Understanding Temperature and Humidity for Books

A range of environmental factors can harm collections, such as radiation causing photochemical damage, vibrations, particulate or gaseous pollution, and insect pests. Apart from these, a major harmful environmental factor that all collections experience, independent of the circumstances of location, building or enclosure, is the variation of the ambient temperature and relative humidity (RH). Inappropriate levels and fluctuation of these two factors can cause mechanical (physical) damage as an object’s moisture content equilibrates rapidly to changes in air temperature and RH, causing swelling and shrinkage. These dimensional changes accelerate deterioration and lead to such visible physical damage as cockling paper, flaking ink, warped covers on books, warped text blocks, warped wood, brittleness, tearing and cracking of materials, etc.

The area of research—safe limits for RH—has received considerable attention in the fine and decorative arts, but not for the complex and diverse mechanical structures of bound volumes.  The surrounding air ultimately governs how much moisture is absorbed into, or desorbed from, collection materials. Because equilibration relies on diffusion of moisture throughout the body of the collection objects, the rate of moisture equilibration varies considerably, but is generally much slower than temperature equilibration. The time period required before the entire object has reached equilibrium (in terms of water content) can range from an instant to months depending on the ability of water to diffuse into the object. Composite objects (which are comprised of more than one material bonded tightly with another) are more vulnerable to environmental changes in RH and temperature because the different materials that make up the object respond differently to environmental conditions swelling a different amount and/or over a different time scale. This is known as differential expansion. As one material expands it can exert force on another to which it is adhered or attached. In this situation the risk of mechanical damage is often thought to be particularly significant for rare books.

To overcome the difficulty of studying the mechanical behavior of complex book structures, IPI employed time-lapse imaging to gain insight into the subtle changes books undergo when RH and temperature fluctuate. This initial research has been concerned with the book alone in a situation where it will be most exposed to the changing environment, not shelved and not buffered by any enclosure. This method has illustrated that for bound volumes, the cover material absorbs and desorbs water in seconds leading to rapid dimensional change over the slowly reacting board material. This change effectively follows the RH profile instantaneously and leads to behavior as illustrated in the following video.

This observed behavior exists independently of how quickly the RH change occurs. In the next video we see the same RH change for many different types of books taking place over 25 hours, 250 hours (about 10 days), and 2500 hrs (about 104 days). Using digital image manipulation software, the behavior of the books show very little variation despite the difference in the duration of the environmental RH change (shown in black and white below). Text blocks seem largely stationary apart from the pull and push from the movement of the vellum binding and board.

An added complexity that time lapse imaging sometimes observes is the interaction of the slowly changing boards of books with that of the surrounding cover material. This differential expansion of the board and cover ‘envelope’ in time and space can have significant effects.

As an example, the following video shows two books equilibrating to around 86% from 55%. After the initial relaxation of the vellum cover, both boards fall closer to the text block. After this the board of the book on the right swells and moves higher while that on the left remains stationary.

For the same books, once a near equilibrium is reached for the board at 87%, a rapid drop to 60% and return to 87% results in a rapid tightening of the cover and a jerking up of the boards and then a release back. This response can be seen in the next video.

Another aspect to better understand the behavior of the book in reaction to environmental conditions, is the hygrometric half-life. This is defined as the time required for the equilibration of the object to change by half of the difference between the initial RH and the new RH of the new environment. An important thing to note is that the inside of an enclosure such as a phase box, case or cabinet will also have a half-life which can slow the pace of the equilibration of a book contained inside. For the core of a book, previous experiments have illustrated that books possess a hygrometric half-life of around one day or more. The time it takes to equilibrate increases with the square of the thickness of the text block and also the size of the book (i.e. Quarto, Folio, Octavo etc.).  

The next video shows a large text block changing shape as it desorbs water. To make it easier to see the changes the book is marked with grid lines and analyzed with digital image manipulation software created at IPI.  We slowly see the altering of the shape over ten days as it equilibrates from 87% RH to 55% under a constant temperature.

The figure below illustrates this further (from research conducted by Dr. Eugene Salesin of IPI). The plot shows the results of our research on the percent weight gain of four leather and vellum bound 17th Century books as they equilibrate to a new RH condition (from 22% to 77% RH) over 23 days. For these smaller objects the time it takes to reach about 50% of the final percent gains (the top of the curves) is around 1-2 days.

From this knowledge we can conduct a quantitative analysis. For illustrative purposes it is possible to simulate the equilibration state of books (in this case assuming a constant human comfort temperature for simplicity) using simulation software created for this purpose by IPI. This was done using real data collected over many years with a PEM2® datalogger. The following plots illustrate (in red) what the core of the books experience. The half-life of the object varies from 1 day, 6 days, and 11 days (indicative of the book size and how much it is buffered from the changes, for example by an enclosure).

The objects equilibration (shown in red) ‘bounces along’ constantly pulled and shifted by the environmental conditions outside the object, and the book rarely reaches full equilibrium. Even with a half-life of one day as shown in the first graph, the core of the object does not equilibrate to the peaks and troughs that the skin of the object experiences. The larger the half-life (possibly aided by an enclosure) the less of the extremes it experiences. As the half-life increases, the extremes of seasonal high and lows that the inside of the book experiences reduce. As we discussed earlier, the surface equilibration is so rapid that it can be considered to follow that of the environmental RH profile.

These videos and models help provide a limited but intuitive understanding of the movements of the ‘skin’ and ‘core’ of books in changing RH conditions. This information proves valuable in two ways: 1) for preservation, to help avoid damage; and 2) for sustainable and efficient operation of HVAC systems.