Preliminary investigations by Raman microscopy of prehistoric pigments in the wall-painted cave at Roucadour, Quercy, France

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.