Charlie, I just returned minutes ago from consulting on this very subject for a group of 6 museums in historic buildings. A lot of research has been done recently on the effects of environmental fluctuations or changes on historic objects and materials. While it is a complex subject, several general rules apply, subject to specific exceptions. 1) Inorganic objects are less sensitive to environmental conditions than organic objects. Therefore, control is based upon the more sensitive organic objects which have the most restrictive needs. 2) Humans are sensitive to temperature. We can sense the difference between 65 degrees and 70 degrees. However, we are not sensitive to RH variations, and can't tell the difference between 45% RH and 50% RH. Organic objects are sensitive to RH, and can "sense" the difference between relatively small RH changes. They are fairly insensitive to temperature differences. Thus, needs of objects are almost always at conflict with needs of people in historic buildings. 3) For collections preservation, RH should be controlled as a priority if only one environmental variable (T and RH) can be safely controlled. For historic structures, a range of 40%-60% RH is adequate. Tighter control than this generally is not reasonable or possible. For this reason, humidistatically-controlled heating (narrow RH range) is often used in historic structures that contain collections, rather than controlling the heat thermostatically (narrow T range). 4) Humidification of historic buildings generally is not recommended. Doing so in the winter can cause condensation in the walls and other areas that can deteriorate the building fabric. Thus, it can be difficult, if not impossible, to provide both steady RH and steady T during the heating season. 5) A number of organizations/individuals have attempted to develop deterioration calculations that will indicate the overall effect of the various factors affecting preservation of collections. With respect to temperature (temperature is energy, which accelerates deterioration reactions), the best current metric indicates that for every 5 degrees temperature DROP, the life of the object is DOUBLED. Thus, at 65 degrees, the life is doubled over 70 degrees. At 60 degrees, it is doubled over 65 degrees or 4 times longer than at 70 degrees, and so forth. 6) For historic buildings, forget the standard of 70 degrees and 50% RH. That was developed for purpose-built museums that had adequate insulation, vapor barriers and other construction features that allowed maintenance of these conditions. It also was heavily weighted toward human comfort. If you are concerned with object preservation, significantly better preservation would result with conditions of 40 degrees and 50% RH. If you need to accommodate human needs, you will have to accept that sacrifices to preservation need to be made by raising the T to human comfort levels, but no higher than absolutely necessary. Sorry for the primer, but to answer your specific question, if you lower the T to 68 or 66 or even lower, as long as you maintain 40%-60% RH, you will NOT hurt your collections, but you will actually enhance their preservation. Marc American Conservation Consortium, Ltd. 4 Rockville Road Broad Brook, CT 06016 www.conservator.com <http://www.conservator.com/> 860-386-6058 *Collections Preservation Consultation *Conservation Assessments & Surveys *Environmental Monitoring & Low-Tech Control *Moisture Management Solutions *Collections in Historic Structures *Collections Care Grant Preparation *Conservation Treatment of: Furniture Painted Wood Horse-Drawn Vehicles Architectural Interiors Marc A. Williams, President MS in Art Conservation, Winterthur Museum Program Former Chief Wooden Object Conservator, Smithsonian Institution Fellow, American Institute for Conservation (AIC) > -----Original Message----- > From: Museum discussion list [mailto:[log in to unmask]]On > Behalf Of Charlie Knight > Sent: Thursday, December 11, 2008 10:18 AM > To: [log in to unmask] > Subject: [MUSEUM-L] Temperature > > > I have a question regarding keeping a constant temperature/RH in > exhibit areas. Our main museum is housed in a mid-19th century > Court House (2 floors) and currently we keep the temperature > between 68-72, usually right at 70, although some galleries run > much higher temps than others. However, we have been instructed > by our Dept Head (we are a City bureau) to reduce our energy > consumption to cut our costs - obviously lights can be turned off > when we're not open, but the temperature controls must remain on > for the welfare of the artifacts on exhibit. So my question is > would there be any harm in lowering our temperature by two (2) > degrees, for a target range of 66-70? > > Charlie Knight > MacArthur Memorial > > ========================================================= > Important Subscriber Information: > > The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . 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