The Abbey Newsletter

Volume 18, Number 8
Dec 1994


Bugs, Mold & Rot II (A Workshop on Control of Humidity for Health, Artifacts, and Buildings), Nov. 16-17, 1993. Edited by William B. Rose and Anton TenWolde. National Institute of Building Sciences, Washington, DC, 1993. 152 pp. $35 from NIBS, 1202 L St., NW Suite 400, Washington, DC 20005 (202/289-780, fax 289-1092).

Reviewed by Ellen McCrady

This workshop was sponsored by the Building Environment and Thermal Envelope Council (BETEC, part of NIBS) and Oak Ridge National Laboratory. BETEC is one of five Councils of NIBS, a congressionally chartered non-governmental organization.

Stefan Michalski has a paper on "Relative Humidity in Museums, Galleries, and Archives: Specification and Control," but none of the other names is familiar in the conservation or preservation world. The papers are nevertheless relevant to that world, and some provide crucial, hard-to-find information. Almost all of them are technical and fact-filled.

Two questions provided the motivation for this conference:

"What is the ideal relative humidity for the house envelope, for the inside space, for the contents, for people, and for other things that live in a house?" and

"What is the best strategy for controlling humidity?"

The papers that are particularly valuable and useful are:

The names and addresses of all attendees and speakers are in the back of the book. (2C1.3)

"Relative Humidity and Temperature Guidelines: What's Happening?" by Stefan Michalski. CCI Newsletter No. 14, Sept. 1994, p. 6-10.

Reviewed by Ellen McCrady

Author Michalski says that CCI scientists have shifted from a "single, simplistic standard" to more flexible recommendations that are more specific to types of materials. He may be overstating the rigidity of earlier CCI recommendations on relative humidity, however. A 1975 CCI Technical Bulletin by K.J. Macleod, "Relative Humidity: Its Importance, Measurement and Control in Museums," does not recommend one simplistic standard for all types of artifacts, climates and buildings, but instead recommends a range, say from 45% to 65% or 35% to 55% RH, within which the museum climate may vary from season to season, if it must. Daily variations should be much smaller than the seasonal variations, which should also be as small as possible. Ultra-sensitive articles, it says, can be given special housing to protect them.

If the 1975 Technical Bulletin is any indication, past CCI recommendations have not been rigid, though CCI scientists, like other advisors of practitioners, may have occasionally been obliged to simplify their message in order to get their point across. What is different now, if Michalski can speak for the other scientists, is that they are being more specific. They recommend different museum environments for different types of materials and identify the different types of damage done to these materials by different temperature and relative humidity stresses.

On the other hand, they also open up the recommended range of permitted variation to ± 25%, which is even wider than the controversial limits announced by the Conservation Analytical Laboratory (CAL) of the Smithsonian in August.

Table 1 gives an overview: organic and inorganic materials of all sorts are susceptible to mold above 75% RH; acidic paper, acetate films and color photographs can be preserved longer at lower RHs, all the way down the temperature scale. (Oddly, the life expectancy of other materials, including non-acidic paper and textiles, parchment, and stable black and white photographs, is not represented as being affected by either high or low RH, aside from mold.) Flexible-but-restrained (e.g., framed or pasted) organic materials are said to be only slightly affected by fluctuating RH up to ± 20%.

Temperature that is too low is said here to embrittle all organic materials. Nothing is said about the temporary nature of the effect. Temperature fluctuations are said to have virtually the same effect as humidity fluctuations.

There is a problem with this article. Although it may be right on the mark when it comes to paintings and objects, it appears to have been written without adequate knowledge of research on paper and photographic materials, or of the experience of curators and conservators.

Fluctuating relative humidity ages paper rapidly, according to the Library of Congress research. Although book paper is usually protected from this effect by the book covers, there is reason to suspect that fluctuating RH is responsible for some of the yellowing and weakening of page margins. Accelerated aging research at the Image Permanence Institute has shown that the life of film can be extended from four to ten times by incubation at 20% rather than at 50% RH. This means that each 10% reduction of the RH will at least double the life of the film, and may quadruple it. (Michalski's article says, however, that it takes a decrease of 20% RH to double the life of acetate film.) The ANSI standards for film storage say that fluctuations in temperature and relative humidity must be minimized.

Anyone who makes recommendations (like those in this article) that are different from those currently accepted in preservation of paper and film needs to give readers an explanation of why the old recommendations and the research they are based on are no longer valid. They need to describe, explicitly and in detail, the research the new recommendations are based on, and make clear the connection between the research and the recommendations.

The best way to make these basic changes in environmental guidelines is not by simply publishing conclusions. Scientists need to get together with other researchers and practitioners, repeatedly if necessary, review all relevant background information, make direct observations, reach a consensus, and then publish the conclusions. In effect, this is what is done in standards committees and to a lesser extent in focus groups.

In this and all other applied fields, information has to flow both ways, as it did in the 1980s on the matter of whether light bleaching degraded paper. In that case, the debate went on for several years, and was finally settled in favor of the practitioners.

Science has been enriched by interaction with technology since the beginning. Daniel Kevles, in the third chapter of The Physicists (1979), says thermodynamics owed much to the steam engine. Other examples of the interaction between science and technology can be found in the history and philosophy of science, despite the popular belief that knowledge is generated in science and that engineers and others in "technology" only apply it.

The author's originality is refreshing, and he is fun to read, even if it is hard to take seriously everything he says about paper and film. Here is part of his argument for low humidity:

"On the scale of an entire building, low RH requires far less energy or building modification, and people can work in a building with low humidity more easily than in a building with low temperature. Also, Canadian libraries and archives can achieve mass desiccation (as compared to mass deacidification) for free during the winter by using heating systems with no humidifiers."

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