Distorted oil paintings and wax-resin impregnation – A kinetic study of moisture sorption and tension in canvas:Multidisciplinary study of the Sarno Baths in Pompeii

Wax-resin impregnation and lining are former widely used conservation methods. It is well-known that these methods slow down moisture diffusion into the canvas and changes the corresponding development of tension. However, the rate at which these processes happen are not well characterized and it is therefore unclear how long treated paintings are protected from high moisture environments. In the present work, moisture sorption characteristics of wax-resin impregnated linen samples were measured using dynamic vapor sorption (DVS) and tensile tests. Samples of untreated linen, wax-resin impregnated linen and Berger's Ethyl Vinyl Alcohol (BEVA) treated linen as well as an aged wax-resin treated lining canvas from 1958 were measured. The samples were in equilibrium at 42% relative humidity at 23°C in DVS and tensiometer when the relative humidity was stepped to 69% RH while monitoring the development of mass and tension respectively in the canvases. This showed that there is no or little delay from the time moisture is taken up by linen fibers until swelling and the possible tension build up sets in. Both wax-resin impregnated and non-impregnated samples took up moisture when the relative humidity was stepped up, but the wax-resin impregnated samples did so at a much slower pace than the non-impregnated ones. Tension built up in some canvas samples already at 69% relative humidity whereas others stayed unaffected until a relative humidity of 82% was reached. The findings confirmed that a fine weave canvas, tightly spun thread and the presence of wax-resin matrix in the voids between fibers all are factors that characterize a painting at risk of climate related shrinkage damage. It was also demonstrated that BEVA gel treatment had very little effect on the rate of moisture sorption as there was no penetration of the canvas. In the aged lining canvas, moisture was taken up at a rate that was intermediate between untreated and wax-resin impregnated linen, which was ascribed to cracks in the wax-resin coating