Cu(II)-binder complexes in azurite and malachite pictorial mixtures: An EPR study

Degradation processes of paintings, frescos, illuminations, and painted decorations represent a relevant challenge in the cultural heritage field for both conservators and scientists dealing with the causes of these processes. It is very important understanding the molecular origin of colour changes, like fading, yellowing, blackening, as well as the molecular origin of damages due to degradation, like cracking and fall of pictorial layers. Metal complexes, often indicated as “metal soaps”, when referred to metal carboxylates, play a relevant role in degradation. However, their identification is not easy, and the involvement of the metal ions in the degradation mechanisms is often not clear. Continuous Wave Electron Paramagnetic Resonance (CW-EPR) spectroscopy is a very sensitive tool towards paramagnetic species (e.g., transition metal ions like Fe(III), Cu(II), Mn(II), radicals, etc.), providing relevant information on these species, not always well-characterized with other methods. In this paper, we use CW-EPR to study the formation of Cu(II)-binder complexes in natural malachite and azurite pigments mixed with several organic binders. We show that CW-EPR technique is able to detect easily Cu(II) complexes with organic ligands, and can additionally provide information on the pigments themselves. In particular, the first coordination sphere of the Cu(II) complexes is characterized by applying the Peisach-Blumberg method. Moreover, CW-EPR shows that binders can affect the structure of the raw pigments.