Copper-Binder Complexes characterization in Blue and Green Pigments and in Illuminated Medieval Manuscripts by EPR/SERS/RAMAN spectroscopies

The degradation of copper-based pigments is very well known since ancient times, and many efforts have been produced to assess the chemical mechanism and the reactions on which the process lays on. Unfortunately, a complete and wide characterization is not yet available. The aging processes of copper pigments, their role on the notching of the supporting materials upon which these pigments were applied (e.g., parchment, paper, wooden boards, etc.) and the chemical modification of organic binders (e.g., collagen, linseed oil, etc.) represent ones of the most relevant problems in the cultural heritage conservation. As said before, the degradation proneness of these pigments is known since ancient times, and is very well reported in one of the most famous books on pictorial techniques: “The Book of the Art” by Cennino Cennini. In this text, the author refers to the copper acetate, also called verdigris, using these words: “Grind it with vinegar, which it is its nature to retain; and it makes a green for grass perfect and beautiful to the eye, but not durable.” In recent year, with the developing of many spectroscopic and analytical techniques new pieces of knowledge in cultural heritage chemistry have emerged. Nevertheless, many aspects of the reactivity of these systems remains hindered. The inhomogeneous degradation of supporting materials, the darkening of binders, and the change in color and appearance of the pigments suggest that the pigment itself is not simply dispersed in the ligand matrix as in a colloidal-like material, but the metals ions (Cu(II) in this case) are dissolved, creating a metal-ligand complex able to react and catalyze degradation processes. It appears evident that giving proofs of the formation of these complexes, and assessing their reactivity represent a priority to conserve the cultural heritage and study new strategies for conservation. In this context, the aim is to study the copper-organic ligand complexes in raw pigments and pigment-binder mixtures, and their reactivity via a multi-technique approach. The main novelty is to exploit the high-selectivity and specificity of the Electron Paramagnetic Resonance (EPR) spectroscopy towards the detection of paramagnetic species (radical, Cu(II) cations, etc.). In addition, the EPR spectroscopy allows us to gain more information on the chemical surroundings of paramagnetic species by the implementation of pulsed methods. In particular, the so-called hyperfine spectroscopies (ESEEM, ENDOR, HYSCORE, etc.) allow monitoring the first and the second coordinative shells of the metal-binder complexes, which give the molecular “fingerprint” obtained by the EPR methods. The data obtained by the latter are then coupled with largely used spectroscopic technique (Raman, FORS, ATR-IR, etc.), theoretical calculation methods (DFT), and spectrometric methods (GC/MS, Py-GC/MS) shedding light on the molecular and bulk material characteristics.