**6. Wood and paper**

Wood as a material has been widely used as a structural element for many types of constructions or for ritual, religious and decorative purpose. Historically, human usage of wood is embedded in wood cultural heritage reflecting past and present human life, culture, ideals, symbols and values. Wood cultural heritage objects can be classified as: moveable (musical instruments, frames, furniture, sculptures, iconic altar etc.), immovable (temples, churches, chapels, royal palaces, pagodas, wooden bridges etc.) and underwater (shipwrecks, foundation piles, wooden cargo or contents which were partially or totally underwater, periodically or continuously for at least 100 years), according to UNESCO [79]. Lignocellulose is the major component of wood biomass and consists of three types of polymers, cellulose (40– 55%), hemicelluloses (24–40%) and lignin (18–35%) that are strongly intermeshed and chemically bonded by non-covalent bonds and by covalent crosslinkages [19]. Organic nature and optimal water content make the wooden substrate suitable for microbial attack [80]. However, microbial deterioration of these materials occurs only under poor conservation conditions: high humidity level, soil contact, poor ventilation, and rare maintenance [81]. Even though deterioration of wood cultural heritage is a process conducted by all groups of microorganisms, fungi have the most significant potential to affect this type of historic artworks [82]. Biodegradation and biodeterioration of wood materials is predominantly dependent on its moisture content, requiring a minimum of 20% of water. Despite dry wooden objects are considered to be resistant to fungal degradation due to low moisture content, occasional wetting, leaks and flooding can increase humidity, enabling conditions for fungal growth. The mechanism of biodeterioration implies the development of fungi on the surface (**Figure 1G** and **H**) or between internal structures, the production of extracellular enzymes, the structural change of basic biopolymers, which ultimately results in visible changes of the object [81]. Generally, fungi that attack wooden material can be distinguished as white-rot, brown-rot and soft rot fungi. White rot fungi are the only organisms that can completely depolymerize and degrade all lignin components as well as cellulose and hemicellulose. The largest number, of about 1500 species, belongs to the Basidiomycota and a smaller number belongs to Ascomycota. Most commonly found species are from genera *Bjerkandera*, *Donkioporia*, *Fomes*, *Irpex*, *Phanerochaete*, *Pholiota*, *Pleurotus* and *Trametes*. Brown rot fungi decompose cellulose and hemicellulose while lignin degradation is limited to the process of demethylation of methoxyl groups, partial oxidation and depolymerization in a non-enzymatic catalytic cycle of the Fenton type where the free radical reaction is initiated by hydroxyl radicals (OH•). Only 6% of the total number of species that have been confirmed to be able to decompose wood mass belong to this group, and almost all representatives inhabit coniferous wood (species from the genera *Antrodia*, *Aspergillus*, *Coniophora*, *Coriolellus*, *Fusarium*, *Gloeophyllum*, *Merulis*, *Paxillus*, *Poria*, *Postia*, *Serpula*, etc.). Soft rot fungi decompose cellulose and hemicellulose, while the process of lignin modification is limited to demethylation. It is typical for this group of fungi to attack wood mass with high levels of humidity and low lignin content, forming a microscopic cavities inside the wood, sometimes leading to discoloration and occurrence of cracking pattern similar to brown rot (species from the genera *Alternaria, Chaetomium, Daldinia*, *Humicola*, *Stemphylium*, *Xylaria,* etc.) [79, 82, 83]. Soft-rot decay has been described

frequently from construction timbers, ancient Egyptian wooden coffins, wooden structures of Buddhist temples, waterlogged archeological wooden material, which can be related to more tolerant growth conditions. In comparison to soft-rot species, other two groups of wood decay fungi have a relatively narrow spectrum of growth conditions (preferring moisture content between 35% and 50%) but were documented in wooden churches, historic timbers and various historic buildings [79]. Objects of wood cultural heritage in the outdoors have been more disposed to deterioration process than those in the indoor as they are exposed to the relative humidity of 70% or higher, which is stimulatory for fungal growth. Additionally, the physical contact with moisture-absorbing surface can also provoke fungal decay [79]. On the other hand, fungal degradation of waterlogged and buried wood is much slower than for that found in dry environment, but once excavated decay can occur rapidly [82]. Finally, it should also be emphasized that in some cases fungal decay can be observed in extreme conditions such as in wood with 17.4% of moisture content [79].

Additionally, paper, which is mostly produced by mechanical and chemical processing of cellulose fibers, originating from wood, is the most important material on which cultural achievements in the whole world are recorded and preserved. Since it is created as a product of the wood industry and consists of 90–99% of cellulose fibers, in the ecological sense, paper is considered to be a cellulose substrate. Books, documents, writings, old maps, photographs, etc. are objects made of paper that are most often kept in libraries, archives and museums. Apart from paper, cotton and linen are fabrics which main components are cellulose fibers. Also, paraments, defined as hangings or ornaments used for decorations of Christian churches' interiors are often tailored of cotton and linen. In that sense, it should be emphasized that, art objects made of cellulose fibers can be colonized by cellulolytic fungi. These fungi can degrade cellulose fibers via process off cellulolysis, defined as an enzymatic hydrolysis of cellulose polymer into glucose units. In that sense, the fungi capable for production of cellulolytic enzymes are frequently isolated from paper, especially from old books or documents kept in libraries, archives and museum depots. Among the frequently encountered species on the paper substrates are the members of genera *Chaetomium*, *Penicillium*, *Aspergillus*, *Eurotium*, and *Trichoderma* [84, 85]. Some authors reported the presence of *Fusarium* sp., *Humicola* sp., *Paecilomyces variotti* and *Trichoderma viride* on deteriorated art photographs which were part of the collection of the Museum of Contemporary Art (Belgrade, Serbia) [86]. Due to their ability to degrade cellulose fibers, these fungi are referred to soft rot fungi [87]. Ascospores and conidia of different cellulolytic microfungi ubiquitously present in the environment worldwide could easily be deposited on papers and other cellulolytic materials (books), and when optimal conditions are met, they could germinate, elongate and proliferate and consequently lead to fast book decay. Also, fungi can deteriorate the paper-based materials mechanically via hyphal penetration or through production and excretion of pigments and organic acids [88]. A specific and irreversible phenomenon in the form of brown to red spots on paper material has been described in the literature [89]. Since the color of these spots resembles the color of fox fur, the phenomenon is called "foxing". The origin of this phenomenon on paper documents is explained by two theories - abiotic and biotic. According to the abiotic theory, "foxing" is a consequence of natural chemical processes, most often oxidation, which takes place on paper material, as well as a consequence of the deposition of certain compounds on the paper surface [90]. According to the biotic theory, "foxing" is caused by microorganisms, especially fungi that produce organic acids which deteriorate the paper, permanently damaging it [89]. Isolation of a large number of fungal species from the parts of the paper on which the symptoms of "foxing" are

*Fungal Deterioration of Cultural Heritage Objects DOI: http://dx.doi.org/10.5772/intechopen.98620*

observed, speaks in favor of biotic theory. The famous piece of art affected with foxing symptoms is self-portrait of Leonardo da Vinci's drawn in red chalk on paper and deposited in Royal Library in Turin. As a main culprit responsible for "foxing" spots on this famous piece of art, certain authors reported the fungus *Eurotium halophilicum*, which spores are documented near the foxing spots via SEM analyses, along with oxalates of fungal origin [91]. Additionally, they pointed out that tonophilic fungi can germinate on paper materials and also can metabolize organic acids, oligosaccharides and proteins, which react chemically with the material at a low water activity, forming brown products and, via oxidative reactions, leading to foxing spots.
