Cleaning Oil Paintings: NMR Relaxometry and SPME to Evaluate the Effects of Green Solvents and Innovative Green Gels

Gels have been recently introduced in conservation to better control the cleaning action. The slow solvent release is of the utmost importance to selectively remove varnish layers of few microns thickness, applied on painting layers which may have similar chemical properties. To evaluate the interactions of the underlying painting with solvents, diagnostic methods should study the physical and chemical changes that occur during and after the treatment. In this paper, we propose an innovative analytical protocol based on the combination of solid phase microextraction (SPME) and nuclear magnetic resonance (NMR) relaxometry to evaluate the solvent entry and retention in the paint layer, as well as the induced microscopic and macroscopic changes to the elasticity of the binder. Moreover, FTIR and optical microscopy are used to evaluate the cleaning efficacy. The proposed analytical protocol can be used to identify solvents or gels which provide minimal interaction with the painting In particular, we have tested the protocol to evaluate the performance of non-toxic and biodegradable green gels on mock-ups and on a painting dated 19th century showing that they perform better than solvents applied with swabs. Five-minute gel treatments were found to be as effective at varnish removal as thirty-second swabs treatments following traditional cleaning methods. Cleaning with gels limits the mechanical stress, the solvent entry and retention in the paint layer, and reduces the embrittlement due to lower mass transfer. An unusual “sandwich” system was used to avoid the presence of gels residues on the surface with the gels contained into two sheets of Japanese papers - one on top and the other on the bottom.