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
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.
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.
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.
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.)
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.
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.
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