of crystallization of rubber increases with decreasing temperature, reaching a maximum at approximately –25°C. Rubber that is crystallized is characteristically inelastic and may have hard or “crunchy” cracked surfaces. This alteration is sometimes reversible upon warming (Baker 1995).

Allotropes are polymorphs of elements, and some occur at low temperature. Tin disease (tin pest, tin blight, tin plague) is one such example. One pure tin allotrope, beta tin, is the shiny stable white metal seen at room temperature. The alpha tin allotrope (a nonmetallic crumbly gray powder) becomes the more stable form as temperatures decrease, reaching a maximum at –30°C. Upon warming, crystalline faults form, exacerbating the problem (Elzey 2001). Tin disease is inhibited by as little as 0.1% bismuth, antimony, or lead, the typical alloying metals used with tin. Most of our museum materials (such as tin cone tinklers found on Native American artifacts from the Great Plains) are alloys and therefore safe from polymorphic phase change in the freezer. However, the textbook example of tin disease involves Napoleon’s attempted 1812 winter invasion of Moscow, which failed in part because of the disintegration of the tin buttons on the soldiers’ clothing.

2.2.5 Molecular Alteration

The technology for studying proteins at low temperature in the absence of ice formation has been developed only within the past decade. Previously, scientific knowledge of low-temperature “denaturation” or unfolding of proteins was based on extrapolations from high-temperature experiments. Current research indicates that the denaturation of proteins has different causes at high and low temperatures and results in different disruptions of the molecule (Fahy 1995; Franks 1995). “Low temperature” studies of biological phenomena rarely involve temperatures below –70°C and often involve temperatures just below –0°C (Douzou 1977; Taylor 1987). Conformational stability in proteins is dependent on a complex energy balance involving a variety of intermolecular forces. Cooling weakens some forces, such as hydrophobic interactions, but enhances others, such as hydrogen bonding. These kinds of changes in the molecule may not be completely reversible upon warming and could alter some of the identifying characteristics of the protein (Taylor 1987). Many of these studies, however, involve freezing proteins with significant moisture content and suffer from the associated concentration effects. Simple exposure to low temperature exclusive of moisture-related complications is thought to cause a general instability that renders the protein susceptible to the influence of other factors leading to denaturation. However, most of those factors involve water content and ice formation (Taborsky 1979; Taylor 1987).

Although the rate of most chemical reactions tends to decrease with decreasing temperatures, according to the Arrhenius equation (Mills and White 1987), oxidation of lipids is an important exception. Autoxidation of unsaturated fatty acids, however, can be accelerated by low temperatures in the range used for pest control (Franks 1985). Autoxidation of lipids in foods is associated almost exclusively with unsaturated fatty acids such as are found in vegetable oils (Karel 1985). Lipids contain a wide variety of fatty acids that differ in chemical and physical properties as well as in their susceptibility to oxidation. Lipids in the NMAI collection include animal fats, plant waxes, beeswax, avian preen oil on feathers, and lanolin in wool. These materials have complex combinations of lipids that usually include a percentage of unsaturated fatty acids. The autoxidation of saturated fatty acids is very slow and slower still at low temperature. Oxidation may also be catalyzed by enzymes, although the definition of enzymes as “proteins produced by living organisms functioning as biological catalysts in living organisms” (Roberts and Caserio 1977) calls into question whether there are any active enzymes remaining in museum objects. Furthermore, some enzymatically catalyzed oxidation in lipids is influenced by solute concentration effects that allow enzymes and substrates to come into contact (Karel 1975; Reid 1987). Museum objects that cannot form ice are unlikely to face these problems.

Caution must be exercised in extrapolating data from other fields. Agricultural research, for example, is concerned with the longevity of biological tissues as a nutritional resource and issues such as flavor and texture preservation. The behavior of fresh fish muscle at low temperatures and its purpose upon thawing is very different from the behavior of an aged, dried fish skin artifact. The cryogenics field is concerned with the viability of tissues at low temperatures in the colder range of –80 to –196°C (Reid 1987). Both fields focus closely on the continuation of biological function and address issues of decomposition and cell death from a point of view that considers loss of structural integrity a somewhat secondary concern. For these fields, their objectives have already been lost at the level that museum freezing for pest control is addressing.

