Non-invasive characterisation of varnished modern paintings: comparative insights and conservation implications

• Results of non-invasive analysis of modern artworks using MOLAB platform presented. • Novel insights into varnish and paint layer detailing through multimodal approach. • Accurate polymer identification informing creation of research mock-ups for testing. • Enhanced artwork material data boosting conservation strategies in museums. • Multimodal study pioneering non-invasive methodology for conservation research. This study presents the results of a non-invasive analytical investigation into eight varnished modern paintings from the Norwegian National Museum, dating from 1966 to 1982. These works, executed in either waterborne acrylic dispersion resin or polyvinyl acetate (PVAc) media, have not been examined before. They form part of a broader research initiative aimed at documenting the use and degradation of synthetic, waterborne varnishes over acrylic and vinyl paints in the context of Norwegian modern art, and the implications for conservation treatments. The eight paintings were examined through the mobile laboratory (MOLAB) platform of the European infrastructure for Heritage Science (E-RIHS). Multiple complementary techniques, including optical coherence tomography (OCT), external reflection infrared spectroscopy (ERIRS), x-ray fluorescence (XRF) mapping and optical micro-profilometry, were employed to identify binders and pigments, assess varnish thickness and distribution and document the layer structure without sampling. The results of a central case study, Winter Sun , 1966, by Gunnar S. Gundersen (1921–1983) (MS-02579-2988, acrylic on canvas, 165×110 cm), are presented including characterisation of the layer structure, and identification of the paint and varnish compositions. Comparative analysis between the case study paintings highlights both prevalences and differences within the materials used, informing material history and artistic practices within the mid-century Norwegian context. This in turn informs conservation approaches to the as-yet unresolved challenge of varnish removal from waterborne modern paints. The findings have the potential to inform the future design of representative mock-ups to test potential varnish removal systems. This multimodal, non-invasive approach offers new tools for understanding artist materials and supports conservation decision-making by clarifying when and how varnish removal may be appropriate.