Volume 17, Number 1, Jan 1995, p.23

CAL scientists revise guidelines for museum climate control

Reprinted with permission from The Torch, Dec. 1994, p.3. The Torch is published monthly by the Office of Public Affairs of the Smithsonian Institution for distribution to Smithsonian employees.

For decades, museums have kept their thermostats at a steady 21 degrees Celsius (70 degrees Fahrenheit ), with a relative humidity of 50 percent. Now, a team of Conservation Analytical Laboratory researchers has found that most museum objects can safely tolerate a wider range of both temperature and relative humidity.

In fact, according to the teams research, there can be as much as plus or minus 15 percent fluctuation in relative humidity and as much as 10C (50 F) difference in temperature. Within that range the scientists say, any object -- whether it's Leonardo daVinci's painting "Mona Lisa" or an installation of Jeff Koons' vacuum cleaners -- may be safely stored or placed on exhibit.

The researchers' insights could save museums, archives and libraries millions of dollars in construction and energy costs necessary to maintain rigid environmental controls.

The CAL researchers -- Marion Mecklenburg, Charles Tumosa, David Erhardt, and Mark McCormick-Goodhart -- reached their conclusions during a series of investigations of the chemical, physical, and mechanical properties of materials common to a wide variety of museum objects. The objects ranged from natural history specimens and archaeological artifacts, for example, to 19th century landscape paintings and photographic prints and film.

In the past year, the researchers have presented their work in a variety of papers and presentations for organizations such as the Materials Research Society, the American Chemical Society, and, most recently, at a meeting in Ottawa, Canada, of the International Institute for Conservation of Historic and Artistic Work.

"As scientists, we don't work from the idea that each object in a museum is unique," Mecklenburg says. "Rather, we start by looking at the whole picture -- examining and understanding all of the materials found in the vast majority of museum objects."

Through informal discussions of their work, the researchers say, came the understanding that materials such as wood, cellulose, various polymer coating, fibers, minerals, pigments and the like share an overlapping range of tolerance to temperature and relative humidity.

"Up to 50 percent of construction costs for new museums and archival storage facilities may go toward highly overbuilt heating and cooling systems," Mecklenburg says. "Our research shows that such specialized systems are unnecessary. Most museums can adequately protect their collections with commercially available technology, such as the heating and cooling systems used in grocery or retail stores."

Moreover, Mecklenburg says, specialized heating and cooling systems that keep temperature and humidity stable can be expensive to operate. Seasonal variations in outdoor temperature and relative humidity, particularly in temperate climates, he says, can mean monthly energy costs that soar to tens of thousands of dollars in order to maintain strict environmental controls.

For older or historic buildings, Mecklenburg adds, making use of conventional equipment avoids the structural damage that might result from installing more elaborate heating and cooling systems.

The materials research at CAL that has let to the new insights about temperature and relative humidity involves laboratory tests of the properties (physical, mechanical, and chemical) of materials commonly found in museums. The overall goal of the CAL researchers is to apply the best scientific knowledge about various materials to the treatment and conservation of cultural, historic, artistic, and scientific artifacts.

Chemist Tumosa has measured the effects of changes in relative humidity on acrylic paints. For example, he has cooled and dried samples of acrylic paint on canvas to document responses to lowered temperature and humidity (if temperature and humidity are too low, many paints and coatings become brittle and crack). Tumosa also considers changes on stretched canvas in response to changing temperature and humidity, which might cause paint to crack and fall off.

Other materials -- wood, photographic emulsions, paper -- are subjected to high humidity, or they undergo accelerated aging through exposure to many potentially damaging environmental factors, including heat, humidity, light and various pollutants.

For example, McCormick-Goodhart has tested the effects of temperature and relative humidity on photographic prints and film, especially motion picture film. Results show that temperatures below freezing provide the best storage for maintaining the film (particularly color film) and that commercially available freezers are adequate, despite fluctuations in temperature that might occur with such off-the-shelf equipment. Precautions must be taken to guard film against high humidity, he says. For motion picture film, McCormick-Goodhart places each reel inside a zip-lock freezer bag, which is encased in a cardboard box.

In general, the CAL researchers say, for most materials the low end of the temperature / relative humidity range prevents biological damage from microbial growth and minimizes chemical reactions that occur naturally within objects over time. At slightly higher values for temperature and relative humidity, they say, physical damage is minimized.

"This work is capable of defining the tolerance limits for temperature and relative humidity of large classes of materials represented in museum collections," McCormick-Goodhart says. "It means we don't have to study every single object. That's the breakthrough."