The loss of moisture-regain ability due to changes on the molecular level is another realm of potential problems for organic materials (Kronkright 1990; Pool 1997). The term “hysteresis” is used to describe nonlinear input-output systems because of material memory. Imagine an experiment measuring water content at different relative humidities. The experiment could be set up in two ways: the material may begin dry and measurements taken as it adsorbs water, or the material may begin wet and measurements taken as it desorbs water. Interestingly, at a given RH, the water content of the material is higher when it is in the process of desorbing than when it is in the process of adsorbing (Tímar-Balázsy and Eastop 1998). This finding is thought to be because polymers are at a more stable energy state with higher moisture content and are not as willing to give up moisture as they are to take in moisture. This phenomenon of adsorption and desorption rates relying on moisture history is one example of hysteresis. At low temperatures, molecules with potential water-holding sites may draw closer together and bond, creating a reduced capacity to hold water in the future. Water activity and moisture content are related by a curve known as a “moisture sorption isotherm.” It has been reported that sorption ability decreases with increase of cold storage time (Wolf et al. 1972). Long-term cold storage of furs and skins in an open system (not in a sealed bag) has been reported to cause a loss in moisture-regain ability (Pool 1997). Another study suggests that low-temperature treatment for pest control (a comparatively brief period of time) has a minimal initial effect on shrinkage temperature of collagen (Williams et al. 1995). There appears to be a difference between damage from short-term low-temperature exposure and long-term cold storage.

3 VULNERABLE MATERIALS IN ARTIFACTS

Several categories of artifacts were systematically observed in this study because they exhibited characteristics or flaws making them potentially vulnerable to the damage mechanisms described above. The following sections will discuss the potential for damage in artifacts with multiple material compositions, cracks, lamellar structures, waxy or oily elements, or cultural sensitivity.

3.1 MATERIALS IN COMPOSITE OBJECTS

The vulnerability of composite objects involves two areas of concern. The first category includes objects featuring different materials in close contact inhibiting independent movement of each material according to change in temperature or humidity. Tension may be built into an object during natural formation of the material (such as a tooth) or human creation of an object (such as a drum). Cracking, splitting, or warping may occur when one material is restricted by another. The second category involves materials that are not generally frozen (particularly inorganics such as metals, glass, ceramics, or stone) attached to materials that are good candidates for freezing. Garments with metal, glass bead, or tooth adornments are common. If the packing material or the more thermally robust part of the composite cannot mitigate the effect of the freezing environment, there is the potential for the vulnerable material to suffer from cracking or surface condensation. Condensation from poor packaging may result in staining, corrosion, or other changes in surface characteristics. Examples of vulnerable materials in composite objects include rawhide or sinew wrappings (arrows, hammers, snowshoes, etc.), metal (buttons, cone tinklers, brass tacks, inlay, etc.), tooth (garments, masks, jewelry), ceramic (beads, pipes), stone (beads, pipes), glass (beads), wood objects with inlay or tight joinery, and wax on wood or other organic substrates.

3.2 CRACKED OBJECTS

Shrinkage, swelling, embrittlement, and other phenomena in the freezer may lead to propagation of existing cracks or formation of new ones, especially in materials made up primarily of inorganic compounds. Ivory, tooth, and bone are of special concern since they have been shown to crack under changes in humidity, apparently from the stresses of shrinking and swelling. Tooth, bone, ivory, and baleen contain both inorganic and organic components that behave differently under temperature and humidity changes. Bone, for example, is anisotropic (it has a higher percentage of change in the long axis) and responds to environmental changes differently in different directions (Williams et al. 1993). Cracking is most easily caused in thick objects (such as stone, glass, and ceramic) having poor thermal conductivity, a high CTE, and high elastic stiffness when they are subjected to large, sudden changes in temperature. Beads with the tiny cracks associated with glass disease may be at additional risk because of their sensitivity to changes in humidity. Improper packing for low-temperature treatment could theoretically expose the glass to the elevated RH that occurs with decreased temperature (see sec. 2.1.3). Glass disease involves deterioration of the glass structure from leaching out of water-soluble components with flawed compositions created during manufacture (Lougheed 1988; Erhardt and Mecklenburg 1994). Michalski mentions that craquelure on painted or coated wood may crack further at –50°C (Michalski 1996). Candles are thought to crack if placed in the freezer, suggesting thick wax layers may be vulnerable. Some botanicals such as seeds or gourds might have the potential to crack, although the National Museum of African Art recently treated 60 gourds, including some with crack and ethnographic repairs, and noted no visible damage (Hornbeck 2001). The cracking danger in the freezer does not seem to be from low temperature alone as much as from mishandling while cold objects are embrittled. Examples of materials vulnerable to cracks include bone, tooth, ivory, diseased glass, painted or coated wood, plant materials, wax, and inorganic materials such as metal, ceramic, and stone.

