Insights into deformation and recovery of gypsum plaster under wet and humid environments

Long-term deformation and creep of plasterboard coatings in wet and humid environments can threaten the conservation of wall paintings. This research focuses on the significant deformation of gypsum plasterboards after exposure to water, altering the paintings on their surface. In order to propose a safe and appropriate restoration protocol, mechanical softening of gypsum plaster can be used to flatten the deformed gypsum coatings. To gain a better understanding of the response of gypsum plaster to wet and humid environments and its ability to sustain such flattening, creep tests were carried out under different environmental conditions. We focused on the maximum deflection amplitudes measured by macroscopic threepoint bending under constant load and their variations with liquid water and high humidity, during an initial deformation phase and a recovery phase. Thanks to this research, we have improved our understanding of: i) the relationship between the deformation of gypsum plasters and the environmental stresses involved, ii) the plasticity of deformation and the recovery ability of gypsum plasters, iii) the humid and wet conditions and environments that could be proposed to carry out a safe restoration of the wall paintings. • The deformation of gypsum plaster during creep test measurements is studied. • The influence of liquid water and high humidity levels on the creep deformation of gypsum is presented. • The shape recovery ability of gypsum plaster with respect to the hydric and hygric conditions is confirmed. • An achievable restoration protocol based on the mechanical softening of gypsum plaster can be proposed for wall paintings.