Preventive conservation is becoming an increasingly important part of our work as conservators, but it often seems that many important questions about environmental control have yet to be answered. Questions such as to what degree are fluctuations of temperature and RH humidity damaging to collections, and are they more or less damaging than strictly maintained but not ideal conditions?
Jean-Louis Bigourdan addressed some of these uncertainties in his talk on temperature and moisture equilibration in storage spaces containing significant quantities of hygroscopic materials. He focused on reconciling the need for climate-controlled storage with the quest for sustainability and the pressure of budgetary limitations. His introduction was reassuring: the current thinking on storage climate is that relatively stable low temperatures are desirable (“cool storage”), but there is little benefit to maintaining a perfectly stable climate (i.e. without fluctuations). Rather, a certain degree of cycling is acceptable, so long as the shifts are not extreme.
Following from this fact, Jean-Louis presented the concept of “dynamic management” of HVAC systems. Dynamic management entails shutting down the HVAC for short periods, such as overnight, and adjusting climate set points seasonally. This would save on energy, and thus reduce the environmental impact and cost of operating such systems. Of course, we as conservators are immediately concerned with the effect on collections materials during such shutdowns: How extreme are the fluctuations in temperature and RH resulting from periodic shutdowns of the HVAC?
This is the questions Jean-Louis attempted to answer through two phases of testing. He was particularly focused on the possibility that collections containing large quantities of cellulosic and/or hygroscopic materials might buffer against large or sudden shifts in temperature and RH. Jean-Louis undertook two phases of testing to understand the extent of the self-buffering capabilities of such materials. The first round of testing was conducted in the laboratory, and the second in library and archive collections storage rooms.
In his laboratory tests he exposed different types of materials to large fluctuations in temperature and RH. The materials included things like closed books, matted photographs and drawings in stacks, and stacks of unmatted photographs. He also tested the effects on these materials when they were placed inside cellulosic microenvironments, such as archive storage boxes, measuring the temperature and RH at the surface of objects, and at their cores. His results indicated that the RH at the core of books or stacks of cellulosic material does not change as rapidly as the exterior environment. Temperature equilibration occurred over a period of hours, and moisture equilibration occurred over the course of weeks or even months. Microenvironments increased the time to equilibration, mostly by controlling diffusion of air.
Another useful result of this laboratory experimentation was that it demonstrated that the moisture content of paper-based and film collections was more affected by environmental temperature than by environmental RH. In other words, at the same exterior RH, the moisture content of the collections object was lower at higher exterior temperatures. The laboratory testing therefore suggested that storage spaces with significant quantitates of hygroscopic materials will be buffered against large changes in RH and temperature due to moisture exchange with the collections materials.
Jean-Louis found that field testing in collections storage spaces returned many of the same results as his laboratory tests. 6-8 hour shutdowns of HVAC systems had little impact on environmental RH, and many of the systems they examined were already following seasonal climate cycles without causing dramatic shifts in the temperature or RH of storage environments. He encouraged conservators to take their collections materials into account when evaluating the buffering capacity of their storage environments.
I was very encouraged by these findings, although I have some remaining questions about the potential effects on collections materials. How much moisture is being exchanged with collections items in such a scenario? Is it enough to cause dimensional change in hygroscopic materials, especially on exterior surfaces, and will that contribute to more rapid deterioration in the long term? Regardless, I was happy to be prompted to remember that collections materials are an active part of the storage environment, not an unreactive occupant of it.
The talk wrapped up with Jean-Louis raising a few areas of further research. He hypothesized that changes in storage climate which are achieved through a series of small but sharp changes would result in slower moisture equilibration between environment and collections than would a change made on a continuous gradient. He also raised the possibility of predicting the internal moisture fluctuations of collections materials using their known hygroscopic half-lives. Both of these areas of research could be extremely helpful to conservators attempting any dynamic management of their climate control systems.
A particularly thoughtful question by an audience member provided the opportunity for more climate control wisdom. A Boston-area conservator of library and archive collections wondered whether it made sense to use dew point as the set point on HVAC systems in the winter to save money on heating costs, but during the summer to use RH as the set point to insure against mold growth. Jean-Louis felt this would be an unnecessarily complicated method of control, but offered a general rule for the storage of hygroscopic collections. He suggested thinking of lower temperatures as the primary goal, and of RH as important to maintain within a broader range. Lower temperature slow degradation reactions inherent to such materials, and so generally lower is better. However, RH need only be high enough so as not to embrittle material, but low enough to prevent mold growth. Essentially he suggested that if your RH and temperature are too high, you are better off reducing temperature slightly, which will slow degradation reactions, and as a side-effect your collections may absorb a small amount of moisture, thereby lowering the RH in the building environment.
Jean-Louis’s talk left me intrigued and excited about the possibility of taking advantage of hygroscopic collections materials to provide a more stable and sustainable storage environment.
Skip to content
AIC Blog Archives: Conservators Converse
Former Blog of the American Institute for Conservation