3.3 LAMELLAR OBJECTS

Delamination is the peeling apart of materials with a layered structure. Examples of particular concern include painted objects such as masks and furniture, particularly if the pigment is well bound in media that will behave differently from the substrate. Amorphous and semi-amorphous polymer media (oil, varnish, glue, gum) can suffer shrinkage on the scale of 0.7% per 70°C decrease in temperature. Leaner paints with less binder and a larger percentage of pigment, however, fare better with shrinkage in the range of 0.4% per 70°C decrease in temperature (Michalski 1996). It is interesting to note that certain woods, such as cedar, cypress, and redwood, hold paint better than others, due in part to their dimensional stability but also because they are relatively porous and the wood-paint bond is thought to be largely mechanical (Mecklenburg et al. 1997). Another area of concern involves adhered or glued objects such as feather tipping on war bonnets, inlaid objects, furniture joinery, and past treatments. Most adhesives are stronger, more brittle, and more reactive to increases in RH at low temperatures (Erhardt and Mecklenburg 1994; Erlebacher et al. 1992). Joins that are under stress are at additional risk for failure as the adhesive becomes brittle at low temperature. The adhesive and adherend may also shrink or swell at different rates, causing failure. Objects with accretions from burial or use may be vulnerable for similar reasons. There have been reports of successful low-temperature treatment of leather with adhesive repairs, including BEVA 371 film, silicone adhesive SF2, and wheat starch paste mixtures (Kite 1992), as well as the successful low-temperature treatment of Japanese lacquer wares (Tanimura and Yamaguchi 1995). Shell (turtle shell, marine shell, snail shell) is thought to be potentially vulnerable to damage at low temperature due to its lamellar structure. This natural lamellar structure provides areas of weakness for stresses to be released if shrinking or swelling occurs. This damage could manifest as an opening up of these layers, with associated peeling and loss. Byne’s disease is another source of concern: the appearance of powdery deterioration on the surface of a shell could indicate that the calcium carbonate has reacted adversely with acid vapors off-gassing from wooden shelves or cabinets, forming hygroscopic salts that could swell in elevated humidity (Tennent and Baird 1985). Horn, as a keratinaceous material similar to hair, has the ability to absorb limited amounts of moisture. However, the lamellar structure of its growth makes it prone to crack with age, and it is these cracks and microcracks that may be propagated if stressed. Baleen is similar to horn, but further calcified. Changes in RH between 25% and 85% do not seem to affect the dimensional stability of skull bones in mammals (Williams et al. 1993). Teeth, which are hygroscopic, anisotropic lamellar structures, suffer more from low RH than from temperature changes. If damage occurs, canines are more prone to crack than molars, in part because the hollowness of molars constrains movement less than the more solid interiors of canines (Williams 1991). It is interesting to note that industrial cleaning techniques recently developed for large wooden surfaces such as floors employ low temperature expressly to force failure between layers, including wood, dirt, wax, varnish, and overpaint (Piening and Schwarz 1999). It is worth repeating that proper packaging should eliminate the elevated humidity that occurs with low-temperature treatment (see sec. 2.1.3). Examples of materials vulnerable to delamination include adhesive joins and repairs, painted or gilded objects (masks, furniture, beads), turtle shell, marine shell, snail shell, horn, baleen, bark, resins, and accretions.

3.4 WAXY OR OILY OBJECTS

The possibility of waxy, powdery, or crystalline formations developing on the surface of some materials during treatment is another area of interest (see sec. 2.2.4). Waxes, oils, and fats found in some objects (oiled ropes, food bowls with residues, dressed leather) may undergo a polymorphic (solid-to-solid) phase change during cycled changes in temperature, resulting in an opaque, powdery wax formation on the surface (Pearlstein 1986). Another explanation suggests that bloom may be the result of having materials with different coefficients of thermal expansion in contact with each other. For example, if a high CTE wax is sandwiched between two low CTE fibers (or vice versa), the wax will be squeezed (extruded) from between the fibers as temperature is increased (Elzey 2001). Spew from dressed leather exposed to low temperature for pest control has been reported (Baughman 1999). Cold temperatures may also cause waxes to become brittle (Victoria and Albert Museum 1970). Examples of materials vulnerable to bloom include bark, botanicals, wooden food dishes, dressed leather, wood with waxed surfaces, and oiled ropes.

