Study of the mechanical behaviour of a 16th century wooden painted panel: Effect of the cradle on its shape

• The thickness of the panel controls its deformation and the stresses induced by both the horizontal crosspieces and the glued vertical uprights. • The uneven and reduced thickness of some areas of the panel indicates weakness and, consequently, an uneven effect of the cradle on the wooden panel. • The lower horizontal crosspieces play an important role in holding the panel flat. • The parameters of the experiment, from the position of the panel (vertical or horizontal) to the direction of removal of the horizontal crosspieces (lower to higher or higher to lower), will determine the results. In the past, conservation treatments such as thinning and cradling painted panels to stabilize them and prevent warping have inadvertently introduced considerable stress. These systems have often proved overly restrictive for the wooden support, leading to substrate failure and numerous, sometimes irreversible, alterations to the paint layers. This study, conducted on a 16th century Flemish painted oak panel, seeks to improve our understanding of the behaviour of cradled panels. More broadly, it aims to establish foundational elements for the development of a treatment protocol that may guide conservators in making informed and pragmatic interventions. The methodology implemented attempts to approach the mechanical behaviour of the panel by confronting two experimental phases, one using mock-ups and the other with measurements directly on the historical panel, in a stable relative humidity. To understand the heterogeneous shape and out-of-plane deformations of the obverse and reverse sides of the panel, non-invasive optical methods were employed, specifically the Fringe Projection Profilometry technique and the stereoscopic marks tracking method. Initial shape characterization revealed that, although the panel appears flat under normal visible light inspection, it in fact exhibits subtle but significant deformations, resulting in an uneven distribution of internal stresses. Subsequently, a gradual removal of the cradle’s horizontal crosspieces, from top to bottom, was carried out, partially releasing compressive, shear, and tensile stresses that had accumulated over time due to asymmetric moisture exchange within the conservation environment. Photomechanical approach was employed to quantify and characterize these stress releases and resulting deformations. The results indicate that the lower horizontal crosspieces play a particularly critical role in maintaining the panel’s deformation. Additionally, the glued vertical uprights of the cradle exert a notable influence that must be taken into account when interpreting the results. Several of these observations were corroborated by results obtained from the mock-ups, especially regarding the influence of the horizontal crosspieces on panel deformation.