Spectroscopic ellipsometry as a tool for the optical characterization and ageing studies of varnishes used in Post-Byzantine icon reconstructions

Abstract The subject of picture varnishes has concentrated the attention of numerous researchers from various scientific fields, during the last 15 years. Although several analytical chemistry techniques have been used in the field, information on the optical properties of picture varnishes is not extended. Most of the optical measurements on picture varnish refractive indexes, have been done by the immersion method (at 589 nm) while ageing studies are usually based on UV-Visible transmission curves. The aim of this paper is to present the potential of spectroscopic ellipsometry as a powerful tool in picture varnish studies. The main advantage of the technique is that it can measure refractive index, absorption coefficient and varnish thickness at the same time. Furthermore, it gives more reproducible and accurate results than traditional optical techniques, since relative instead of absolute values are measured. It is also very sensitive to surface roughness and thickness inhomogeneity. That is why it is more effective on Christian icons, which traditionally have very flat paint surfaces. Several examples on the application of the technique on fresh natural and synthetic varnishes (dammar, mastic and egg white, rosin and Paraloid B72) are presented in this work, showing the potential of the technique on varnish characterisation and alteration due to ageing.