Agent of Deterioration: Incorrect Temperature
Table of Contents
- Definition of Incorrect Temperature
- Occupants, Energy, the Environment, and Sustainability
- Deterioration by Incorrect Temperature, and the Most Vulnerable Collections
- Sources of Incorrect Temperature
- Control of Incorrect Temperature
- Vignette 1. Letting Collections Freeze and Fluctuate in the Yukon
- Vignette 2: Small-scale Cold Storage for Archival Records
- Glossary of terms
Definition of Incorrect Temperature
Temperature, unlike fire, water, pests, etc., cannot be considered an agent of deterioration — we cannot speak of avoiding "temperature." From a collection risk and deterioration perspective, we must speak of incorrect temperatures.
Three practical categories of incorrect temperatures arise. Different collections have different sensitivities to each one.
- Temperature too high. This category can be subdivided into chemical, physical, and biological phenomena. The most important one for museums and archives is chemical: normal room temperatures are much too high for the long-term preservation of unstable humanmade materials, especially those carrying images, sound, and text. In fact, for most museums, only these archive collections require any thought about incorrect temperature.
- Temperature too low. Overall, low temperature is beneficial to collections, but polymeric materials, such as paints, become more brittle and fragile. Fortunately, careful handling mitigates most of the risk.
- Temperature fluctuation. This is the temperature issue that has most vexed museums and driven many requests for climate control (together with concern for fluctuations in relative humidity). This emphasis on temperature fluctuation has been out of all proportion to its significance for collection preservation.
Although these three categories overlap in terms of control measures, it helps to keep them separate when assessing the risks to collections.
Occupants, Energy, the Environment, and Sustainability
Temperature control in a museum raises issues of human comfort, energy cost, environmental impact, and sustainability. Conventional recommendations on museum temperature control emerged from a blend of human comfort needs, a limited amount of science, a considerable number of assumptions about possible damage from uncontrolled conditions, and an unfortunate tendency to generalize to a single rigid target. In a time of greater concern for the wise use of our planet's resources, such assumptions cannot be left unchallenged. For smaller museums that never achieved such control anyway, the question becomes, where should it focus its temperature control efforts, and why? This chapter sketches out what we reliably know about the needs of collections, and where, typically, the large risks emerge.
Deterioration by Incorrect Temperature, and the Most Vulnerable Collections
Temperature too High
Cumulative chemical damage from all exposures to temperature that is high enough to cause rapid decay
Many products manufactured from the mid-19th century onwards, in particular paper, photographic materials, rubber, and many plastics, chemically self-destruct within a single human lifetime. To this list, we can add modern electronic records — from analogue tapes to digital discs. Table 1a and 1b lists museum and archive objects by their chemical sensitivity to a very important form of "temperature too high" — normal room temperature — and provides their approximate lifetimes. Because acid hydrolysis drives most of this decay, relative humidity (RH) plays a role as well (consult the chapter on "Incorrect Relative Humidity"), but temperature remains the most important factor to control. In addition to the materials that are acidic from the date of manufacture, we must add those materials such as textiles, paper, and leather that became acidic after exposure to certain internal or external pollutants, especially the sulfur dioxide of industrial air pollution of the 19th and 20th centuries.
An extreme example of high sensitivity is cellulose nitrate film and sheets (produced mainly from to ). These become powdery or sticky. Heavily deteriorated rolled films can even ignite above 38°C. Museums should identify these items and isolate them.
Table 1a et 1b. Chemical sensitivity of materials to room temperature and the lifetimes of the materials at various other conditions. All lifetimes are for ~50% RH. For the combined effect of RH and temperature on lifetimes, consult "Incorrect Relative Humidity." Sources for most materials are reviewed in Michalski ().
|Low sensitivity||Medium sensitivity||High sensitivity||Very high sensitivity|
|Wood, glue, linen, cotton, leather, rag paper, parchment, oil paint, egg tempera, watercolour media, and gesso. Serviceable examples of all these exist that are 1–3 millennia old from dry burial or dry enclosures at ~20°C. These examples were protected from any acid exposure, such as air pollution in the Industrial Revolution, and have never been damp. Skin, bone, and ivory of the Wooly mammoth have survived intact for over 40 millennia while frozen.||Current best estimate for stable photographic materials to remain usable as images with little or no change, e.g. 19th century black-and-white negatives on glass, 20th century back-and-white negatives on polyester film.||Acidic paper and some film become brittle and brown, difficult to access, e.g. newsprint and low-quality books, papers, post-. Acetate film shrinks, image layer cracks. Celluloid and many early plastics, become yellow, crack, distort.
