Strains in gesso on painted wood panels during humidity changes and cupping

Strain gauges were attached to both faces of 10 mm thick Poplar wood panels before applying a traditional gesso-based ground layer and varnish on only one face. The application and setting of the ground layer left high strains on both faces of the wood panels. Later measurements, followed by geometrical calculations, showed that for step-humidity changes the strains in the outer surface of the gesso ground layers were small. These results were followed by numerical calculations to indicate the behaviour of thicker panels and with different stiffnesses. These indicated that the strain levels in the ground layer are strongly dependent on the ratio of the thicknesses of the ground layer to the wood, and also weakly dependent on their stiffness ratios. Further calculations showed that the prevention of cupping by mechanical restraint can increase by many times the strains in the ground layer surface, but this also is dependent on the panel thickness. In this paper, the authors have tried to provide some background information about the strains that can result either from moisture changes or from external restraints; to help the conservators make a decision in any specific case.