# NMR Relaxometry and IR Thermography to Study Ancient Cotton Paper Bookbinding
## 1 Introduction  
Protection of precious library archives includes specific tasks of the same relevance. In particular, the characterization of the books supporting materials is crucial for pointing out degradation phenomena, for improving micro-climatic parameters, and for suggesting restoration activities [1]. Considering the large number of factors involved and the vastness of the collections, the ongoing degradation processes are often identified once clear and irreversible. Knowledge of biological, physical, or chemical deterioration pathways is essential in preventing the increase of structural defects, so to improve the permanence of library heritage [2]. The present work aims at surveying the state of conservation of a 19 th-century book, focusing on its paperboard cover. We used the cover as a case study for a reliable non-invasive diagnostic procedure. The front-board cover is made of cotton-based paper, a hygroscopic material that absorbs and releases water depending on the ambient conditions, i.e., temperature and relative humidity (RH). The cover was analyzed in terms of spatial distributions of moisture content, through two-dimensional nuclear magnetic resonance relaxometry (2D \({}^{1}\)H-NMR-R) [3], and thermal diffusivity via higher-order statistics thermography (IRT-HOS) [4]. We performed both methods using non-destructive and portable instruments which enable in situ investigations [5; 6], likewise precious book collections [7; 8; 9]. Cotton paper is a random network of hydrated cellulose fibers. The cellulose chain is the elemental unit of the cellulose fiber comprising a linear homopolymer of glucose monomers linked with glycosidic bonds. Possible inter- and intra-molecular hydrogen bonding provide the interaction between the different cellulose chains and their aggregation into hierarchicalarrangements, from micro fibrils up to fibers [10]. Chains pack into a micro fibril with regularly arranged crystalline (with two different allotropes, I\({}_{\alpha}\) and I\({}_{\beta}\) cellulose) and irregularly aggregated amorphous domains. The microfibers, in turn, assemble into a fiber. The degree of polymerization (DP\(\sim\)10\({}^{4}\)), the relative amounts of the two polymorphous (I\({}_{\alpha}\)/ I\({}_{\beta}\)\(\sim\)40%) and of crystalline material in cellulose (CI-80%) influence the mechanical and physical properties of cellulose. Based on this architecture, the amorphous regions in a microfiber along with microfibers-microfibers and fibers-fibers inter-spaces allow considering cellulose as a porous system. Therefore, water molecules play a crucial role in establishing its chemical and physical properties [11]. In soft drying conditions, at least a minimum quantity of water (hydration water) stabilizes the porous network by bridging different cellulose chains via hydrogen bonds, wherever hydroxyl groups are available. Hydration water and water molecules filling cellulose pores are characterized by different dynamics, affected by their spatial confinement [12; 13; 14; 15]. Water can swell or shrink the porous structure depending on its volume under controlled thermo-hygrometric conditions [16]. Although water is required to achieve the paper's flexibility, water vapor adsorption/desorption mechanisms are responsible for solubilizing and carrying atmospheric reactive compounds, soluble salts and minerals, contributing to the deterioration of cellulose. To describe the conservation status of the cellulose-based paper, many authors suggested correlating the water molecule dynamics, physically and chemically hindered by the cellulose network, with the thermo-hygrometric response and hence with the ageing processes of the material [17; 18]. \({}^{1}\)H-NMR-R measurements allow characterizing the dynamics of water molecules confined in cellulose amorphous phase, acting as hydration water or filling pores [19; 20; 21; 22]. Protons belonging to the polymer chains, in fact, are seldom experimentally accessible since their NMR signal's coherence time is too short.