## 2 Materials Used for Restoration and Conservation of Wooden Elements
### Consolidants Used for Wooden Elements  
Historically speaking, one of the first used consolidants were the vegetal oils (such as linseed or Tung oil) and natural resins (such as colophony) [19]. Having advantages such as being easy to obtain and apply, the main disadvantages (such as the aesthetic changes induced by the colophony, extensive drying periods of the linseed oil leading to wood structure softening or inhomogeneous film structure obtained using Tung oil) led to the search for alternative treatments [20; 21; 22]. A similar approach, is represented by the use of sugars or sugar alcohols, which, through controlled drying, give rise to crystals consolidating the material. However, development of large crystals could damage the wood materials; in the same time, sugars can be a very good source of nutrients for the development of microbial growth [23; 24]. One of the first inorganic treatments of the historical wooden elements is represented by the use of potassium aluminum sulfate dodecahydrate, developed in the middle of the XIXth century. However, its major drawbacks (among which the acid degradation of cellulose is one of the most important) led to abandoning the treatment, but not the general use of inorganic consolidants [25]. More recently, several types of inorganic materials (including nanoparticles) were applied for the consolidation of wood materials. As some such types of particles also possesses antimicrobial properties, their application can also cover the biocidal need in restoration procedures [19; 26; 27; 28]. With the development of polymer science, several polymeric materials or polymeric resins were successfully tested for the conservation of wood materials. Among those, several types of commercially available materials are currently applied in our days [29; 30; 31; 32; 33; 34]. Finally, in recent years, the use of biobased materials (such as keratin, cellulose or chitosan) was supported by several literature studies [35; 36; 37]. Besides the materials previously presented, [PERSON] et al. [38] proposed the addition of microcrystalline cellulose (5%-20%) into a UV-light curable methacrylate-siloxane resin formulation, with potential application as historical wood coating. The addition led to the increase of the dynamic moduli, flexure stiffness, and hydrophobicity, accompanied by a decrease of the thermal expansion coefficient, which suggest that the photo-curable micro-composites could be used to recover the mechanical and physical properties of damaged wood, being a good alternative to the traditional resins currently used. Another interesting approach was presented by [PERSON] et al. [39]. Using polyester acrylate styrene free resin and gamma curing, the authors observed the increase of thermal, chemical and photochemical stability. In the same time, the gamma curing offers the advantage of disinfection of the material. However, great attention must be payed upon gamma treatment, as the ionizing radiation can damage the cellulose structure [40]. Additionally, considering the radioprotection issues, the method cannot be usually applied for in situ interventions [19]. In our opinion, the previously described materials present advantages such as slowing the fungal growth in treated samples, in situ application (for most of the proposed solutions), recovery of damaged properties of wooden materials, etc. The main disadvantage of the current approaches is represented by the lack of long-term protection (the need for repeating the treatment). Additionally, the future research should be focused on the development of multi-phase treatments, which could adapt to environmental changes in real-time, besides offering a protection for multiple degradation causes [19].