Natural materials acidified by pollution (textiles, leather) weaken, may disintegrate.
|So-called "unstable" materials. Typical magnetic media begins to be unplayable, e.g. tapes of video, audio, data; floppy discs. Least stable of the photographic materials decay, e.g. colour prints fade (in the dark), poorly processed items yellow, disintegrate; cellulose nitrate yellows, disintegrates, faster when packaged in large amounts. Many elastic polymers, from rubber to polyurethane foams, become brittle, or sticky, or disintegrate.
Some acrylic paints on some canvas supports yellow rapidly.
|Temperature||Low sensitivity||Medium sensitivity||High sensitivity||Very high sensitivity|
|1 Lifetime is defined here in terms of the effects or utility described for each material listed in the top row. Whereas the lifetimes expressed in each row have considerable uncertainty, the relative improvement from top to bottom rows is certain.|
|Heat treat, sun ~60°C||~4+||~1||~6 months||2 months|
|Hot room ~30°C||~250 years+||~75 years||~25 years||~7 years|
|Warm room ~25°C||~500 years+||~150 years||~50 years||~15 years|
|Normal room ~20°C||Millennia
|A few centuries ~300 years||One human lifetime
|One human generation
|Cool store ~10°C||~5,000 years+||~1,500 years||~500 years||~150 years|
|Cold store ~0°C||20,000 years+||~6,000 years||~2,000 years||~600 years|
Note that the majority of materials in a mixed collection fall in the low-sensitivity category and have lasted centuries, even millennia, without "modern" care of their storage temperature. These materials have been preserved by a mix of moderate temperature conditions plus protection from industrial pollution, either due to a rural location or some form of enclosure, such as a building, a box, or the object's own structure, as in the binding that protects a closed book.
A practical rule of thumb for the benefits of lower temperature states that each reduction of 5°C doubles the lifetime of the object (as can be seen in Table 1a and 1b when comparing steps of 5°C). There may be controversy over the criteria defining an object's "lifetime" — how much yellowing, distortion, or disintegration — but given a selected criteria, there is no doubt that the rule holds.
Physical damage during events when the temperature is too high
Some objects contain materials that will deform and weaken, or even melt, above a certain temperature. Table 2 lists some known temperature transitions and examples of the damage possible to objects. Aside from exotic examples of foods, cosmetics, wax, and the occasional problem of repair adhesives letting go, we can conclude that the most significant example in Table 2 is the irreversible distortion of modern plastic items. Many electronic media tolerate very little distortion before they become unreadable. An example from our daily lives is the rapid distortion of video cassettes, CDs, and DVDs when left in direct sunlight. Note that the temperature necessary for this rapid distortion, ~60°C, exceeds considerably what one might infer from the climate standards for such collections. Extreme caution always drives a large safety margin when writing standards, but from a risk-management perspective, it is useful to know what temperature exactly constitutes a collection catastrophe for short events (in this example, ~60°C for however many minutes or hours it takes each object to warm up).
The section on temperature fluctuations, below, addresses physical damage due to the expansion of materials caused by the rise in temperature itself.
|Temperature Range||Temperature (°C)||Physical effect and
|Too high||Above 60°C||"Heat distortion temperature" of many common plastics (PET, acrylic, HDPE, ABS, nylon in the 65–90°C range)||Plastic objects, plastic cassettes housing electronic media, optical media — all distort quickly and irreversibly at these temperatures.|
|Above 60°C||Release of built-in stresses in biaxial PET that normally takes centuries, will occur over the course of hours to days.||The base of magnetic media, such as video, audio, or data tapes, floppy discs, deforms irreversibly. Records may become unreadable.|
|Above 45°C||Melting or softening of waxes, e.g. paraffin wax 47–65°C, beeswax 60°C, carnauba 80°C.||Paintings: wax-resin-lined oil paintings may slide or detach from their lining. Encaustic paintings soften.|
Wax seals, candles, soaps, soften, deform irreversibly.
|Above 30°C||Mixtures based on waxy components deform, separate, form blooms. Chocolate melts (34°C). Various PVA glues soften significantly, lose strength.||Some foods and cosmetics deform, mixtures bloom, separate.|
Assemblies of paper, wood, repaired ceramics, using "white glues" can fall apart, especially if combined with high RH.
