In search of the optimum Raman/IR signatures of potential ingredients used in San/Bushman rock art paint

Abstract

Vibrational spectroscopy (Raman and FTIR) has played an important role in identifying pigments, substrata and deterioration products in rock art studies worldwide: in the laboratory and on-site. However, the detection of organic binders and carrying agents has so far been scarce and the quality of many spectra recorded on-site inadequate. In this study, possible pigments (charcoal, ochre, raptor faeces, thermally treated ostrich egg shell, etc.), binders (fat, egg, blood) and carrying agents (saliva, gall, egg, water) were selected based on artistic considerations and analysed with FTIR and Raman (514.6 and 785 nm excitation, both available in mobile instruments) spectroscopy in order to determine usable marker bands for each ingredient. The resultant marker bands were then used to analyse five ten year old San replica paints. It was found that FTIR spectroscopy is very efficient to identify organic compounds as there is no fluorescence but the broadness of the bands inhibits the exact assignment of many ingredients. A high fluorescence background experienced for many natural products prevented the recording of Raman spectra for all ingredients, in many instances though the sharp peaks usually associated with Raman spectra make identification easier than with FTIR spectroscopy. Most of the ingredients in the paints could be identified, but it is clear that better results are obtained when more that one technique is used.