Bulging in wax-resin impregnated canvas paintings: Review and cases

This work presents four specific case studies of wax-resin treated paintings that have shown significant distortions after being exposed to a change in the ambient climate. The cases show how certain conditions during loans or other relocation scenarios may lead to bulging or sagging in the canvas paintings that differs substantially from the response by canvases not impregnated with wax-resin. Possible explanations for these deformations are provided and explanations offered as to why they may reduce or disappear over time, once the stretcher has had sufficient time to come to a moisture content equilibrium within a new environment.
The cases are discussed in relation to previous research, including recent lab results of sorption kinetics in wax-resin impregnated canvases and an investigation into the corresponding development of forces in restrained samples. The discussion shows the importance of time of exposure and weave geometry to the development of deformations as an impact of moisture. Also, once the wax-resin mixture has experienced stress beyond the yield point, it takes very little additional development of forces for an impregnated canvas to deform. This points to the importance of including both relative humidity, temperature and time as factors in a risk analysis for wax-resin impregnated or lined canvas paintings.
Based on these case studies and lab tests, as well as a literature review, specific recommendations can be offered to paintings conservators dealing with bulging paintings that have been impregnated or lined with wax-resin.