|Too low||Below 10°C||Ductile to brittle transition in carbon steels, varies widely with carbon content. (Aluminum and copper alloys have no such transition.)||The most famous cases were Second World War naval vessels cracking unexpectedly in the cold North Atlantic. Assuming no external loading, not an issue in museums. Operating or loading machinery in industrial collections more risky in winter than in summer.|
|Below 5°C||Artists' acrylic paints, leathery and tough at room temperature, enter their glassy phase.||Acrylic paintings become more vulnerable to shocks and blows than at room temperature.|
|Below -30°C||Artists' oil paints enter their glassy phase.||Oil paintings become much more vulnerable to shocks and blows than at room temperature.|
|Below -40°C||Ductile to brittle transition in artists' acrylic paints. Many other practical polymers that are rubbery or leathery at room temperature will have become glassy, or even brittle, by -40°C. Shrinkage becomes significant; therefore, any restraint may cause fracture.||Acrylic paintings become extremely vulnerable to shocks and blows. Similarly, most rubbers and plastics that are elastic, or tough and leathery at room temperature, will be very vulnerable. Some plastic components may fracture if restrained, e.g. dial faces attached to wood or metal elements.|
Biological damage during events when temperature is high enough for growth
Above ~4°C, mould becomes active. Above ~10°C (consult "Pests"), insects become active. Canadian collections rarely suffered mould or moths in winter when storage was unheated. Museums in cold climates must recognize that the decision to heat a collection dominated by wool, skin, or feathers, to human comfort levels, will not only cost more, it may also increase the mould and pest risks from a concern of six months of the year to a concern of 12 months of the year.
Temperature too low
"Temperatures too low" may cause physical damage. Many practical polymers designed to be tough at room temperature become stiff or even brittle as the temperature decreases, especially modern paints and coatings. Most of the risk to a collection is not that this change is itself damaging, but that the objects become much more fragile and likely to crack when handled. Table 2 summarizes the known phenomena. The change in brittleness is both sooner, and more dramatic, with acrylic paintings than with oil paintings.
Many ordinary objects in Canada have survived -30°C routinely. Common experience shows that most suffer no noticeable damage. (Consult Vignette 1). With the advent of low temperature "freezer" methods for non-toxic pest disinfestation in museums (consult "Pests"), many objects have been subjected suddenly to temperatures of -30 to -40°C. Recent controlled studies have reported only slight damage to a few items. It is currently unclear whether the damage was due directly to the low temperature, or to collateral effects discussed under fluctuations. Overall, the risks from low temperature to objects most at risk from pest damage, i.e. organic and generally tough or flexible materials, is much smaller than the risks from live pests.
Direct physical effect of temperature fluctuations
The sections on "temperature too high" and "temperature too low" discussed damage that can be attributed to a particular temperature, rather than the process of getting to that temperature from warmer or cooler conditions. In the case of physical damage, this was due to specific transitions in physical properties. This section on fluctuations considers damage caused by the change of temperature itself, regardless of where it starts or ends. This is the parameter the engineer and your mechanical systems are trying, expensively, to control when a narrow range of fluctuation is requested. The mechanism underlying the damage is the expansion of materials as their temperature climbs, and the converse, the shrinkage as it falls. There are two situations that lead to damage: when the components of a complex assembly have different coefficients of expansion, and when an object is subjected to a fluctuation more rapid than its ability to respond evenly.
These are classic engineering problems that have been solved for many complex objects undergoing extreme temperature fluctuations such as engines, long steel bridges, even glass coffee urns, etc. When one uses these models to estimate the risk of fracture, it is clear that the necessary temperature fluctuations to cause this form of damage in most objects in the size range between vehicles and hand-held objects is on the order of a minimum of ~200°C for brittle materials, and much higher for tough materials such as wood, paper, leather, and most paints. Damage due to transitions listed under temperature too high or too low, even charring, will occur before fracture. As noted under "temperature too low," recent studies of the side effects of low temperatures (-30 to -40°C) for pest control have found little to no evidence of physical damage. One researcher with long experience in the field (Padfield ) has observed only a single example of significant damage that can be assigned to the 50°C fluctuation itself, i.e. from 20 to -30°C and back. The damage was delamination of the metal layer from the glass of an old mirror. Thus, a laminate of very stiff, solid materials in a continuous layer, that had a weak bond (the silvering of aged mirrors is often easily peeled), provides a benchmark for high sensitivity to temperature fluctuations: it can survive decades of historic climate fluctuations (probably at least 15°C), but it is likely to delaminate if exposed to a 50°C fluctuation. This is consistent with the earlier estimate that most objects, especially those of more flexible materials than glass and metal (wood, paint, leather) or those with designs that allow relative movements (metal inlay in wood, watch and gauge faces held by clips) should tolerate the fluctuation of 50°C with very low or negligible risk.
