An analytical study to verify the suitability of free and gelled siloxanes for the treatment of modern oil paintings

Since its introduction, about 15 years ago, the cyclic siloxane Cyclomethicone D5 has become increasingly used in the conservation of Cultural Heritage. In free form it allows the temporary hydrophobization of sensitive surfaces that must be treated in an aqueous environment; in the commercially available gelled form it allows the formulation of water-in-oil particle emulsions for surface cleaning or removal of film- forming substances, depending on the conditions of the aqueous phase. Over time, doubts have arisen about the real inertness of this solvent towards modern oil media. In addition, European regulations have restricted its use in consideration of its environmental impact, and so the already difficult availability of the material could further complicate; this advises the search for a substitute solvent. These two ar- guments motivated this study, conducted on two modern oil paintings: one unvarnished on paper, the other varnished on canvas. Samples of these paintings were treated under different conditions with D5 and the potential substitute chosen: dodecamethylpentasiloxane. The GC–MS and SEM-EDS analysis of the treated samples made it possible to evaluate and compare the degree of interaction with the oil binder in terms of leaching and topographic alterations of the surface, for both the free solvents and their gelled forms. Tests conducted on specially prepared paint samples also made it possible to evaluate the degree of solvation and consequent swelling produced by the two solvents on the different binders.