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.
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is in the process of moving approximately 168,000 ethnographic objects and 632,000 archaeo-logical artifacts from outdated facilities in New York City (the Research Branch) to new facilities near Washington, D.C. (the Cultural Resources Center).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).
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.
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.
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).
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.
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.
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
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.
For table, see link
Cultural Resources Center (CRC) Conservation Laboratory “Fix-It Shop” Collection (artifact materials not part of accessioned museum objects)
For table, see link
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).
For table, see link
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.
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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