3.5 OBJECTS WITH NONTANGIBLE SENSITIVITIES

Low-temperature treatment may be inappropriate for objects with cultural sensitivity determined by traditional care. Some objects are considered to be sacred or living members of certain cultural groups. The bagging required for a low-temperature treatment may constitute mistreatment from a traditional care perspective. Some Native American medicine bundles at NMAI are sometimes allowed to deteriorate naturally and be consumed in isolation from other objects within the museum environment. Examples of materials with potential cultural sensitivity include bundles, masks, pipes, sacred or ceremonial objects, medicine objects, fragments of human remains, and associated funerary objects.

4 THE PREVENTIVE FREEZE PROCESS AT NMAI

Both the Research Branch and the Cultural Resources Center are equipped with large walkin freezers. All objects packed at the Research Branch for the move are lightly surface-cleaned with a vacuum and secured on a travel mount. Some are tied down with cotton twill or Teflon tape to corrugated pallets with polyethylene foam supports, while others are placed in small boxes with acid-free tissue, bubble wrap, or polyethylene foam padding. These housings are then bagged in clear polyethylene plastic sealed with tape and grouped in larger cardboard boxes. These large boxes are wrapped with an additional layer of polyethylene and securely taped. During the move process (but not during this study), the boxes are placed in the Research Branch freezer at approximately –20ºC for five days. Boxes are then loaded on a climate-controlled truck for shipping to the Cultural Resources Center.

For the purposes of this study, freezing for pest management was done at the Cultural Resources Center instead of the Research Branch to eliminate possible damage in transit as a variable and allow condition after travel and before freezing to be assessed. The freezer at the Cultural Resource Center is a Bally pre-engineered walk-in freezer averaging a temperature of –40ºC. From the perspective of insect mortality, there is no benefit to temperatures below –40ºC, and lower temperatures may put the objects at greater risk of damage (Strang 1997). This temperature is lower than the –20ºC routinely used at the Research Branch for the NMAI move process. The freezer used for the observational study is a two-stage 10HP refrigeration system with a 45-minute defrost cycle every 6 hours. Freezers with defrost cycles are not typically recommended for pest control because during the defrost cycle they tend to rise above temperatures required to kill pests (Florian 1990a). Two ACR Systems Smart Reader 2 dataloggers were placed in the freezer with the objects on two occasions. Data from this equipment indicate that the temperature in the freezer drops from approximately 20ºC (room temperature) to approximately –40ºC over a two-hour period. This rapid drop is key to preventing cold acclimation in insects (Florian 1986b; Strang 1997). Temperatures were recorded as low as –45.8ºC. Defrost cycles were never warmer than –23ºC, safely below the recommended temperatures for insect mortality. The air inside the boxes returns to room temperature over a period of three to six hours after removal from the freezer. Relative humidity below 0 degrees is difficult to monitor because of the reliance of the datalogger on nonfrozen moisture for accurate readings.

5 THE OBSERVATIONAL STUDY

Objects for this study were selected at the Research Branch from among those slated for freezing and that fell into the previously described categories of concern. Additional samples were taken from the collection of NMAI “fix-it shop” materials used in the past for restoration purposes but now kept in the conservation laboratory for mock-ups and experiments. Table 2 lists the categories of materials observed for each object, and table 3 provides a description of “fix-it shop” materials observed. None of the study objects had been previously frozen at the Research Branch. Records for earlier low-temperature infestation treatments were not available. Upon arrival at the Cultural Resources Center, each object was inspected and documented with 35 mm color print details. For this project, a Nikon 6006 automatic 35 mm camera, a Nikon AF Micro Nikkor 60 mm lens, and a cameramounted Nikon Macro Speed Light SB-29 ring flash were used to ensure consistently controllable lighting. Kodak Royal Gold 200 color print film was chosen for its high quality and fine grain permitting good sharpness and contrast. Measurement of cracks, particularly on anisotropic materials, proved highly subjective with the available equipment. Even with a fine pair of calipers, fading of cracks into the grain of the mate-rial made it difficult to have confidence in the measurements. Visual comparison became the basis of observation. Detail photographs were taken as close as the lens would allow, and this distance proved to be at the threshold of easy visibility with the naked eye. Objects were packed according to normal move protocols described above, and put into the freezer for 6 days. After removal from the freezer, boxes were allowed to acclimate for at least 24 hours. Objects were then unpacked and compared to the prefreezing color prints. In cases where the outcome was ambiguous, a postfreezing photograph was taken under identical lighting conditions to compare with the initial print. In cases where the outcome was particularly illustrative, a postfreezing photograph also was taken. Table 4 documents the results of the three object groups put into the freezer during this study.

