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

摘要

In th emajority of the literature describing green coloured materials used on ancient painting layers (15th or 16th century),two copper greens are mainly cited:malachite [CuCO_3 centre dot Cu(OH)_2] and verdigris [Cu(CH_3COO)_2 centre dot[Cu(OH)_2_3 centr dot 2H_2O].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 [CuSO_4 centre dot 3Cu(OH)_2 centre dot H_2O].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 form a carbonate or acetate but also the different sulfates themseleves;in this respect,we found that the high wavenumber region (2800-4000cm~-1),characteristic of H_2O 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 empolyed 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 palettes of the ancient artists.