Experimental and comparative studies for evaluation of green solvents in cleaning the aged dammar varnish from oil paintings

Abstract

Recently, the pursuit of green cleaning practices has been induced with the aim of the replacement of the toxic materials with safer ones for the conservators' health and environment. This study aims to investigate the efficiency of using the green solvents as alternatives to the toxic solvents commonly used for the removal of the discolored dammar varnish from the artificially thermally aged oil-painted mock-ups. The research mainly focuses on the investigation of the sensitivity of two different oil-painted formulas toward dimethyl carbonate and dimethyl glutarate as green solvents during the cleaning procedure. The microscopic and analytical protocols were established for the varnished, unvarnished, and cleaned mock-ups to evaluate the effectiveness of the cleaning test. Complementary investigation of different microscopy techniques at a multiscale started with Stereo Microscope (SM), Atomic Force Microscope (AFM) and followed by Scanning Electron Microscope (SEM). The colorimetric measurements were carried out to compare the cleaned mock-ups with their varnished and unvarnished reference equivalents. The analytical protocol of cleaning assessment involved Gas Chromatography-Mass Spectrometry (GC/MS), Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and Thermogravimetric analysis (TGA) to respectively study leaching phenomena, the possibility of carboxylates formation and retention time of solvent inside the oil painted layer. Although the results varied, dimethyl carbonate exhibited promising results with respect to removal of the discolored aged dammar varnish with reducing the risk of paint removal and retentive time of the solvent inside the painted layer compared to dimethyl glutarate.