What about many fluctuations? Repetitive stresses can give rise to fatigue cracking. Beginning with the "single cycle stress" that causes fracture, engineering data from many materials shows that at about one quarter of this stress for brittle materials (glass, ceramics, old oil paint) and one half of this stress for tough materials (wood, paper, leather), fatigue cracking will occur after about a million cycles. By about one eighth of this stress, fluctuations will be tolerated indefinitely, but because it will take 3,000 years to reach a million daily cycles and because most objects cannot respond fully to cycles faster than this, then we can take the million cycle/one quarter stress combination as a very cautious extrapolation of how much to worry about multiple fluctuations. Thus, we can extrapolate that if a high-sensitivity (brittle) object is damaged by one 50°C fluctuation, then daily fluctuations of 10°C will take many millennia to cause the same damage (hence the tolerance of prior history by the old mirror). For the great majority of archive and museum objects that are many times less sensitive, we can cautiously increase these permissible daily fluctuations to 20°C or even 40°C. The geological modelling on erosion of poorly bonded and brittle sandstone exposed to the temperature extremes of the desert for millennia suggests that this estimate is cautious because the day–night fluctuations experienced by such surfaces is well over ~50°C.
One author has reported that cracks in experimental paintings on canvas were due to small temperature fluctuations, but examination of the data shows that a combination of faulty temperature measurement and condensation on the canvas is a more likely explanation. The heavily cracked and cupped Krieghoff painting in Figure 4 in the chapter on "Incorrect Relative Humidity," subjected to daily historic fluctuations in RH and temperature, shows an absence of cracks over the stretcher bars. Calculations of the thermal response, as well as the moisture response of the area over the stretcher bars (Michalski ), shows that this region was protected primarily from daily RH fluctuations, not daily temperature fluctuations. Therefore, the daily temperature fluctuations of a historic Canadian house do not appear to be responsible for the cracking in this object. On the other hand, some authors' computer modelling of old oil paintings suggests that low winter temperatures, as a seasonal fluctuation, are indeed responsible for certain patterns of cracks commonly observed.
Using the practical concept of "proofed fluctuation"
The analysis of the risks from temperature fluctuations is complex and many uncertainties remain. For practical purposes, one can draw instead on the concept of a "proofed fluctuation." Any object known to have been at least once at some very low temperature, say -30°C, or at least once at some high temperature, say 40°C, is not susceptible to further mechanical damage from one more event of the same magnitude because any fractures, delaminations, and irreversible compressions will have already taken place (unless the object is known to have weakened significantly from other causes in the interim). The fatigue effect does mean that one must modify this simplistic "proofed fluctuation" by noting that one should say that risks from single fluctuations should be predicted in light of the "proofed single fluctuation" and that the risk from repetitive fluctuations must be predicted in light of the "proofed repetitive fluctuations."
In other words, any future pattern of fluctuations that is similar to the past pattern of fluctuations cannot be expected to cause significant fluctuation damage. A practical corollary is that even modest improvements on past climate conditions will eliminate the risk of physical damage. It is important, therefore, to be accurate about assessing past climate control, and not to underestimate how bad it was because the worse one knows the past to have been, the easier it will be to make the future better. (The same is shown for RH fluctuations.)
Balancing the Risks from Conflicting "Correct" Temperatures
This variety of incorrect temperatures almost always means that one cannot find a "correct" temperature condition with zero risk to the collection, that one can only find the temperature conditions of minimum risk. The most common dilemma arises when considering cumulative chemical damage from "temperature too high," plus the mechanical damage from "temperature too low," plus the effects of the inevitable seasonal fluctuation. If, for example, a Canadian museum has a 20th century archives, a warehouse of newspapers, a collection of rubber dolls, the original rubber tires and plastics on its vehicles or agricultural equipment, and boxes of woollen textiles, should it take advantage of low winter temperatures, even if some materials may become brittle? Given that it cannot afford the machinery to maintain these cold conditions in the summer, will the large seasonal fluctuation outweigh the benefits of occasional cold? Is there a "compromise" that works best?
