Many of us are aware that seasonal changes affect the wellbeing of our collections. Changes in humidity can harm many different types of materials within our collections. So how does dryness really affect collections?
Dryness is one of the main culprits of mechanical damage. Objects tend to reach equilibrium with the air within the space or room where they are located. When changes in humidity occur, objects react in different ways to compensate for the change, and, at the same time, are affected in different ways. Dryness can cause contraction, and it can increase stress as well as brittleness. Dryness has the most impact on collection objects that are organic materials. Organic materials are hygroscopic in various degrees, which means that these materials absorb or desorb moisture according to environmental changes. As humidity increases, the material absorbs the moisture, which can cause the material to expand and/or soften. On the other hand, as humidity decreases, the material desorbs moisture, which can cause the material to contract and become brittle. In addition, when objects are composed of multiple layers from various materials, some layers might absorb or desorb moisture at a different rate, causing stress between these layers. Sometimes this stress can cause irreversible damage.
A Closer Look at the Effects of Dryness on Collections
Museum, library, or archive collections might contain a wide range of materials, such as books, paintings, furniture, and photographs. Each of these materials will react differently to winter dryness. Let’s explore the effect of dryness on a few materials.
Rare Books and Paper
Rare books often have vellum (goat or sheep skin over wood or cardboard) covers. When exposed to dry conditions, vellum tends to contract. Figures 1 and 2 illustrate the effects of dryness on a rare book.
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Figure 1: Example of the effect of moisture loss on a rare book. (a) The state of the book at equilibrium under normal conditions. (b) The state of the book at equilibrium at low RH. The contraction of the vellum was the result of moisture loss, which caused the cover to curl.
Figure 2: Example illustrating a crack in the book's cover due to dryness.
Paper
Dry conditions have similar effects on paper as it does on books. Paper tends to become brittle when exposed to low RH, so any type of stress, such as folding, can cause the paper to break. Furthermore, if the paper was also affected by natural aging (often indicated by discoloration), exposing it to dry conditions exacerbates its brittleness, sometimes to a point that it can crumble to pieces if handled.
Figure 3: Example of paper materials affected by natural aging. Dryness will further affect the state of the material by causing it to become even more brittle.
Photographs
The effect of dryness on photographs can be curling. When looking at the structure of a silver gelatin print, it contains several layers: the paper as well as the so-called emulsion layer (silver particles in a gelatin binder). As the print is exposed to dryness, the emulsion layer contracts, causing the print to curl, sometimes to an irreversible state. Figure 4 below illustrates a similar behavior in a silver gelatin print as well as Albumen prints.
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Figure 4: Example of a silver gleatin print (a) as well as Albumen prints (b) that were exposed to prolonged dry conditions.
Glass Plate Negatives
The basic structure of glass plate negatives is similar to that of the photographic print layer. It includes the actual glass plate or support as well as the gelatin emulsion. The glass itself is not affected by dryness; the emulsion, on the other hand, is. The gelatin expands/swells when exposed to high humidity and contracts when exposed to low humidity. This environmental change causes stress between the glass support and the emulsion layer. The result of this stress can be delamination or flakiness of the emulsion layer (Figure 5).
Figure 5: Delamination of a glass plate negative. The glass plate negative was exposed to prolonged high RH followed by dry conditions.
Wooden Objects
Wooden objects are greatly affected by winter dryness. When exposed to low RH, the wood contracts, causing the structure to crack and shrink.
Figure 6: Example of a cracked wooden object due to prolonged exposure to dryness.
In addition, dryness can also cause the structure of a wooden object to fail. For example, often quality furniture uses the tongue and groove technique for drawers or any other connections. When the wood contracts, the connection is no longer secure, causing the drawer or object to come apart. Additionally, veneered, gilded, or painted furniture is also susceptible to cracks, delamination, and flaking from inappropriate RH, since these objects are composed of various layers of different materials.
Managing Dryness
So what can one do about winter dryness? The overall main goal is to minimize the impact of the dry winter months. To increase the RH, you can humidify the air or heat the air less. Furthermore, it is important not to allow the object to reach the state where it is permanently affected or deformed.
To determine what conditions would be appropriate for your particular institution, you can use IPI's Dew Point Calculator [1]. The Dew Point Calculator [1] allows you to predict how changes to the environment would affect your collections, by setting different Temperature, RH, and Dew Point values. In addition, plotting different scenarios on the Dew Point Calculator [1] allows you to consider and evaluate the impact of changes before they are implemented.
Unfortunately, winter dryness is just one of the commonly encountered preservation problems. Spring, summer, and fall bring their own preservation problems and challenges. Seasonal variations in outdoor conditions are the most significant source of variation in indoor climates. It is seasonal variations that produce the most damaging extremes and that pose the largest challenges to environmental management. Since having a long-term view is the ideal in preservation management, gathering and analyzing data throughout the year is very important.
Look for more information about seasonal preservation problems and how to cope with them in future issues of the Climate Notes newsletter.
Help is Here
Are you interested in starting a preservation management system in your institution? Do you have existing data, but you are not sure how to interpret it? Contact the Image Permanence Institute to discuss how we can help you.
Phone: 585-475-5199
Email: ipiwww@rit.edu [2]
In addition to the Dew Point Calculator [1], IPI has developed other tools that can help you with your preservation management. These tools include the PEM2 and PEMData [3].