Revisiting the identification of commercial and historical green earth pigments

Abstract

Green earth is a common green pigment based on celadonite and glauconite, used since Antiquity by artists. Two geological minerals, eight commercial green earth pigments and a sample taken from a historical location in Monte Baldo were characterized. A set of different techniques including X-Ray diffraction (XRD), scanning electron microscopy coupled to energy dispersive spectroscopy (SEM-EDS) and numerous spectroscopies: spectrophotocolorimetry, near and mid infrared, Raman, Mössbauer were used to identify the structure and composition of the different earths. The results highlight complex composition with the presence of various phases, which can be due to the pigment sampling at a different location in the same deposit. Mobile and non-invasive analyses were carried out in order to suggest a protocol for the identification of green earth in artworks, and more specifically to distinguish celadonite and glauconite. With the available mobile non-invasive techniques, and the above analyses on the raw pigments, the green area in Nicolas Poussin's painting, Bacchanales d'enfants (Galleria Nazionale d'Arte Antica (GNAA), Rome) was examined as a case study.

Time-of-Flight Secondary Ion Mass Spectrometry Imaging of Cross Sections from the Bacchanals Paintings of Nicolas Poussin

“…The detection of Al + , Si + , Fe + , Mg + and K + ions in several round shaped areas indicates a green earth, as previously observed on Poussin paintings. 18,19 It is a natural mineral green pigment, composed of celadonite K(Mg,Fe)(Fe,Al)[Si4O10](OH)2 and/or glauconite (K,Na)(Fe III ,Al,Mg)2[(Si,Al,Fe III )4O10](OH)2, depending on the geological origin. Large dark green particles with very precise edges are visible, in a lighter green mixture.…”

“…However, according to Fanost, et al the garland contains a celadonite-rich green earth. 19 Therefore, either Poussin used two kinds of green earth in the painting depending on the area, or TOF-SIMS would not be able to distinguish between glauconite and celadonite in mixtures. The rest of the layer is a green-yellow mixture containing many pigment particles.…”

Clay Minerals in European Painting of the Mediaeval and Baroque Periods

“…In conclusion, therefore, we can note that recently commercially available Verona green earth is brownish-green and contains mostly smectite, as the celadonite from this source is almost exhausted (similarly to Kadaň a hundred years ago) and that under the name "Bohemian green earth" light green glauconites from Prague area are provided by commercial suppliers. Not in all cases these facts are considered even in recent comparative studies [48].…”

Green Earth pigments aqueous dispersions: NMR relaxation rates dataset

“…Their main mineral composition is listed in Table 1 of JCIS article according to Fanost et al. [1] . As France GE was not studied in Fanost et al.…”