Implementation of EPR spectroscopy for the study of different synthetic copper green-bluish pigments obtained from antique recipes

The preparation of different green-bluish copper pigments via synthetic routes is reported in several treatises, from antiquity to the beginning of 19th century. The most famous preparation is the one for the production of verdigris, a bright green pigment resulting from the corrosion of pure copper plate by vinegar. The recipes are numerous, and many differences in terms of ingredients and procedures emerge from a comparative study. Such a variety of preparations corresponds to a chemical variety. Many works have already investigated such systems with the most common spectroscopic techniques (e.g., ATR-IR, Raman, etc.). However, recently the Electron Paramagnetic Resonance (EPR) spectroscopy has emerged as a new tool for the investigation of these pigments, since new and more complete information on the chemistry of such systems can be gained. In this work, three different recipes have been selected as more representative, reproduced, and characterized with EPR, micro-ATR-IR and micro-Raman spectroscopies. Our experiments highlight the advantages in the use of EPR, with respect to simple Raman and IR investigations. In particular, the EPR spectroscopy evidenced the presence of different bimetallic and monometallic species in the samples, allowing us to differentiate and characterize the Cu(II)-complexes among the studied samples. • The ingredients included in ancient recipes for green/bluish pigment preparations actually impacts on the pigment chemistry. • The ATR-IR and Raman are not always the most suitable tools. • CW-EPR spectroscopy is a very informative, non-damaging technique that can unveil the coordination of Cu(II) cations. • CW-EPR can overcome some ATR-IR and Raman spectroscopies drawbacks and limitations. • EPR spectroscopy must be included into a multispectroscopic protocol to study ancient copper-based pigments.