Damage Caused by Incorrect Temperature and RH
The damage caused by incorrect temperature and RH falls into three broad categories:
The biological threat directly related to RH is mould growth. Mould is always undesirable in a collection. It causes irreversible and often devastating damage.
Mould spores are naturally present in the air around us, and it is impossible to eliminate them. However, it is only when the mould spores have sufficient nutrients, time, and moisture that they grow into destructive mould. As suitable nutrients are readily available in almost any environment, the route to controlling mould is to control moisture. Without moisture, mould spores cannot grow.
The safe RH boundary usually cited to prevent mould growth is 65%. Below the safe RH boundary, mould will not grow at any temperature. In contrast, mould is very likely to grow at high RH. For example, at 85% RH, mould will probably appear in less than a week. Establishing what happens in borderline conditions (in RH between 60% and 70% with different temperatures) is a bit more challenging. Graphs that provide more precise answers to these questions can be found in the ASHRAE chapter mentioned previously and in Technical Bulletin 23 (Guidelines for Humidity and Temperature for Canadian Archives).
It is relatively easy to maintain ambient RH and temperature levels that will preclude mould growth. The danger, which is often hidden, lies in microclimates that can inadvertently be created within a building.
Many rooms have temperature gradients, and colder spots have higher RH than warmer ones. For example, during cold weather, exterior walls or floors may become considerably colder than the rest of the room, creating pockets of high humidity or even condensation. The tools designed to condition the air can create their own problems: near the output of humidifiers, RH will be too high; and near dehumidifers, there is the risk of water leaks. Even attempts to protect objects by enclosing them in a protective wrapping can backfire. Consider what happens if a package containing an object is stored where the temperature is uneven (for example, one end of the package is touching a cold wall). The cold end will have much higher RH than the warmer end, leading to mould growth. In summary, protective wrapping is beneficial in many ways, humidifiers and dehumidifiers can play important roles, and most storage rooms are better for collections than no storage room at all. Each of these improvements to collections, however, needs to be applied carefully so as not to create new sources of damp and mould.
Chemical damage is caused by chemical reactions taking place within a material. The key reactions are hydrolysis and oxidation, which account for most natural aging processes.
"Hydrolysis" is a reaction between a substance and water that results in the chemical breakdown of the original substance and the formation of one or more new substances.
"Oxidation" is a reaction between a substance and oxygen, often resulting in physical breakdown.
Temperature and RH both affect chemical processes:
- heat speeds up any chemical reaction (the rule of thumb is that the rate of reaction approximately doubles for every 5°C increase in temperature)
- RH is significant because some reactions require moisture before they can take place
Acid hydrolysis is a primary concern for museums and, especially, archives. It affects cellulose-based materials, including photo negatives and cine film, magnetic media, and paper. In paper, acid hydrolysis splits the long cellulose chains into shorter ones, making the paper less flexible, more brittle, and more susceptible to damage. This breakdown process will continue as long as acid is present and, because the process itself produces acid, the degradation will actually accelerate as it progresses.
The lifetime of paper is highly variable. It depends on the make-up of the paper, with fibre type, fibre length, degree of beating of the pulp, chemicals used in manufacture, basis weight, and thickness all affecting how long the paper can be expected to last. Although none of these factors can be changed, the temperature and RH conditions of the environment where paper is stored can influence its relative permanence.
Isoperms or "record lifetime multipliers" provide a graphical representation of the relationship between the lifetime of materials that deteriorate rapidly from acid hydrolysis, and the temperature and RH at which they are stored. Isoperms can be used to plot the relative lifetime changes associated with changes in temperature or RH. Isoperm graphs and further explanations on their use can be found in the ASHRAE chapter and in Technical Bulletin 23 (Guidelines for Humidity and Temperature for Canadian Archives).
Extremes of temperature or RH make many objects vulnerable to mechanical damage. For example, materials that become stiff or brittle when cold are more likely to break at low temperatures. Luckily, extremes are usually easy to avoid, except in rare cases such as sudden failure of an environmental control system.
It is fluctuations in temperature or RH that cause the most mechanical damage. Unfortunately, these are less easily controlled and not as well understood.
The explanation for the dangers of fluctuations lies in the basic mechanical properties of materials:
- inorganic and organic materials (e.g. metal, stone, paint, and wood) respond to temperature changes by expanding when hot and contracting when cold
- organic materials, which are hygroscopic, respond to changes in RH by shrinking when RH drops and swelling when RH climbs
These responses do not cause damage in and of themselves. However, damage can occur when objects combine materials that respond in different ways, placing one material under restraint from another. For example, a wooden panel that is not restrained in any way can expand and contract in response to changing conditions without harm. However, if the wooden panel is part of a chest of drawers, or an agricultural machine, or an architectural element, and it is attached to other elements that restrict its movement, then damage is likely when the wood attempts to move during RH changes. A painting presents a more complex example. A typical painting is composed of many layers that expand and contract in differing ways. The internal stresses thus created can cause cupping and craquelure over time.
The damaging nature of very wide fluctuations in RH and temperature has been known for a long time. The early specifications for acceptable fluctuations in museums assumed that even very small fluctuations would produce some form of damage. Specifications such as 50±3% RH and 21±1°C emerged. Since the early 1990s, a consensus has grown among scientists and conservators with wide experience of collections that objects have a range within which they tolerate fluctuations without damage. The ASHRAE chapter currently provides the best review of the issues and the relevant literature.