May 22, 2000 Chemistry study shows how to protect, when to clean fine art
Old artwork never dies, it simply fades to black. That's because soot - tiny pieces of black carbon emitted from trucks burning diesel fuel or factories burning coal - accumulates on paintings, causing the image to darken over time.
How much of the airborne particles it takes to show on artwork is the subject of research reported in the current (May 15) print edition of Environmental Science and Technology, published by the American Chemical Society, the world's largest scientific society.
The research can help control damage to priceless works of art, frescoes and family heirlooms. Once it sticks to artwork, soot is difficult and sometimes impossible to remove, according to Leon Bellan, lead author of the study. Bellan, a high school student at the time, was named a national finalist in the prestigious Intel science competition for his work.
The findings can be used to determine how often to clean some works of art and how to protect others, said Glenn Cass, a professor at the Georgia Institute of Technology, who oversaw Bellan's research.
"This research gives museum designers the information needed to protect a collection from changing over the years," Cass said.
Using a laser printer that randomly deposited microscopic dots on eight different colored samples, the researchers developed two tests to compare how people detected differences in appearance. They found that more than 12 percent of a sample must be covered with dots before people could accurately see the difference between clean and soiled sheets separated from one another. When the sheets were placed side by side, differences between "clean" and "dirty" samples were easier to discern: people detected a difference when less than four percent of a sheet was covered with dots simulating soot.
Soot is made up of microscopic particles typically too small for most museums' air filters. There are currently no U.S. Environmental Protection Agency standards for indoor airborne soot deposits.
Soot deposition rates ranged from .08 to 2.7 micrograms per meter of artwork per day, according to previous measurements. The rates were based on conditions in Southern California museums, but are applicable to a modern building anywhere in the world, Cass said.
At the rates found, it would take between five and 300 years for a painting to reach the visual threshold of darkening. Higher rates would be expected in Europe, where more diesel fuel adds additional soot to the environment, and during the winter, when fuel use goes up, he added.
"The present findings may mean that management of soiling problems is more difficult than previously expected," Cass said. "The rate of change on the appearance of the objects in a museum will be so gradual as to possibly escape the notice of people who see the collection on a daily basis."
Cass worked at the California Institute of Technology at the time of the study. Bellan is now a student at the same university.
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