In short, yes, winter cold will help collection preservation in general. All the historical evidence we have comparing collections from different climates implies that the condition of our collections will be much better in the future if a collection goes cold every winter, and that any risks from the cold, or from the seasonal fluctuation, are either very small or non-existent.
A more nuanced approach to winter's benefits is possible. Once the winter temperature is below about 5°C, then the benefits of even lower winter temperatures are insignificant in terms of total annual chemical decay because the summer period of decay is unchanged. The mechanical risks, though, do continue to increase as the winter temperature drops below 5°C. Thus within a low-energy approach of a little winter heating and a little summer cooling, a "sweet spot" does exist in terms of total risks: keep summer below 25°C; keep winter above 5°C. To improve preservation of chemically unstable materials such as newspapers, film, tapes, plastics, etc., further than this, one must consider special year-round cold storage.
RH Problems Caused by Fluctuating or Uneven Temperature
Museums and their consultants usually lump temperature and RH together under "climate control" or "the environment." Here we discuss incorrect temperature and incorrect RH as separate agents because both the damage to collections and the means of control are more dissimilar than similar, and because the entanglement of the two under "climate control standards" has led to many false generalizations and wasteful simplifications. For photographic and electronic records, for newsprint and office records, and for self-destructing plastic objects in modern art collections, the key issue is "temperature too high" with risks from RH fluctuations negligible in comparison. The complete reverse is true for collections of furniture, ivory, metals, and oil paintings — RH fluctuations are important while most forms of incorrect temperature are not. Furthermore, the low-cost, low-energy, passive solutions for each are distinct and become sidelined in the quest for a single engineering solution to "climate."
That being said, two practical linkages between fluctuating temperature and fluctuating RH must be noted. One is the problem of temperature fluctuations over time; the other is the problem of fluctuations over space, which can be more simply referred to as the problem of "uneven" temperature.
In a closed and empty room or display case at 20°C and 50% RH, with no humidity-buffering materials, a fluctuation of 1°C causes ~3% RH fluctuation. A fluctuation of 5°C causes ~15% RH fluctuation. The most dangerous fluctuation, a temperature drop over 10°C will cause 100% RH and condensation. Fortunately, for most spaces in most historic house museums, these effects are greatly moderated by the moisture buffering of the room or case surfaces. (The limitations on temperature fluctuations in the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE ) chapter specifications were determined more by this linkage to RH risks than by the temperature effects themselves).
Uneven temperatures from place-to-place in a building are generally a bigger problem than fluctuations over time, especially for museums in less than ideal buildings. The chapter on "Incorrect Relative Humidity" explains the various ways in which uneven temperatures become a source of incorrect RH. In short, the most important form of incorrect RH, damp, is more often than not caused by humid air reaching localized cold spots.
Sources of Incorrect Temperature
The single most damaging source of incorrect temperature is direct sunlight. Surface temperatures of dark insulating materials oriented towards the sun, such as dark wood, textiles, and plastics, can quickly reach 40°C above ambient air temperature; therefore, on a warm summer day, surfaces can reach 75°C. If these surfaces are enclosed by glass, as in display cases and picture frames, even higher temperatures are possible. Clearly, sunbeams have the power to exceed all temperatures listed as "too high" in Table 2, as well as making the rate of decay of all categories of sensitivity in Table 1a and 1b increase a hundred fold if routinely hit by a sunbeam. Paper sheets in glazed and backed frames are especially vulnerable because the RH needed for rapid decay is also maintained by the otherwise beneficial buffered enclosure.
Outdoor air temperatures in Canada range between the extremes of 40°C and -40°C. A change in the weather of 10°C usually takes many hours; a larger change usually takes many days. Outdoor air temperature is, thus, not a source of rapid fluctuations from the perspective of objects, nor a source of the severe high temperatures that can cause physical damage of Table 2. It is a source during summer of "temperatures too high" for unstable materials, but it is extremely benign to these same materials for the winter, and more so as one proceeds north. Some of the best-preserved samples of early movies have been found in the garbage in the high North because shipping them back at the time was not worthwhile.
