Uncovering of scalar oxidation within naturally aged varnish layers

The present work originates by a recent study on the uncovering of a scalar trend for the spectral characteristics related to the oxidation/crosslinking in an artificially aged resin film. It has been found that by moving from the surface of the aged resin towards its interface with the paint layer, crosslinking decreases. In the present work, the existence of this trend is tested on original samples, three 19th century Byzantine icons covered with natural aged varnish layers of different type and thickness. The varnish layers have representative problems due to aging and preservation. The verification of the deterioration trend was possible by measuring the solubility of the material at subsequent depth-steps created within the same varnish layer. The realization of a number of satisfactory homogenous depth-steps, with enough space to perform the necessary solubility tests, has been possible through UV photo-ablation using a KrF excimer laser. The results of the chemical tests revealed that there is, indeed, an exponential gradient of the deterioration across the film thickness, which is directly related with an equivalent gradient in the solubility within the same varnish layer. Consequently, the deeper one goes into a varnish layer, progressively more dilute solutions of the appropriate solvent in an inert medium are required to remove it. The benefits of a combination of chemical with laser cleaning of resin-based varnishes are demonstrated, and at the same time, lasers are established as a complementary and useful tool in conservation.