In situ X-ray fluorescence analysis of rock art paintings along the coast and valleys of the Atacama Desert, Northern Chile

Results of exploratory in situ analyses with a portable X-Ray Fluorescence (XRF) applied to six rock painting sites, located from Lluta to Camarones coastal valleys in the Atacama Desert, northern Chile, are presented. The results indicate the use of iron oxide as a main component in all paintings, alone or combined with aluminum silicate. Arsenic, a component highly present in water in the region, was also identified, indicating the use of water as a possible binder or thinner. These output allow unraveling painting technology, a step forward to define and understand the stylistic variability of rock art (engravings and paintings) in the context of the social changes and continuities of prehistoric communities. Additionally, it illustrates the effectiveness of portable XRF, a non-destructive analytical tool, applied for the first time in the Atacama Desert. 1-3 , specially in Europe and the United States, because of its ability to identify the presence of elements in major and minor amounts, and its non-destructive and non-invasive method of application. The improvement of portable equipment has contributed over the last 40 years to increasing its use, as, in many cases, it allows in situ analysis without sample extraction. This technique is also valued for its relatively low cost and short period of analysis 3 . The use of portable instruments, such as Raman and especially X-ray fluorescence, has proven quite useful for the physical- chemical characterization of Paleolithic art and more recently for rock paintings by contributing to their preservation and preventing unnecessary sampling 4-11 . In contrast, until recently, most of the work undertaken in Chile to identify paint components has involved invasive and destructive laboratory sample analysis methods, by means of X-ray diffraction 12-14 or scanning electron microscope combined with energy dispersive X-ray spectroscopy 15-19 . Moreover, rock art paintings have been generally studied as images or iconographic representations, while the material aspects of these visual manifestations have been ignored because, among other issues, the lack of trained people in these methods and techniques, and the absence of specialized laboratories in Chile, of the kind where these samples were processed. Paintings are embedded in complex production processes, from obtaining raw materials up to applying them onto a wall, followed by its subsequent use, maintenance, recycling, discarding or abandonment 20-22 . The selection or use of specific formulas in rock art painting can be as stylistic as the production of the images themselves. A figure is stylistic when it displays a selection of attributes of the real referent combined and arranged in a regular and specific way 23 . The composition of the panels, the use of certain techniques, and the selection of certain platforms or spaces can also reflect stylistic decisions. Thus, XRF analyses can help to find out whether dissimilar painting components or combinations of elements were used in rock art styles. In future research, we aim to strengthen the relation between physic and chemical results and cultural changes in the prehistory of northern Chile. Along the coast and valleys of the Atacama Desert, in northern Chile, rock painting is a rare form of expression—only 8 sites have been identified to date— which contrasts with the large number of engravings and geoglyphs recorded. These images were created during a wide temporal range, from the Formative period or "Andean Neolithic" time (ca. 2500 B.P) up to the beginning of the European colonial era (16th century)