As does sunlight, incandescent electric lamps can cause surface heating due to their high content of infrared (IR) (this includes quartz-halogen lamps). Incandescent lamps have even higher ratios of IR to light than a sunbeam. After direct sunlight, excessive incandescent lamps used in display cases are probably the most common cause of extreme temperature fluctuations in museums (and the concurrent fluctuations in RH). The wide split in a rare First Nations saddle placed on display in the Museum of Man (Ottawa) during the s was almost certainly the result of not only low winter RH in the building, but also of even lower RH in the case caused by lamp heating.
Buildings and Their Climate Control Systems
Aside from whatever temperature the climate control system is delivering in the centre of the room on average, many forms of incorrect temperature will be located near local heaters and air vents. In rooms with poor or non-existent air circulation, exterior walls experience larger fluctuations than the room average, the ceilings are always warmer, and the floors always colder. Figures illustrating these zones of uneven temperature have been placed in the chapter on "Incorrect Relative Humidity" because the RH effects from these temperature differences are generally a bigger problem than the temperature differences themselves, especially when they cause damp.
Objects in Transit
Along with many other risks, the risk of incorrect temperatures is high during transit, especially for paintings. Temperature in uncontrolled vehicles in summer can be much hotter than the air outside. In winter, truck interior temperature falls well below the listed "temperature too low" values given in Table 2 for acrylic paintings. Even just a "quick run" between storage and display buildings in winter can cause lightly wrapped paintings to become much more brittle than staff expect, at the same time that the painting is flapping and bumping into walls and floors.
Control of Incorrect Temperature
Stages of Control
Identify incorrect temperature values, and specify correct temperature values
Unlike other agents of deterioration (such as pests, pollutants, fire, etc.), where one wants none, or zero "agent," one cannot plan a goal of "zero temperature." One must determine the various incorrect temperatures before one knows what to control. Collecting large numbers of thermohygrograph records, and worrying whether the wiggly lines mean anything, is impossible without first assessing a collection's sensitivities (consult Tables 1 and 2). One thing is clear: humans are a very poor reference point. We like a temperature near 21°C, with no more than 2°C fluctuation if we are sitting. This setpoint is incorrect for most archival records and unstable modern plastics. The fluctuation limit is much more stringent, and resource wasteful, than needed for any collection.
- Avoid placing organic objects, or fragile inorganic objects, in locations receiving direct sunlight. Even in outdoor storage of large objects, avoid areas with full sun exposure if the item contains wood, paint, leather, rubber, textiles, or plastics.
- Avoid creating sources of incorrect temperature during the design stage of purpose-made buildings. "Passive thermal control" amounts to making well-insulated walls, with high thermal mass, so that exterior temperature fluctuations are smoothed out over many days and weeks.
- Avoid selecting and installing mechanical systems that lack reliability and that cannot be easily maintained by local resources and local funding. Inform any consultants of this need. It is far more important to avoid a few extreme conditions due to system malfunction over the years, than to avoid routine small fluctuations from day to day.
- Avoid heating collections in winter that contain unstable materials, as noted in Table 2. Take advantage of the cold winter hiatus (if your region has one) from chemical decay and pest activity.
- Block sunlight, both by shutters and blinds for indoor areas, and by simple overhangs and roofs for outdoor items.
- Use insulation or at least provide an air space (10 cm or more) between objects and external walls, cold floors, and hot ceilings.
- Insulate artworks in transit, either in the crate, or at least with blankets if moving by hand a short distance outside.
- Monitor temperature. Of all the agents of deterioration, temperature is probably the simplest and cheapest to measure precisely.
- Detect signs of chemical damage, such as brown brittle paper and decaying photographs. These examples can be used as a general indication for decision makers unfamiliar with unstable materials. If you wish to establish that the temperature control in the recent past was especially incorrect, you will need accurate condition reports and good temperature records for the period in question.
- Detect signs of old mechanical damage, but interpret this carefully before drawing conclusions about current temperature control. Collection managers often point at cracked furniture or cracked paintings and cite them as proof that they need new climate systems. While this may (or may not) be true of the RH control, it is almost never proof that temperature control itself was inadequate.
- Respond via mechanical systems, such as heaters and air conditioners controlled by a thermostat. Reliability is essential.
- Respond to the issue of unstable materials that will disappear in one generation by lowering temperature, (or by other archive strategies, such as transferring the information to stable media).
