BIODETERIORATION OF NATURAL AND ARTIFICIAL STONE
Stone is one of the most intenstly studied materials in conservation. Understanding the deterioration of stone requires knowledge of various mineralogical and physical characteristics and of stone wealthering response in different climates and environment. The alteration and weathering of stone is affected by natural and artifical elements, physical, chemical or biological damaging factors. Biodeterioration of stone is coupled with environmental factors that induce the decomposition of stone structure, either directly or indirectly as a form of catalysis. Many elements contribute to the deterioration of stone monuments and others objects of cultural value such as pogudas and status of Buddha. This report concentrates on action of biodeterioration factors including bacteria, algae and higher plants.
Biodeterioration, defined as damage caused by living organisms such as fungi and termites, endangers cultural property all around the world. The destruction results in enormous economic loss” (Khadelwa, 2003) and the irreversible cultural and artistic loss of the information of the objects affected. The loss is greatest among materials derived from plants and animals, since the food value of the cellulose or protein that comprise these materials make them particularly vulnerable to biological attack (Allsop et al, 2004). As a result wooden objects and structures, bamboo artifacts, manuscripts, books, textiles leather, paper, basketry and various types of paintings are most at risk. Stone and mansonry are also vulnerable however, and stone momuments and outdoor sculptures may become severally damaged. Cultural property is even more endangered. While kept under adverse environmental condition that invite biodeterioration, typically hot and humid surroundings.
The importance of microbial activity in the alteration and deterioration of stone and concrete walls has been frequently neglected. However, active biofilms are found anywhere where there are micro organisms and humidity and a complex biofilm system can develop in external walls of buildings. The microbiota on building stones represents a complex ecosystem which develops in various ways. Depending on environmental conditions and the physiocochemical properties of the material in question. Some investigations have begun to elucidate the essential role biological agents play in the deterioration of stone [Bock and Sand, (1993), Warscheid and Krumbein (1996)], and it is clear that many physical, chemical and biological factors act in both synergistic and antagonistic association to affect the durability of the material [Koestler et al, (1994), Vatention (1993)].
The colonization of external surfaces of building by unacceptable appearance of staining of the stone surface by biogenic pigments [Urzi et al (1992)] and the production of extracellular polymeric substances (EPS) that cause mechanical stresses to the mineral structure due to shrinking and swelling cycles of these collecdal biogenic stones inside the pure system [Dormeden et al, (2000), Warscheid (1996)]. This can lead to the alteration of pore size and distribution, together with changes in moisture, circulation pattern and temperature response [Warscheid (1996), Krumbern (1988)].
Microorganisms may also alter the water permeability of the minerals by the deposition of surfactants [Gaylardie and Morton (1999)]. Last but not the least, it has been shown that the early presence of biofilms, on exposed stone surfaces accelerates the accumulation of atmospheric pollutants [Witheburg (1994)]. Thus microbial contamination acts as a precursor of the formation of detrimental crest on rock surfaces caused by acidolyptic and oxidoreductive (bio) erosion of the mineral structure [Blaschke, (1987), Krumbern and Peterson, (1987)].
Organisms present on stone monuments can include photolithoantotrophs, such as algae, cynobacteria, moses and higher plants. Also present are chemolithoantotropic bacteria that can release acid such as nitros acid (e.g. Nitrosomonas spp.) nitric acid (e.g. Nitrobacter spp). Or sulphuric acid (e.g. Acids thiobacillus spp) thus changing the local pH [Bock (1986), Bock et al (1989)] and chemoorganotrophic bacteria and fungi, that may release chelating organic compounds [Ascaso et al (1990), Palmer (1994)] or weaken the mineral lattice by the oxidation of metal cations such as Fe2+ or Mn2+ [Vourmen et al (1981), Eckhardt (1980)].
Literature data show that the phototrophs dwelling on stone monuments are represented by a rather large number of genera and species in microbial consortia [Pantazidon and Theoulakis (1997)]. Although chemolithotrophic micro organisms have often been described in association with damaged inorganic materials [Bock and Sand (1993), Milde et al (1983)] and were first suggested to play a role in stone deterioration in the 19th century [Muntz, (1890)]. More recent studies have emphasized the significance of chemoorganotrophic bacteria and fungi together with photoantotrophs [Eckhardt (1988), Gorbushma et al (1993)]. Even in the absence of a primary colorizing film of phototrophs, heterotrophic bacteria and fungi can grow on painted surfaces using organic compounds from the paint as substrates and produce acids that cause degradation of the paint and the underlying surface [Shirakawa et al (2002)]. Because of their ability to survive repeated drying and rehydration cycles [Whitton (1992)] and high UV levels [Chazal and Smith (1994), Matsnnaga et al (1993)], the cyanobacteria are particularly important on exposed surfaces.