Experimental tests for mechanical properties of wooden ‘buttons’ used for attaching auxiliary supports behind panel paintings

Abstract

When wooden supports of panel paintings have been severely altered or damaged and the original crossbeams are missing, new crossbeams or other types of auxiliary supports can be connected to the panel’s back-face in order to achieve desired effects including panel strengthening and control of deformations. In order to make such connection, the Restorers of Opificio delle Pietre Dure use small wooden blocks, each glued on the back face of the panel, and holding freely the head of a screw connected (often by means of springs) to the auxiliary support; the English term ‘buttons’ has been proposed for such blocks. This research examines in depth how the shape (conical or cylindrical) and dimensions (diameter in the 20-30 mm range, and thickness in the 3-6 mm range) of the buttons, all made from oak wood, influence their stiffness and load-carrying capacity. Each button was glued onto a dummy beech board by means of epoxy structural resin; a 3 mm diameter steel screw was inserted in each button’s hole prior to gluing, so that its head remained ‘trapped’ between the button’s cavity and the board. A short-term mechanical test was performed with a universal testing machine by axially pulling out the screw from the button. Load and displacement were recorded, and the load–displacement curves were analysed. The results showed that within the tested range: a) the ultimate load-carrying capacity of the buttons is only influenced by the residual thickness above the screw head, b) nor the shape nor the external diameter of the buttons have any influence on their mechanical properties, c) the stiffness (slope of the straight part of the load-displacement curve) of the connection depends only on the embedding of the screw head into the wood (i.e. mainly on the wood density).