Hygromechanical behaviour of wooden panel paintings: classification of their deformation tendencies based on numerical modelling and experimental results

Abstract

Wooden panel paintings are among the most important historical and artistic artworks from the Middle Ages and the Renaissance period. Currently, they represent a challenge for conservators and scientists who face complex issues related to their conservation. Panel paintings can be considered multilayer objects, that for brevity can be considered to consist of a wooden support and various paint layers. The wooden support is known to be hygroscopic and is continuously seeking hygroscopic equilibrium with the humidity of the environment, thus it tends to deform. Based on various hygroscopic tests carried out on 6 real panel paintings chosen by expert restorers to represent different periods and construction techniques, this paper describes the deformation tendencies of the selected panel paintings. Among possible variables, three most important variables were identified: (a) tree ring orientation of the wooden support, (b) stiffness and (c) emissivity of the paint layers. The internal equilibrium of the forces, governed by the moisture gradients across the thickness of the wood, changes drastically according to the varying characterisation of these factors. To observe their individual contributions, the 6 panel paintings underwent various humidity cycles, were completely free to deform and were always in complete safety. To characterise the stiffness and emissivity of the paint layers, the 6 panel paintings underwent a few humidity cycles with the front face totally waterproofed; thus, the moisture exchange was forced from the back only, and one of the three variables was eliminated. A complex system emerges where the tree ring orientation of the wooden support, the stiffness and emissivity of the paint layers are strongly coupled and determine the deformation modes of the panel paintings. A numerical analysis was conducted to classify the various general deformation modes of panel paintings and the specific classification of the 6 real panel paintings analysed experimentally. The complexity of the interaction of the variables studied suggests that experimental procedures must be conducted in preparation for numerical analyses of real panel paintings.

Coupling numerical and experimental methods to characterise the mechanical behaviour of the Mona Lisa: a method to enhance the conservation of panel paintings

“…On the one hand, these difficulties have confirmed the uniqueness (i.e. non-generalizability) of each artwork also from the material and behavioural viewpoints (21). On the other hand, it appears that this complexity can be optimally addressed not in a preconceived way, but rather by integrating in an innovative way (as in this paper) the experimental results and adequate computational tools.…”

“…Some have investigated the hygro-mechanical behaviour of the main materials making up such artwork, tested on new materials [1][2][3][4][5], others the hygro-mechanical behaviour of mock-ups tested in the laboratory [6][7][8][9][10] and some others numerical modelling based on mock-up behaviour or literature data to interpret their behaviour [11,[12][13][14]. The present research rather focuses on an authentic artwork, as already proposed by [15][16][17][18][19][20], following the consideration that each panel painting shows a specific hygro-mechanical behaviour which should be known for its preventive and remedial conservation, as highlighted both by testing and numerical modelling in [21,22]. Since 2004, the wooden panel on which Leonardo da Vinci painted his Mona Lisa has been studied by an international research team under the request of the curators of the Louvre Museum.…”

“…Previously published work [23][24][25][26][27] confirmed the relevance of integrating computational tools with experimental data directly obtained from the original artwork being studied. This approach, which has been gaining ground in recent years [20][21][22]28], has necessarily required collaboration among scientists, conservators and restorers, and can be considered as an important step towards fully integrating this kind of research for the in-depth knowledge and conservation of the physical structure of the wooden artwork. Starting from such approach, the considerable difficulties found in fully interpreting the results of the experimental observations and measurements have highlighted the complexity of the phenomena observed, and the variability from case to case of the observed behaviours.…”

Fracture toughness of dried oil paints

“…So far, those properties have been determined predominantly for new materials, and an assumption has been made that they represent the properties of historical materials. Only a few sets of data have been obtained for historical materials (10,11,12) but due to the limited number of tested specimens, their statistical relevance is still to be con rmed. A recent compilation of material parameters for oil-based pictorial layers illustrated that they evolve over decades and might continue to evolve even over centuries (1).…”