Analysis of green copper pigments in illuminated manuscripts by micro-Raman spectroscopy

Abstract

In the majority of the literature describing green coloured materials used on ancient painting layers (15th or 16th century), two copper greens are mainly cited: malachite [CuCO₃·Cu(OH)₂] and verdigris [Cu(CH₃COO)₂·[Cu(OH)₂]₃·2H₂O]. It is shown, by micro-Raman spectroscopy, that the artists were actually employing more than these two copper greens, in particular various copper sulfates, among which the most common pigment found is posnjakite [CuSO₄·3Cu(OH)₂·H₂O]. In contrast to the PIXE (particle induced X-ray emission) technique, Raman spectroscopy is a technique of choice, able to distinguish not only a copper sulfate from a carbonate or acetate but also the different sulfates themselves; in this respect, we found that the high wavenumber region (2800–4000 cm⁻¹), characteristic of H₂O vibrations, is of particular interest. It is also shown that numerous green areas were created with mixtures of a copper sulfate mixed with other pigments, for instance to enhance the colour depth. Finally, in some cases, no green pigment is actually employed but the colour is obtained by intimately mixing yellow and blue pigments. All these results led to a new look at the pigments which were in use on the palettes of the ancient artists.