- Segregate especially unstable materials in collections, such as badly processed negatives, pieces of urethane foam, and rubber items, mixed in with more stable materials. These often stand out visibly as sources of yellowing, and should be removed and stored separately (or discarded) because their degradation products damage adjacent materials.
Recover and treat
- Mechanical fracture can often be repaired, although disfiguring lines may remain.
- Physical deformations, from extreme forms of high temperature listed in Table 2, cannot be treated.
- Chemical aging, listed in Table 1a and 1b, cannot be treated.
Control Strategies For Different Degrees of Preservation
Basic control: No moving parts, no machinery, no energy consumption!
- Ensure reliable walls, roof, windows, and doors have good insulation, and preferably high mass walls.
- In addition to the above, ensure that direct sunlight does not strike any materials, especially those listed as sensitive in Table 2, which will suffer from a single day of direct sun. With these two steps in place, almost all physical risks from incorrect temperature are avoided, but high-sensitivity materials, listed in Table 1, will still have very short lifetimes.
- Inspect archival film collections and separate rapidly decaying negatives (due to poor processing) from the bulk of the collection. Segregate all cellulose nitrate films, to prevent acidic attack of adjacent materials, and to reduce fire risk.
- Inspect mixed historic collections and modern art collections, and remove any rapidly decaying nitrates, plastics, rubber, and urethanes that may contaminate adjacent objects.
Optimum control: Different collections, different situations, different control measures
- Follow basic control as above, plus integrate the following as needed:
- For archival collections and modern materials in mixed historic collections, identify the stability of the materials as listed in Table 1a and 1b, and provide cool or cold storage as required within the institution's mandate. Cold storage can range from a single-chest freezer storage (consult Vignette 2) to building-scale storage (see Wilhelm ). Groups of small museums should consider shared cold storage.
- For a mixed historic collection that has remained in an old building for many decades without noticeable change within the last decade, do not "improve" the control systems (e.g. add new components, or change their operation such as heating more in the winter than before), without carefully considering exactly what the current incorrect temperatures are and what evidence you have to believe they will cause more damage than the "improvements." Begin by ensuring the reliability and long-term maintenance of any current building elements and control systems, rather than changing the climate targets.
- When considering full-scale "building climate control," recognize the building envelope's limitations especially if it is of historic value itself. A table of five building types and their ability to tolerate climate controls is provided in ASHRAE (). For an overall philosophical approach to the dilemma, refer to the New Orleans Charter for Joint Preservation of Historic Structures and Artifacts of the Association for Preservation Technology International/American Institute for Conservation of Historic and Artistic Works (APT/AIC). Then select and implement an appropriate ASHRAE control setpoint and fluctuation level (ASHRAE ). (Consult Table 2 in the chapter "Incorrect Relative Humidity.")
- When the objective is the display of travelling exhibitions, recognize that some major lending institutions require ASHRAE level A control, or sometimes AA (ASHRAE ). (Consult Table 2 in the chapter "Incorrect Relative Humidity.") and LINK TO CCI ASHRAE PAGE Purpose-built rooms or buildings are usually required. Consider a "room within a room" or cocoon approach (ASHRAE ).
Not only are we poor judges of an object's response to temperature, but we also tend to personify the collection. It is difficult to feel that our most precious furniture or paintings are relatively content at a frigid -20°C, if left quietly alone. We ourselves much prefer to be heated in winter even if the result is a very low RH. We need to understand that our historic collections "feel" the opposite. They prefer to be cold accompanied by a moderate RH.
We also find it difficult to accept that the old materials found throughout most of humanity's history are quite stable, and that it is our own era's objects (late industrial through to the electronic era) that are the problem, that are as ephemeral as our own selves. While cold storage for us, or migration of our memories, may be a science fiction fantasy, they are the only practical solutions for much of our recent material heritage.
Traditional museum specifications for temperature were based not on any detailed consideration of collection needs, but on the superficial observation that collections did not seem so uncomfortable at the temperatures that we ourselves found comfortable. The fact that our human comfort temperatures appeared, conveniently, not to be incorrect for our collections, transformed into the notion that they were the correct temperature for our collections, which was not actually true for any known collection and completely false for many, such as libraries and archives. Permissible fluctuations, given the observations of bad things happening at very large fluctuations, were predicted to be very, very small, because less and less of a bad thing must be better and better. Unfortunately, such flawed logic has led us to greatly overrate temperature stability as a guiding principle, to spend unnecessary resources on its achievement, to gut the fabric of historic buildings in its implementation, and to use scarce energy resources fighting the benefits of cold winters.
