The detection of copper resinate pigment in works of art: contribution from Raman spectroscopy

Abstract

Copper resinate is a green pigment widely used by the 16th century painters, as many surveys on Italian and European paintings proved. The pigment is a transparent green glaze, and its color is due to copper salts of resin acids. The oldest recipes suggest the preparation of copper resinate by mixing verdigris with terpenic resins as Venice turpentine (conifer resins) on hot ashes. The detection of copper resinate in paintings is up to this time an analytical challenge. We examined the Raman features of copper resinate (powder and in a mock paint film with linseed oil) and compared them with verdigris. Six laser sources (488, 514, 532, 633, 785, and 830 nm) were used in different laboratories to highlight the drawbacks and advantages of a specific excitation source. The obtained results were applied in the analyses of a famous Caravaggio's painting. For a detailed interpretation and full exploitation of the Raman spectra, Fourier transform infrared spectroscopy measurements were carried out as well. Copyright © 2014 John Wiley & Sons, Ltd.

Raman spectroscopy of green minerals and reaction products with an application in Cultural Heritage research

“…Raman spectroscopy proved successful in identifying these compounds, with some significant differences related to the excitation wavelength used . The best overall results seem to be obtained with a 785‐nm laser (detection of both verdigris and copper resinate, even when mixed with linseed oil) …”

“…In case OH groups are present, bands appear at 3050–3650 cm −1 . Slightly different band positions and assignments can be found in . Also, the results from the study of the pigments themselves are not straightforwardly applicable to real samples, as the bands of binding media overlap with the organic part of these greens, and binding media alteration because of ageing can also have an effect .…”

Identification of artistic materials in paintings and drawings by Raman spectroscopy: some challenges and future outlook

“…Nastova et al analyzed a green pigment in a medieval Slavonic illuminated manuscript by Raman spectroscopy and SEM‐EDS and concluded that it contained an organo complex of copper, and that further identification of the compound/s was not possible by these techniques . Conti et al acquired Raman and FTIR spectra on two fresh copper resinate commercial samples and on a commercial verdigris, as powders and mixed with linseed oil, using different excitation wavelengths, and reported the analysis of a cross‐section sample removed from a painting by Caravaggio, where they suggest the presence of a copper resinate. These authors state that differentiating between copper resinate and verdigris when mixed with oil becomes complex, and it is expected that this task is even more so in aged samples.…”

Raman Spectroscopy of cultural heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing

“…Research focused on reconstructing the chemistries of production of traditional pigments following ancient recipes also includes the characterisation of deterioration products [70,79,101,270]. Occasionally, spectra of pigments that have not been previously characterised are still presented in the literature and studied in depth, significantly enriching available libraries [28,55,290].…”