Agent of Deterioration: Incorrect Temperature

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 section 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 1896 to 1952 ). 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 and 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 (2000).

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 2006 ) 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 section 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 1991), 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 2003 ) 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 section 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.

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.

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 1993 ). 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 (2003). 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 2003). (Consult Table 2 in the section 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 2003). (Consult Table 2 in the section 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 2003).