An integrated approach for the climate-induced risk assessment within historic libraries combining microclimate data and modelling

The durability of historic libraries is affected by deterioration processes driven by the environmental conditions in which the collections are kept. The present research aimed to infer meaningful information from the combination of microclimate observations (i) and modelling (ii) so to outline an integrated approach to assess the climate-induced risks (iii) in historic libraries. Four historic libraries in Italy, associated with different climates, were used as case studies: theMeteorology Library of CREA (Consiglio per la Ricerca in Agricoltura e l’Analisi dell’Economia Agraria) at Collegio Romano and the repository of the Alessandrina Library in Rome, the Ca’ Granda Library in Milan and the Delfiniana Library in Udine. The study of the microclimate (i) enhanced the understanding of the interaction between the heritage material properties and the environmental forcing. For the first time, a comparative study was carried out to provide useful insight into the impact on the climate-induced conservation risks for paper collections of conditioned and unconditioned microclimates in historic buildings. Modelling (ii) allowed to simulate the microclimate inside historic buildings housing the libraries using both sinusoidal heat and moisture transfer functions and whole-building dynamic simulation. For the latter method, the thermal model of the Collegio Romano Library was built in IDA ICE (Indoor Climate and Energy) environment and calibrated using on-site measurements. Then, the capability of IDA ICE extended with HMWall (Heat Air and Moisture) model was tested in the simulation of 1D hygrothermal exchanges across a wall made in paper. To this aim, the physical and hygrothermal properties of ancient and modern papers were investigated through a sensitivity analysis to identify the most influential parameters in the simulation of moisture gradients. Finally, dose-response/damage functions for library materials and NDT (Non-Destructive Testing) measurements were used to evaluate the climate-induced risks for paper collections (iii). The climate-induced risk assessment involved mechanical, chemical and biological mechanisms. The allowable microclimate bands to avoid mechanical stress to organic hygroscopic materials and the risk of wear and tears due to handling were studied to draw recommendations to limit mechanical deterioration in case of consultation and loans. The isochrones of equal expected lifetime of paper allowed to evaluate the chemical risk for different paper-based collections as a function of their intrinsic vulnerability (i.e., acidity and degree of polymerisation) and considering the typical response time of paper books to the environmental temperature and humidity changes. In addition, the Time Weighted Expected Lifetime (TWEL) index was defined to explore the effect on paper conservation of changes in the microclimate conditions resulting from climate control strategies, retrofitting measures and/or the possible future climate change. The biological threats were estimated by using Sedlbauer curves for mould germination and growth and the Brimblecombe model for potential production of cloth moth eggs. Finally, the colorimetric change on some faded book covers in the repository of the Alessandrina library was monitored over a year to estimate the photodeterioration rate due to solar exposure. The results highlighted that, at all climate zones considered, the historic libraries were characterised by high thermal inertia and moisture buffering capacity due to the combined effect of massive building envelopes, low air exchanges and large total volumes of hygroscopic materials. The modelling of the hygrothermal conditions inside paper collections showed that the relative humidity levels and fluctuations collected in the libraries would affect only the first layers of the books, showing a low impact on their conservation. Temperature was found to be a key microclimate stressor to be studied for preventive conservation of paper collections, as it controls the rate of cellulose hydrolysis and favours insect proliferation. In terms of paper chemical deterioration, the annual microclimate conditions inside the libraries would lead to the loss of their acidic collections in less than 300 years. The use of winter heating markedly reduced the expected lifetime with respect to that estimated in the unconditioned microclimates in the same season. For this reason, the natural microclimates within historic buildings in winter could be suggested as a sustainable preservation strategy for paper collections. This result was confirmed in simulation environment reconstructing the natural microclimate in Collegio Romano Library through its validated whole-building model. The observed summer overheating is particularly alarming in view of the predicted climate change, as the expected lifetime for acidic paper was found to potentially decrease up to 46% in the Far Future (2071-2100) if compared to the Recent Past (1981-2010) scenario. Finally, although spore germination could be excluded, the risk of insect proliferation was high in all the libraries. The annual discolouration rate of the faded book covers in Alessandrina was too low to be measured; however, the estimated luminous exposure was found to be incompatible for the conservation of photosensitive materials. The integrated approach followed in this thesis enabled a wide-ranging study on the conservation of historic libraries, enhancing the understanding of the role of climate on the deterioration risks and supporting the design of rational and sustainable mitigation strategies. The same approach could be effectively adapted to most library and archival collections made of paper.