Hygromechanical Behaviour of Painted Wooden Panels from the Cultural Heritage

Provided it has been properly processed and implemented in structural applications, including protecting against biological attacks and accounting for moisture induced movements, wood is able to withstand considerable duration of loading thanks to its good resistance to fatigue and sophisticated multi-scale composite structure. For that reason, it is still present as the structural element of various types of ancient objects belonging to our cultural heritage, from buildings to painted panels or musical instruments. The requests by conservators to contribute to the maintenance of these old objects offer stimulating challenges to the wood scientists. Long-term creep predictions, essential for the design of wooden structures, would be safely validated when they manage to describe the present state of timber in an old building, while the better understanding of the ageing processes responsible for the sound quality of an old instrument could inspire novel approaches for wood drying and improvement by hygrothermal or chemical modifications. This paper will focus on the case of wooden panels serving as the support of paintings, that usually require preventive conservation in order to tackle with microclimatic aggressions consecutive to visitors, transportation and generalization of heating and air conditioning. Moisture induced movements of the painted surface might have damaged the painting and tend to detach it for the support, while restrictions imposed by the frames to prevent these movements often provoked major cracks in the wood and the paint. One of the challenges is the explanation of the present state of the observed objects, based on the knowledge of the long term response of a wood part submitted to hypothesized boundary conditions. Another challenge is the prediction of the consequences of a restoration, considered as a modification of these boundary conditions. These questions can be formulated for specific cases, such as highly valuable objects, or in more general terms to evaluate the long term impact of conservation choices. The wooden support of Mona Lisa, made of a single poplar plate with a pre-existing crack starting from the upper edge, has been previously modelled based on data gathered in October 2004 (fig. 1). The instantaneous response of the panel to the application of the crossbars on the back frame was rather correctly predicted, which ha l-0 07 95 91 1, v er si on 1 1 M ar 2 01 3 Author manuscript, published in "'Mechanical Modeling of Wood and Wood Based Materials', mini-symposium WCCM8 ECCOMAS Italie (2008)"