## 1 Introduction  
Many books, historical documents, sketches, paintings, music scores, architectural construction documents, postage stamps, and photographs are susceptible to destruction [1, 2, 3, 4, 5]. This destruction can be a result of the influence of endogenous factors such as acidity, metal ions, lignin, or paper degradation products, and exogenous factors such as heat, humidity, radiation (light, UV), oxygen, pollutants, or biodeteriogens [6, 7]. Filamentous fungi have been considered to be the most important microorganisms responsible for the biodeterioration of collections in museums, archives, galleries, and libraries [6, 8, 9], causing severe material and information losses in libraries' and archives' collections [8]. Fungi possess the capacity to grow at average relative humidity levels, inducing several chemical and physical decomposing processes through the excretion of metabolites and enzymes that affect the structural organization of the paper [2, 10]. These metabolites comprise a wide variety of substances such as colorants, enzymes, acids, chelating agents, etc. [11]. However, one of the major concerns regarding fungal colonization of paper is that it undergoes degradation that affects its mechanical and aesthetic properties [12] since the stains excreted by fungi are colored [13] (Figure 1). Several reviews and research papers can be found describing fungi, their colorants, and the foxing phenomenon causing biodeterioration of historical paper (Figure 1) [2, 14, 15, 16, 17], but very little data is presented about the acceptable methodology of how to treat and remove the stains from paper. To address these demands, first, it is important to collect available information about the fungal species that stain paper and the chemical nature of those stains. Another important demand is to develop new cleaning methods that would not bleach and destroy the aesthetics of the paper itself. Apart from fungal stains, another problem for the restorers is the colored stamps placed on a specific sheet (Figure 1), marking the book or the document as a property of the library. These stamps are usually composed of ink containing different kinds of synthetic dyes [18], and when restorers need to restore or digitalize a book or a document, they also need to remove the stamp from the sheet. Therefore, the ink of the stamp can also be considered as a potential colored stain necessary to be removed from the paper surface (personal communication, restorer [PERSON]). Generally, microorganisms are considered to be a threat to cultural heritage items in the form of biodeterioration. On the contrary, in the last few decades, these living organisms have been shown to be valuable restoration tools by means of their metabolic properties [19]. Many studies have reported the application of microorganisms and their enzymes in biocleaning, such as the removal of dirt, animal glue, and other organic residues from a variety of artworks [20; 21; 22; 23; 24; 25; 26]. Figure 1: Several types of stains on the paper surface. (**A**) red circle, probably black fungal stains; many black spots are spread on the book page. (**B**) red circles showing presumable yellowish and pinkish fungal spots. (**C**) the phenomenon of foxing. (**D**) colored stamps on the books’ pages. For safety reasons, enzymatic cleaning is preferable in many cases over the application of living cells that can leave residues on the surfaces that are about to be restored and which can later serve as nutrients for dangerous microbes [19; 27]. In the work of [PERSON] and colleagues [22], hydrolytic enzymes, in particular proteases, nucleases, and phosphatases, have been identified and assayed in biocleaning tests for degradation of animal glue performed on different kinds of surfaces such as wood, glass, and stone. These extracellular enzymes were excreted from _Exiguobacterium undae_ and were used for the successful removal of different types of animal glue.