Materials Observed

For table, see link

http://aic.stanford.edu/jaic/articles/jaic42-02-002.html

Cultural Resources Center (CRC) Conservation Laboratory “Fix-It Shop” Collection (artifact materials not part of accessioned museum objects)

For table, see link

http://aic.stanford.edu/jaic/articles/jaic42-02-002.html

6 CONCLUSIONS

The observational study of vulnerable ethnographic material frozen at NMAI indicated no structural failure or visible surface changes to the objects. Low-temperature treatment conditions for this study were more extreme than recommended, in terms of both temperature and duration. Further evidence that encourages a cautiously optimistic attitude toward freezing can be found in several articles addressing museum storage in cold climates (Gates and Thorp 1982; Lafontaine 1982). So-called “humidistatically controlled heating” aims to control the RH of museum storage buildings in northern climates during the winter months, based on the principle that the RH of a constant volume of air with a given moisture content can be controlled by adjusting the temperature. No damage to objects has been reported, although vulnerable materials do not tend to be stored in these buildings. It is important to make the distinction between open systems such as these and the more controlled closed systems achieved with a sealed bag in low-temperature pest control.

Exploration of the literature and consideration of materials science issues raise two areas of concern. One involves the likelihood of repeated freezing cycles for some objects, particularly those actively loaned or exhibited and therefore subjected to low-temperature treatments with each re-entry into the museum collection. Experiments involving wood (Starecka and Mieczyslaw 1986; Erhardt et al. 1996), textiles (Dawley 1993; Holt et al. 1995; Jansson and Shishoo 1998; Peacock 1999), paper (Björdal 1998), synthetic fishing gear (Toivonen 1992), and insect collections (Rawlins 2001) suggest no significant structural damage occurs with repeated freezing cycles for pest control. It has been asserted that repeated cycling within the range of plastic deformation has no structural effects on most museum objects (Tumosa et al. 1996). Herbaria and natural history museums seem to have more of a tradition of successfully freezing collections on a regular basis than fine art museums do (Florian 1990b;Tanimura and Yamaguchi 1995; Shchepanek 1996;Ackery et al. 2000). One of the most encouraging examples comes from entomologist John Rawlins of the Carnegie Museum of Natural History, where 22,000 drawers of insect specimens are preventively exposed to –28ºC for two cycles every 18 months with no observed damage to pigments or structure and no microfractures seen under a scanning electron microscope (SEM). Of particular interest is the fact that these specimens are positioned while damp and then kept at 30–40% RH. Exposure to high RH or condensation in the freezer would cause wings and antennae to droop, and this change has not been observed (Rawlins 2001).

Freezing Report

For table, see link

http://aic.stanford.edu/jaic/articles/jaic42-02-002.html

The second area of concern involves the permanent physical changes that theoretically could occur (and perhaps accumulate) on a molecular level but remain invisible to the naked eye, such as loss of strength, loss of elasticity, distortion, crystallization, molecular alteration, and denaturation. In some cases there may be synergistic effects in which interrelated damage mechanisms combine to cause further problems.

While low-temperature treatment remains the best solution when ethnographic artifacts are actively infested, the need for preventive low-temperature exposure for objects entering and re-entering the museum environment is less obvious. Museum staff must weigh the potential risk for devastating loss through insect damage against the possible cumulative damage posed by repeated low-temperature exposure. Responsible prevention must include consideration of factors such as the quality of integrated pest management in the exhibit space or requesting institution, packaging and shipping conditions, duration of possible exposure to infested environments, ability of staff to perform adequate visual inspection, and the susceptibility of artifact material to insect pests.

 

ACKNOWLEDGEMENTS

Many thanks to Dr. Dana Elzey of the Department of Materials Science and Engineering at the University of Virginia, and the staff of NMAI, including Marian Kaminitz, Emily Kaplan, Jessie Johnson, and Leslie Williamson, for their support and feedback. I would particularly like to acknowledge Mary Lou Florian for her excellent work on this topic. Thank you to the Andrew W. Mellon Foundation for making this research possible.

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