Green earths' are employed since antiquity as pigments in the creation of artworks. The minerals responsible for the colour belong to four groups: (1) the clayey micas celadonite and glauconite, undoubtedly the most common; (2) smectites; (3) chlorites; (4) serpentines. Whereas there have been several studies on clayey materials, mineralogical analyses in the field of cultural heritage are mainly limited to the identification of the green earth without specific characterization of the mineralogical species. This work shows a preliminary characterization by the multi-techniques approach of some raw minerals (glauconite, celadonite and ferroceladonite). Vibrational analyses have been correlated with elemental analyses, thanks to the hyphenated instrumentation of scanning electron microscopy with EDS and Raman structural and chemical analyser (SEM-EDS-SCA) probes, which permitted collection of EDS and Raman spectra on the same microscopic area. Micro-Raman and Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopieswere able to distinguish between celadonite and glauconite. The use of different lasers revealed resonance effects in the Raman spectra. In addition to pure minerals, archaeological samples and commercial green earths were also analysed, thereby enabling a more precise classification of the green pigments in heterogeneous samples such as wall paintings. Some commercially available green earths were found to contain organic dyes.

Raman microscopy (RM) was applied to an ongoing study of prehistoric pigments employed in Palaeolithic wall paintings in the Roucadour Cave (France). Micro-samplings were carried out on parts of red or black painted figures representing animals and also negative human hands.These first analyses showed that in all the red micro-samples, the main pigment is haematite. Where the tone was dark red, well-formed crystals of haematite were identified, but magnetite, Mn oxide and carbon grains were also found. An intermediate mineral phase structurally in-between goethite and haematite was also identified. In red samples, the yellowish hydrated species goethite was also detected. A great quantity of white, greyish-white and yellowish-white crystals were present in the pigment, and many of them were identified as calcite or quartz.The black pigments were constituted of well-formed crystals of Mn oxyhydroxide in some black microsamples. Suggestions are made for the identification of some mineral species (bixbyite, hollandite, nsutite) but there is still much uncertainty on this topic. In others, amorphous carbon grains were utilised as the main pigment. This distinction is important for orienting the research towards a subsequent investigation by radiocarbon dating.Traces of anatase were found in black pigments, as were traces of rutile and gypsum in red pigments. It is interesting that haematite occurs in all the black pigments, black because of the great content of C or of Mn oxyhydroxide, and that several red pigments contain minor carbon, such that each pigment is a mixture with at least five different species in most micro-samples. These data are compatible with the possibility that the prehistoric artists used naturally-occurring impure geological materials (e.g. (Fe,Mn)-ochre rocks) and natural biological materials (e.g. wood or oil to make charcoal or soot), but it cannot exclude the fact that they may have treated the raw materials and/or manufactured mixtures of purer materials.

No abstract

No abstract

No abstract