Multiscale characterization of liquid and dry egg tempera paints based on ochre pigments

Abstract

Understanding the mechanical properties of paints is crucial for their preservation, as these properties determine how paints deform under climate-induced stress and, consequently, affect their durability. This study focuses on egg-tempera paints, whose mechanical characteristics have been minimally explored so far. Various formulations using two natural ochre pigments were investigated. Initially, the pigments were characterized to identify compositional and morphological differences. Rheological analysis was conducted to study the liquid properties of the formulations. Tensile tests and dynamic mechanical analysis were performed on unsupported dry paints to assess their mechanical properties. Additionally, single-sided NMR was used non-invasively to probe the network mobility before and after nine months of aging, providing insights into the network tightness. It was found that ochre-based paints display significant brittleness. The viscoelastic properties of tempera paints are predominantly influenced by the type of earth pigments used and the pigment-to-binder ratio. Importantly, the liquid properties were found to correlate with the solid-state behavior, emphasizing the critical role of formulation in the final performance of tempera paints.