The Who, Why, When and How of Moisture Equilibration

In many ways, it is more difficult to evaluate the impact of fluctuations in the environment's relative humidity than the environment's temperature because Moisture Equilibration is influenced by more variables than thermal equilibration. For example, enclosures or housing situations may act as moisture barriers and thus influence the how quickly (or how slowly) the objects are exposed to the new humidity conditions. The temperature will also influence the rate of moisture equilibration. Furthermore, there is more variation in the capacity of the individual objects or materials to control moisture equilibration than thermal equilibration.

In order to explore a few basic concepts about moisture equilibration, this article will answer several basic questions: the Who, Why, When and How of moisture equilibration.
 

#1) Who?

Only hygroscopic materials – organic materials that naturally contain water – are susceptible to moisture equilibration.

First, while it may seem too obvious to state, moisture equilibration is only relevant for organic materials that naturally contain water, or hygroscopic materials. Only hygroscopic materials will absorb or desorb water to equilibrate with the relative humidity of the environment. Non-hygroscopic materials – materials that do not inherently contain water - will not equilibrate with changes in the environment's moisture because they have no moisture to release, nor the nature to absorb any moisture. For example, organic, cellulosic materials such as paper or textiles will adjust to changes in the moisture of the environment (they will equilibrate) by absorbing or desorbing moisture, while inorganic materials like metal will not. Non-hygroscopic materials such as metal may be affected by the environment's moisture in other ways, (corrosion is an obvious example) but they will not equilibrate with the moisture content of the environment.

#2) Why?

Hygroscopic materials are constantly exchanging moisture with the air in the form of water vapor.

At any given moment, there is a dynamic exchange of moisture occurring between an object's core, its perimeter, and the air of the environment. The moisture is transferred in the form of water vapor by the process of diffusion. The moisture diffuses from the material into the air, then it diffuses back to the material and then back to the air. Roughly speaking, this diffusion is driven by differences in moisture content. So, when an area with more moisture comes in contact with an area less moisture, moisture is transferred from the area of higher concentration of moisture to the area of lower concentration of moisture. Because this exchange of moisture is continuous, eventually enough moisture is diffused from the air into the material (or the material into the air) that the material neither gains nor loses moisture in the exchange. At this point, the object has reached moisture equilibrium with the environment. This dynamic, continuous exchange is why hygroscopic materials equilibrate with the relative humidity of the environment.

#3) When?

When the moisture content of the material is not in equilibrium with the relative humidity of the environment, the material adjusts its moisture content to reach equilibrium.

When hygroscopic materials are moved from one humidity condition to another, (or when they are exposed to humidity fluctuations), the moisture content of the materials is no longer in equilibrium with the relative humidity of the air. Confronted with this difference in moisture content, the material will absorb or release (desorb) moisture until its moisture content reaches equilibrium with the new environmental condition. For example, if the relative humidity of the environment increases, the material will absorb moisture from the environment. In other words, if the moisture content of the air increases, the material reacts so that its moisture content will also increase. During absorption, the moisture travels from the outside of the object inward, affecting the edges and the top of the object before reaching the object's core. Similarly, if the relative humidity of the environment decreases, the material will release (or desorb) moisture into the environment. In other words, if the moisture content of the air decreases, the material will react so that its moisture content will also decrease. During desorption, the moisture travels from the inside of the object outward towards the surface.

The materials thus respond to the changes in the moisture content of the air. But, just as with temperature equilibration, the new moisture equilibrium is not attained instantly. It takes time for the material to respond to the new conditions, to absorb or desorb the appropriate amount of moisture. Only if the new humidity conditions persist long enough will the entire object –from its perimeter to its core - reach a moisture equilibrium with the relative humidity of the environment.
 

#4) How much?
 

The amount of moisture organic materials contain is primarily determined by the relative humidity of the air.

Because hygroscopic materials equilibrate with the relative humidity of the environment, the relative humidity is the primary determinant of a material's moisture content. The amount of moisture a material contains when it has reached equilibrium with its environment is described as the Equilibrium Moisture Content (EMC). Expressed as a percentage, the EMC describes how much of the material's mass is made up by water. For example, if an object is has an EMC of 8% at 60^oF and 40% RH, then there are 8 grams of water in every 100 grams of material when the material has reached equilibrium with that environment. The relationship between EMC and relative humidity can be seen in the representation of moisture equilibrium curves, which graph the EMC of a material at a given RH and constant temperature. Look at Figure 3, which shows the typical moisture equilibrium curves for the components of photographic materials. This representation illustrates that as the relative humidity increases, the moisture content (the EMC) of the materials also increases. Consider one material represented in this figure, paper. At 20% RH, the paper contained approximately 4% water; when the relative humidity increased to 80%, the EMC of the paper increased to 10%.  The same pattern is true for each of the other materials presented here; the higher the relative humidity of the air, the higher the EMC of the materials. Looking at each of the moisture equilibrium curves, we can see that the moisture content of the material is determined by the relative humidity of the air.

Not all materials contain the same amount of moisture.

