New insights to the interactions between amorphous georgite pigment and linseed oil binder that lead to a drastic color change

Abstract

Understanding pigment-binder interactions is very important to predict final paint colors and paint stabilities, which is vital for art-making and art conservation. However, previous effort has been focused on long-term chemical stabilities following exposure to the elements, while initial interactions between the pigment and the binder are less understood. Here, we report an investigation into two polymorph basic copper carbonate pigments, georgite and malachite synthesized by varying reaction time and their interactions with linseed oil directly following the paint curing process. Despite the drastic differences in color of these two pigments, incorporation of them into linseed oil binders, resulted in a surprising color change for the georgite based paints from blue to green. FTIR showed a new peak ∼1575 cm−1 in both paint samples, which was assigned to a Cu2+-carboxylate vibration. Furthermore, EPR spectra of the paints exhibited anisotropic Cu2+ signal, indicating a square-planar Cu2+ molecular complex. These results indicated that carboxylic acid biproducts of the linseed oil autooxidation process were able to efficiently extract Cu2+ from the georgite structure to form new and well-distributed Cu2+ molecular complexes throughout the paint, leading to a color change which resulted in similar final paint color compared to the malachite pigment-based paint. This work has demonstrated the structure-property relationship of two polymorph pigment materials, provided new insights to the interactions between amorphous georgite pigment and linseed oil binder that lead to a drastic color change, and revealed the importance of understanding pigment-binder interactions which can be used to predict final paint colors and enhance pigment stabilities.