Characterization of Varnish Ageing and its Consequences on Terahertz Imagery: Demonstration on a Painting Presumed of the French Renaissance

We report on advance developments of terahertz system data analysis for measuring the thicknesses of multiple films for non-contact control in heritage science. We provide the analysis of time-domain data regarding the measurements conducted on a French Renaissance painting realized on a copper substrate. The piece of art is made of a varnish layer deposited on the different painting layers. We specifically focus on the dielectric characterization of the varnish layer and its ageing consequences on THz images that are also reported here and revealed optically hidden details.