An original molecular approach to the use of clay minerals in the formulation of oil-based dyes and their sensitivity toward polar solvents - A case study

• Published in: Microchemical journal Vol. 147; pp. 142 - 149
• Main Authors: Costa, Thiago G., da Silva, Beatriz Felix Pimenta, Mattos, Lucas Palma, Siebert, Diogo Alexandre, Micke, Gustavo A., Szpoganicz, Bruno, Mangrich, Antônio Salvio
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.06.2019
• Subjects: Clay pigments; Molecular approach; Oil paintings; Water sensitivity; Molecular approach; Water sensitivity; Clay pigments; Oil paintings
• ISSN: 0026-265X; 1095-9149
• DOI: 10.1016/j.microc.2019.03.011

Several restorers of paintings have reported the sensitivity of oil paints toward polar solvents during the chemical cleaning of the surface of the pictorial layer. In this study, a novel mechanism for this phenomenon is described based on the chemical analysis of the pictorial layer of a nineteenth century painting and the swab used in the cleaning of this layer. The pigments used were characterized by FTIR and SEM-EDS and the presence of kaolinite, a clay mineral, was observed in all areas that showed sensitivity. The binder was characterized by GC–MS as a mixture of linseed and walnut oils. SEM analysis showed the presence of pigments adhered to the cotton as well as the presence of kaolinite. GC–MS results for the swab showed the absence of oil in the paint traces present. Thus, the oil and its degradation products are not sensitive toward the polar solvent, but only the pigment and the kaolinite. Since kaolinite is hygroscopic, it aids the leaching of the pigment of the oil layer onto the swab containing the polar solvent. •A new non-hydrolytic sensitivity mechanism of oil-based paints in polar solvents has been discovered.•Water sensitivity is associated with a mechanism described only by intermolecular forces.•Hygroscopic pigments is leached together Kaolinite in polar solvents.•The incompatibility of pigments used in paints causes sensitivity in polar solvents.