Scientific Examination of Art Modern Techniques in Conservation and Analysis (2005) / Chapter Skim
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Biodeterioration of Stone--Thomas D. Perry IV, Christopher J. McNamara, and Ralph Mitchell
Biodeterioration of Stone--Thomas D. Perry IV, Christopher J. McNamara, and Ralph Mitchell
Pages 72-84
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From page 72... ...
McNamara, and Ralph Mitchell Division of Engineering and Applied Sciences Laboratory of Applied Microbiology Harvard University Cambridge, Massachusetts ABSTRACT Stone cultural heritage materials are at risk of biodeterioration caused by diverse populations of microorganisms living in biofilms. The microbial metabolites of these biofilms are responsible for the deterioration of the underlying substratum and may lead to physical weakening and discolora tion of stone.
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From page 73... ...
. Biodeterioration presents conservation challenges that vary as widely as the types of materials themselves, and discussion of the processes causing degradation of these varied historic materials is not possible here.
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Biofilm formation begins with the initial adhesion of microorganisms to a surface. Division of attached cells produces microcolonies containing large amounts of exopolymer separated by patchy areas relatively devoid of growth.
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For example, oxalic acid may have a protective role by the formation of calcium oxalate on stone surfaces (Di Bonaventura et al., 1999)
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Interestingly, in a growth medium of low salt concentration, E nigrum appeared milky white in color, while in a high calcium concentration medium, the fungus produced a red pigment (see Figure 5b)
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The contribution of microorganisms to gypsum formation is unknown. Gypsum crystals combine with dust, aerosols, and other atmospheric particles to form black or brown sulfated crusts, which can tarnish the monument's aesthetic appearance.
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From page 78... ...
In addition to phylogenetic descriptions, the deterioration of stone has been examined using a variety of techniques, which have some utility in the quantification of biodeterioration. These methods include depth measurements using calipers, use of reference surfaces, macro-stereophotogrammetry, ion measurements of water runoff, or acid extraction (Winkler, 1986)
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Marble blocks were exposed to 1 mM sulfuric acid, which is typically associated with acid rain, and the effect was assessed by MicroCT according to McNamara et al.
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Active metabolism, however, requires appropriate levels of relative humidity and temperature. A combination of low humidity and low temperature is the simplest way to control microbial growth, but this treatment may be less effective for control of fungi (Gu et al., 1998b)
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Biocides are a difficult tool for preservation, because many are too caustic for environmental use, they are not strong enough to discourage microbial growth, or the microorganisms ultimately develop resistance. Consolidants Consolidants have been used for some time to conserve archaeological stone from biological and chemical weathering (Selwitz, 1992)
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1997. In Aspects of Stone Weathering, Decay, and Conservation: Stone Weathering and Atmo spheric Pollution Network Conference (SWAPNET)
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1995. In Methods of Evaluating Products for the Conservation of Porous Building Materials in Monuments, pp.
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From page 84... ...
925-941. Los Angeles: Getty Conservation Institute.
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