Photonics Applied to Fresco Preservation
In Roman catacombs, tributes to the dead attract the living. Some visitors are tourists who come to see the frescoes that adorn the walls.
Others, however, don't depart with the guides: Bacteria, algae and other microorganisms stay to dine on the minerals in the paintings, causing irreversible damage to these antiquities.
Organisms in the biofilms formed by photosynthetic bacteria aggressively attack the frescoes on the walls of ancient catacombs.
According to Patrizia B. Albertano, a biology professor at the University of Rome "Tor Vergata," light has been part of the problem. Now, light and photonic technologies are part of the solution used to stop the munching microbes in a study funded by the European Union.
The issue involves colonization on a miniature scale that began with the introduction of artificial light to enable visitors to see. "We have the development of microbial communities that are characterized by the presence of photosynthetic microorganisms, mainly cyanobacteria," Albertano explained. The irradiance in the catacombs may be three to four orders of magnitude lower than that on the surface, but the extremely efficient photosynthetic pigments of the cyanobacteria nevertheless enable them to thrive.
The cyanobacteria form a biofilm --a slime that can contain many species of microorganisms, including those that destroy the frescoes -- when they attach to a surface, which makes their removal difficult. In the case of a biofilm sitting atop delicate artwork, eradication is even more challenging. It would be best to address the growth before it starts.
One method is to reduce the light level, but that may not be enough. Another is to figure out what sort of light a biofilm requires to thrive and to remove that completely. This approach was taken by Albertano and her colleagues in the Cats (cyanobacteria attack rocks) consortium, in work that began in 2001 and that ran until the end of 2003.
Using a commercial portable spectroradiometer, researchers determined that the dominant photosynthetic bacteria in the Catacombs of St. Callistus do not absorb light well in the blue-green region. After two years under 490-nm monochromatic illumination, a previously uncolonized section of the fresco shows no evidence of biofilm growth. Images courtesy of Patrizia B. Albertano.
The focus of their study was on light -- specifically, the illumination from the lamps and the spectra from the biofilms. Albertano, who directed the project, said the latter was used, among other things, to monitor biofilm growth. The emission proved to be a good indicator, she said, because the amount decreased with the thickness of the biofilms as the colonies absorbed more and more light.
The researchers employed a GER 1500 portable spectroradiometer from GER Corp. of Millbrook, N.Y. Covering wavelengths from 350 to 1050 nm, the device has 512 channels, each with a width of 1.5 nm. The instrument measures reflectance as a function of wavelength. For photosynthetic organisms, the light reflected is the light they are not using.
Analyzing their results, the researchers concluded that it is nearly impossible to control microorganism growth under white light; cyanobacteria are just too efficient. However, they did notice a small window between the blue and green regions of the spectrum in which there was little absorption.
Beginning in 2003, therefore, they switched off the white lights in one area of the Catacombs of St. Callistus and turned on monochromatic lamps. Popular in nightclubs, the commercially available lights have emission peaks around 490 nm. Because the bacteria naturally grow slowly, the researchers continued to monitor the effectiveness of the change even after their funding ran out. After nearly two years, they found that the uncolonized part of the paintings showed no evidence of biofilm growth.
As for the future, using this technique at other sites will require work. For one, the illumination wavelength must be changed based on the dominant biocolonizer in that location. Measurements of both the biofilms and the candidate lamps, therefore, must be made. For that, Albertano said, it would be nice to have a more sensitive instrument and better analysis algorithms for the collected data.
Even so, photonics may have provided a promising tool in the battle against the decay of these archaeological treasures.
"Compared with the situation that we usually see in these environments, the results, in my opinion, are quite good," Albertano said of the experiment.
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