Microbial Deterioration of Clay Roofing Tiles as a Function of the Firing Temperature Jonjaua Ranogajec w and Sinisa Markov Faculty of Technology, University of Novi Sad, 21000 Novi Sad, Serbia Jelena Kiurski and Miroslava Radeka Faculty of Technical Sciences, University of Novi Sad, 21000 Novi Sad, Serbia Vilma Ducman Slovenian National Building and Civil Engineering Institute, 1.000 Ljubljana, Slovenia This study illustrates the genesis and growth of the biofilm, known as patina, on the surface of clay roofing tiles after the action of a single fungus species, Aspergillus niger, (90 days) as the main cause of their aging processes. The nanocrystalline products, formed due to the fungus hyphen penetration into the ceramic support, contributed to the formation of an abundant reactive surface area of the existing and newly formed pores, intensifying the aging processes of the clay roofing tiles. The destruction of the ceramic supports, laboratory samples (9201, 9951, and 10551C), and industrially obtained one (11001C) was studied as a direct function of their microstructural and pore characteristics. A relationship among the textural characteris- tics of the studied ceramic segments of the defined microstruc- ture, the tile surface roughness values, and the formation of the biofilm, which was developed after the action of the fungus, was set up. I. Introduction A NY porous surface with an alkalinity below pH 8 is capable of supporting biological life—algae first, and later lichens and mosses. Lichens can look very attractive on clay roofing tiles or stone—but if they are unevenly distributed to variations in the surface or in the way that water is allowed to affect it, they can be an eyesore. 1 Clay roofing tiles support large and diverse communities of microorganisms that colonize both the tile surface and the po- rous interior. Their weathering as a result of organic acid exu- dation by lichens, fungi, and bacteria is recognized as a biogeochemical process. As the growth of microorganisms is affected in different ways by various environmental factors, it is difficult to define the right mechanism of biocorrosion of the defined system. The kinetics of the biocorrosion processes of these materials can be influenced by the biofilm, known as pa- tina, which presents a tile surface-associated microbial growth. The products of the metabolic activities of microorganisms, in- cluding enzymes, exopolymers, organic and inorganic acids, as well as volatile compounds such as ammonia or hydrogen sul- fide, can affect cathodic and/or anodic reactions, thus altering the electrochemistry at the biofilm/metal interface. This phe- nomenon is often referred to as ‘‘biocorrosion’’ or ‘‘microbial- influenced corrosion.’’ However, biological alteration phenomena of clay roofing tiles can occur only if the environmental conditions are favor- able. Moreover, the characteristics of these materials, particu- larly their porosity or hygroscopicity, determine the possibility of the absorption of water by the substrate. The response of clay roofing tiles to temperature changes, moisture, and/or relative humidity cycling is the mechanical breakdown that occurs due to the expansion and contraction of the tiles. This cycling is predominantly diurnal, but seasonal variations, for example, re- lated to freeze/thaw incidence, can be significant and there is some evidence that rapid surface changes in temperature occur in response to fluctuations in cloud cover, wind speed, and rain fall on heated surfaces. 1 The biodeterioration and biomineralization actions of the fungus species Aspergillus niger as the main factors of the pro- cess of aging of clay roofing tiles are illustrated in this paper through the patina formation. The nanocrystalline products, formed in this way, contributed to the abundant reactive surface area of the existing and newly formed pores (due to the fungus hyphen penetration into the ceramic support) intensifying the aging processes of the clay roofing tiles. The destruction of the ceramic supports (laboratory samples treated at 9201, 9951, and 10551C and an industrially obtained product at 11001C) was studied as a direct function of their microstructural and textural characteristics as well as the surface roughness and surface po- rosity values of the tiles. The choice of the fungus, as a model microorganism, was made based on the fact that it is a common fungus on building materials that excretes oxalic acid as a me- tabolite 2 in a short period. Our lichenological analysis of the aged clay roofing tiles collected in the north-east area of Serbia, region of Novi Becej, 3 identified the mixture of alga and lichens on the tile top as biocorrosion agents. II. Experimental Procedure (1) Clay Roofing Tiles Plain clay roofing tiles were prepared from the raw material based on kaolinite–ilite clay, quartz, and feldspath. The com- position of raw minerals was free of carbonates (Table I). The shaping, extrusion (20–22 wt% water, pressure 0.16 MPa) and drying (chamber dryer, i 5 36 h/T max 5 801C), processes were carried out under industrial conditions, while the firing process was carried out under laboratory and industrial conditions: W. Lee—contributing editor Presented at the 10th International Conference and Exhibition of the European Ceramic Society, Berlin, June 17–21, 2007 (Silicates and Traditional Ceramics, Paper No. G-839). w Author to whom correspondence should be addressed. e-mail: janjar@uns.ns.ac.yu Manuscript No. 24231. Received January 17, 2008; approved August 4, 2008. J ournal J. Am. Ceram. Soc., 91 [11] 3726–3731 (2008) DOI: 10.1111/j.1551-2916.2008.02691.x r 2008 The American Ceramic Society 3726