Long-term hygromechanical monitoring of Wooden Objects of Art (WOA): A tool for preventive conservation

Case studies monitoring wooden objects have been conducted for many years. In some studies the monitoring was limited, for longer or shorter periods, to air Temperature and Relative Humidity logging, which can show if extreme values and rates of variation occur. In other cases mechanical monitoring was combined with microclimatic logging, which provides quantitative information directly related to the microclimate; these data are useful to validate mathematical models that eventually may predict the long-term behaviour of the objects. Although the quality of the information obtainable by combined mechanical-hygrothermal monitoring is more directly usable, due to the actual response to the microclimate, using simply logged microclimatic data it is possible to formulate a statistical analysis aimed at defining microclimate variance. Museums all over the world are engaged in lively discussions regarding the long-term conservation of works of art created using hygroscopic materials, which are sensitive to microclimate fluctuations. The climate fluctuations can have both temporary and permanent effects on the hygroscopic objects and they are a potential cause of damage. The current preventive approach is based on a compromise between the technical limitations of the museums' air conditioning plants and the presumed needs of the objects, as determined by conservators and conservation scientists. The primary goal is to keep the climate as stable as possible around standard values, with strict fluctuation ranges usually defined as 20 °C ± 2 and 50% RH ± 5. There is considerable pressure in favor of widening the allowable ranges, based on the need of a lighter carbon footprint as well as to facilitate the loan of artifacts between institutions. Although we have long-term evidence of the generally positive effects of a microclimate within the standard range of allowable fluctuations, we lack experimental data regarding the effects under broader ranges. Wooden works of art are useful in representing the complexity of possible reactions. Because of the mechanical response caused by thermo-hygrometric conditions, the monitoring of Wooden Objects of Art (WOAs) in their exhibition and storage environment is important in order to protect them from potential physical/mechanical degradation. Due to the specificity of each artwork, both from its structural point of view and from its previous microclimatic history (for the most part totally unknown), the analysis of an artifact's response to short- and long-term variations can supply useful information about its "individual" sensitivity to the exhibition microclimate, suggesting the adoption of more or less rigid parameters. Case studies monitoring wooden objects have been conducted for many years. In some studies, the monitoring was limited, for longer or shorter periods, to air T/RH logging, which can verify if extreme values and types of variation occur. In other cases, mechanical monitoring was combined with microclimatic logging, which provides quantitative information directly related to the microclimate; these data are useful to validate mathematical models that eventually may predict the long-term behavior of the objects. Although the quality of the information obtainable by combined mechanical-hygrothermal monitoring is of higher magnitude, using simply logged microclimatic data makes possible to formulate a statistical analysis aimed at defining microclimate variance, obtaining a very efficient schematization of the greater or lesser stability of the climate according to the presumed lower or higher sensitivity of the artifact under consideration. The aim of this paper is to demonstrate how delicate the widening of allowable ranges is and how misleading a generalized approach can be, while the eventual choices of standards relaxation must be based on careful analysis of the long-term response of the objects.