The representation of the moisture equilibrium curves also depicts another important fact: not all materials contain the same amount of moisture. Even though the moisture content of materials is primarily determined by the relative humidity of the air, the inherent characteristics of the material also influences how much moisture it will contain. Look again at Figure 3 and consider the differences in the moisture equilibrium curves of each material. Under identical climate conditions, the different components of photographic materials contain different amounts of moisture. At 20% RH, the paper contains 4% moisture, while at 20% RH the emulsion has an EMC of 5%.  Even more of a difference is seen in the EMC of the materials of the photographic film base. At the same 20% RH, the acetate film base has an EMC of less than 1%, while the polyester film base contains nearly no moisture at all (less than 0.25%) At the same temperature and the same RH, this group of materials exhibits a range of moisture contents from an EMC of <0.25% to an EMC of 5%. Such a comparison gives an idea of how the material's inherent characteristics influence its moisture content as well as its sensitivity to changes in relative humidity.
 

#5) How fast?
 

The rate of moisture equilibration is influenced by each material's inherent capacity to control moisture diffusion.

As stated earlier, materials don't respond instantaneously to changes in the environment. It takes time for the material to absorb or desorb the appropriate amount of moisture to adjust to the new humidity conditions. The length of time it takes an object to equilibrate, however, depends on many variables; the inherent properties of the object, the hygroscopic nature of the material, the dimensional characteristics, and the surface exposure to the environment all influence moisture equilibration. Enclosures and temperature will also influence the rate of moisture equilibration, but this article will discuss the equilibration of materials without enclosures and at constant temperature. This evaluation gives us an idea of how the capacity of objects to control moisture diffusion varies with each object or material.

To see how objects respond when they are freely exposed to humidity changes, researchers at IPI exposed materials without enclosures to two different humidity scenarios at a constant temperature 70^oF (20^oC); #1) a one time humidity change from 20% RH to 50% RH, to see how fast the materials absorb moisture, and #2) a one time humidity change from 50% RH to 20% RH – to see how fast the materials desorb moisture. Figure 4 and Table 2 show the time (in days) for the materials without enclosures to reach 50% equilibration, with the rates of both moisture absorption and desorption represented. The range in demonstrated equilibration times is significant. The fastest equilibration time was seen when the ¼" Magnetic tape was exposed to drier conditions (its rate of desorption). It took the ¼" tape less than half a day to release enough moisture to reach 50% equilibration –half way between the initial humidity and the new humidity. The longest equilibration time was seen when the stack of mounted photographs was exposed to drier conditions (the rate of desorption), which took approximately 15 days to release enough moisture to reach 50% equilibration. Because the rate of equilibration drastically slows down as it approaches 100% equilibration, we can not extrapolate that full equilibration (100% equilibration) will be double the time for 50% equilibration. However, we can use the mark of 50% equilibration to gauge how slowly equilibration can occur.

Despite variation, moisture equilibration is consistently "slow".

The fact that such variation in moisture equilibration rates is seen in just the materials themselves – even without other variables such as enclosures, storage situations or varying temperatures – demonstrates how diverse the rates of moisture equilibration can be. However, clearly and consistently demonstrated in the data is the general rule that moisture equilibration is "slow".  Compared to the rates of thermal equilibration, which were measured in a matter of hours, all of the materials tested reached moisture equilibration in a matter of days. It is important to stress again that the times represented here are for materials tested without enclosures. Although it is not discussed in this article, it has been demonstrated that real-life housing situations of museum and library materials which may act as moisture barriers– books stacked tightly on shelves, objects enclosed in protective housings – will surely slow the rate of moisture equilibration even further.
 

#6) So what!? What does this "slow" rate of moisture equilibration mean in respect to environmental fluctuations?

Let's return to the Pop Quiz:

How long will it take a book, stacked among other books on a library shelf, to adjust to a humidity change from 30% to 50%? Answer: approximately one month.

Just because the relative humidity of the environment suddenly increases 20% does not mean the moisture content of the object simultaneously increases. In fact, if the increase is temporary, the object may not "feel" the change at all.

Knowing that moisture equilibration is a slow process gives us an important perspective on the significance of environmental fluctuations. The example of the pop quiz is a book surrounded by other books, packed tightly on a shelf. Another example presented in the experiments in this article is a hardcover book alone on an open, metal wire shelf. If this individual book – with its surface exposed to the environment on all sides – takes two days of sustained humidity conditions to reach 50% equilibration, even the individual book is unlikely to "see" or "feel" a temporary spike of humidity that lasts only a matter of minutes or hours. If we are concerned about spikes in humidity when analyzing our storage environments, it is the seasonal spikes (of sustained high humidity in the summer, and sustained low humidity in the winter) that we should work to regulate. After all, IPI's Jim Reilly has said, "seasonal changes are the most significant factor in determining the preservation quality of collection environments".

To read more about how the impact of enclosures and microclimate on moisture equilibration in respect to environmental fluctuations, look for future issues of Climate Notes...  

References:

Bigourdan, J.-L., P. Z. Adelstein, and J. M. Reilly, "Moisture and Temperature Equilibration: Behavior and Practical Significance in Photographic Film Preservation," La Conservation: Une Science en Evolution, Bilans et Perspectives, Actes des Troisiemes Journées Internationales d'Etudes de l'ARSAG, Paris, 21 au 25 Avril 1997, (Paris: Association pour la recherche scientifique sur les arts graphiques, 1997) pp. 154-164. (Open PDF file)

Bigourdan, J.-L., and J. M. Reilly, "Effects of Fluctuating Environments on Library and Archives Materials," Final Report to the Institute of Museum and Library Services, IMLS Grant #LL-80088-98, Image Permanence Institute, Rochester Institute of Technology, Rochester, NY, Februrary 15, 2003.