Museums and their advisors adopted these standards throughout the world. Most still insist on very stable temperatures for collections, whether in storage or on display. It will take time to develop the consensus necessary to revise these standards, and, until such time, museums that want to receive travelling exhibitions or obtain various forms of museum accreditation will need to conform. Small museums caring rationally for their own collections, however, can begin to apply a risk-management approach and to use a more subtle and cost-effective logic than most standards allow.
Currently, the only published temperature (and RH) specifications for libraries, museums, and archives that follow this complex and graduated risk approach, that note explicitly that setpoints near 20°C are not beneficial for unstable modern materials, that provide suggestions for seasonal energy-saving adjustments, and that provide an estimate of overall risk from six different fluctuation specifications, is the current edition of the ASHRAE () handbook. These specifications are provided as Table 2 in the chapter "Incorrect Relative Humidity" and are explained in further detail in a special page devoted to the ASHRAE specifications.
Vignette 1. Letting Collections Freeze and Fluctuate in the Yukon
Over 20 years ago, Michael Gates, Curator for Klondike National Historic Sites in Dawson City, Parks Canada, established a small, humidistatically controlled collection storage room. The electric heaters switch on when the RH climbs above 50%, and switch off below 50% RH. The result in winter is stable RH, and temperatures fluctuating in a range well below freezing, as low as -40°C. Energy use is very low. He has monitored the mixed collection of objects and not noticed any damage due to the low temperatures or the fluctuations. Note that the term "freezing," while loosely used in everyday language to mean temperatures below 0°C, does not mean that objects or their moisture content (at 50% RH) solidify. It is only liquid water that freezes at 0°C.
Vignette 2: Small-scale Cold Storage for Archival Records
The British Columbia Archives, now part of the Royal British Columbia Museum, established a modest cost approach to cold storage of its film collections by using a series of stand-up freezers. In order to provide very stable RH control, the items are individually packed using the double-bagging system of McCormick–Goodhart. (A single bag is enough to eliminate any risk of RH damage during freezer failure.) Although the museum finds that a large number of separate, and now ageing freezers, is becoming a significant maintenance issue (and may be replaced by a single walk-in facility), one or two freezers, as used by homeowners, has proven a very effective solution for smaller North American archives and museums.
APT/AIC (Association for Preservation Technology International/American Institute for Conservation of Historic and Artistic Works). New Orleans Charter for Joint Preservation of Historic Structures and Artifacts.
Michalski, S. "Paintings, Their Response to Temperature, Relative Humidity, Shock and Vibration." In, M. Mecklenburg, ed., Works of Art in Transit. Washington, D.C.: National Gallery, , pp. 223–248.
Padfield, T. Personal communication, .
Wilhelm, Henry Gilmer, and Carol Brower. The Permanence and Care of Color Photographs: Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures. Iowa: Preservation Publishing Co., .
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). "Museums, libraries and archives." In, 2003 ASHRAE Handbook: Heating, Ventilating, and Air-conditioning Applications, SI edition. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., , pp. 21.1–21.16. (Note, the edition does contain the same temperature and RH specifications, but lacks the revisions on pollution. Pre- editions do not contain a "Libraries, Archives and Museums" chapter. New editions appear every three to four years. Museums should expect consultants to use the latest edition.)
Michalski, S. Guidelines for Humidity and Temperature for Canadian Archives. CCI Technical Bulletin 23. Ottawa: Canadian Conservation Institute, .
Glossary of terms
Coefficient of thermal expansion: The fractional increase in length of a material due to a rise of one degree in temperature.Consult Michalski () for a review of some values and sources.
K, k: Symbol for degrees Kelvin, the metric unit of temperature used by scientists, where 0 K is absolute zero. The convention for degrees Kelvin does not use the degree symbol. Each change of 1 K is the same as a change of 1°C. One can substitute 5°C for 5 K. Freezing point of water is 0°C or 273.15 K. Although European and Canadian engineers tend to use °C, the SI edition of the handbook, used by engineers in the United States (ASHRAE ), uses a mixture of °C for setpoint specifications and K for fluctuation specifications.
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