Deep eutectic solvent and agar: a new green gel to remove proteinaceous-based varnishes from paintings

The selective removal of thin varnish layers from oil and egg tempera paintings is still an open challenge in the conservation field. In this paper, a new cleaning system was developed by using a gel composed of fully green components as deep eutectic solvents (DES) and agar. The gel, prepared admixing choline chloride -urea with an EtOH-H2O agar gel, exhibited good rheological properties in terms of gel stiffness, allowing easy handling and removal of the gel. The new developed cleaning gel was successfully tested for the removal of proteinaceous layers applied over both hydrophobic (linseed oil as binder) and hydrophilic (egg temperas as binder) surfaces. The procedure consists on two steps: first the choline chloride -urea agar gel was applied and let to act then an EtOH-H2O agar gel was applied for few seconds to allow the removal of the DES residues Moreover, an innovative non-destructive approach based on micro-Spatially Offset Raman Spectroscopy (micro-SORS) allowed to demonstrate how the gel impairs the diffusion of the cleaning agents in the paint layers, highlighting the good solvent retention ability of the gel.