Multi-analytical study of the physical and chemical aspects at the interface between oil paint films and acrylic-based grounds in contemporary paintings

Oil paintings on canvas made in the 20th century are complex systems where organic and inorganic materials coexist as a function of the artists’ choices and preferences. For centuries, oil paint has been the most widely used binding medium by artists and artisans, a distinction it maintains as the most popular binder today. For this reason, the study of film-formation processes of oil paints has been extensively addressed over the last decades. The use of oil paint on ready-to-use acrylic primed commercial canvases is a common practice, offering versatility and fast-drying times. However, the interaction between these layers remains insufficiently understood, particularly at their interface. While acrylic grounds are appreciated for their rapid drying and flexible nature, they have been reported to cause delamination and degradation in some instances, raising concerns about their long-term stability when combined with oil-based paints. Defects such as wrinkles, protrusions, cracks and delamination associated with modern and contemporary oil paintings still present the greatest challenges. However, the existing literature on these conservation issues remains scarce. The objective of this study is, therefore, to study the processes and mechanisms taking place at the interface between oil paint films and acrylic-based grounds. For this purpose, the research was divided into two parts: the first part focused on the study of film formation processes and degradation stages of pictorial layers; in the second part, special attention was paid to the failure mechanisms taking place at the interface between oil paint films and acrylic-based grounds. In the first phase, the research provided a deeper understanding of the interaction at the interface between the pictorial layers. Through the application of empirical (scribe tests), weight-based (ΔW%), spectroscopic (ATR-FTIR), thermal (TG-DSC) and chromatographic (GC-MS) techniques, the research focused on the migration of metal ions and on the study of degradation mechanisms observed in oil paint films, examining the early stages of drying and long-term stability. Furthermore, the application of an innovative imaging approach based on the combination of field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (FESEM-EDX) and Ion Mobility combined with desorption electrospray ionisation mass spectrometry (IMS-DESI-MS) made it possible to detect the spatial distribution and location of both organic oil fraction and inorganic pigment elements across the interfaces of oil paint films. This hybrid experimental approach highlighted that metal ion migration -typically understood as the vertical movement of mobile compounds from the ground layer to the painted surface- can also occur transversely between adjacent colour areas. Key findings revealed that metal ions can migrate, catalysing the drying kinetics, altering the reactivity, and inducing structural changes in adjacent oil-paint films, thereby promoting the propagation of cracks, delamination, and protrusions. The knowledge acquired by this phase of the research led to a shift in focus to a more complex pictorial structure, allowing the interactions between the oil paint film and acrylic substrate to be studied. In the second phase, the research has led to the identification, localisation and characterisation of early degradation phenomena such as cracks, protrusions and opacity, across the entire stratigraphy, even before they become visible to the naked eye. The results evidenced that MBI are effective in understanding such degradation mechanisms. MBI -until now used only for the preliminary characterisation of pigments- provided detailed information on the nature and distribution of degradation phenomena. These findings were then compared and validated at the morphological and elemental level by HR-FESEM-EDX. ATR-FTIR spectroscopy provided further insight into the organic components and potential degradation products on the surface. At the same time, GC-MS identified and quantified organic compounds, providing evidence of oxidation and hydrolysis reactions occurring in the bulk. The long-term monitoring of gloss during the drying film formation processes of oil paint layers revealed a clear correlation between the extent of gloss variation and the type of ground layer employed. This suggests that the choice of ground significantly influences the surface. Additionally, mechanical testing revealed how physical stresses—such as tension and environmental factors—affect the structural integrity of the layers over time according to the pictorial structure. Results suggested that, beyond the interface issues between the acrylic ground and the oil paint layer, the compatibility of additives may also play a crucial role. The results obtained and the methodologies developed in both phases of the research were then applied to real case studies. Close collaboration with conservators turned out to be necessary to develop a more complete understanding of the phenomena observed experimentally in the laboratory. The research provided informed guidance to artists on the appropriate selection and combination of materials and, secondly, a comprehensive methodology for the development of scientifically supported preventive conservation strategies for contemporary paintings.