Single-sided NMR: a non-invasive diagnostic tool for monitoring swelling effects in paint films subjected to solvent cleaning

During solvent cleaning of a painted surface, the control of solvent diffusion into the painting matrix is a primary concern for conservators. In this work, a comparative systematic study of solvent ingress and of the ensuing swelling phenomenon in paint films due to cleaning treatments was tackled using single-sided NMR. Specifically, the effects of a TAC aqueous solution (triammonium citrate in deionized water) applied in both free and gelled forms (by Klucel (R) G) on acrylic emulsion and vinyl-based model paints were evaluated. Notably, the NMR measurements (proton spin density depth profiles and transverse relaxation decays) collected before, during, and after these wet-cleaning tests proved that the use of the aqueous gel did not significantly minimize the penetration and the swelling action of water compared with the free solution. Furthermore, swelling effects associated with the use of an organic solvent (ligroin) were evaluated by NMR profilometry on varnished oil and egg-tempera paints. In this case, by comparing the depth profiles collected before and after the solvent treatments, a moderate paint swelling was observed followed by a width reduction of the paint films ascribable to the removed varnish layer. Overall, the reported NMR results reveal the analytical potentialities of the technique for a non-invasive assessment of the swelling effect of paint films subjected to cleaning (by water or an organic solvent), thereby providing an analytical method in support to the conservators' practice.