Application of Raman microscopy to the characterization of different verdigris variants obtained using recipes from old treatises

Verdigris is an historical pigment of synthetic origin widely used in the artistic scope, from the antiquity to beginning of 19th century. It is a greenish or green-bluish colored product resulting from corrosion of pure copper and alloys caused by the action of different chemical reagents. The preparation recipes are numerous and appear in old texts, such as: treatises of art and texts of alchemy, as well as in books of secrets, natural history and those concerning medicines. A comparative study of these recipes shows significant differences depending on the initial components and the methodology applied in the synthesis of the pigment. Consequently, typical verdigris pigments very likely correspond to a variety of chemical compositions and, in addition, it might contain certain amounts of unknown by-products. To confirm such hypothesis, four different preparation recipes of verdigris have been carefully reproduced in our laboratory, and characterized by Raman microscopy. Our experiments allowed us to establish interesting differences among the studied samples. Some differences are mostly related to the ingredients used in the elaboration of the so-called raw verdigris. In other cases, the observed variations are consequence of the recrystallization treatment of the pigment. In general, all spectra reveal the existence of common component, namely, the copper(II) acetate (hydrated or anhydrous). However, other minority components have been detected in our samples, for instance, copper oxides, copper chlorides, and ammonic salts. In some cases, these compounds allow us to deduce the type of recipe used in the elaboration of the pigment.