Multianalytical characterization of pigments from rock paintings in Guanajuato, Central México

Abstract

Among the numerous displays of rock art that exist in México, the state of Guanajuato in the central part of the country can be noted for its abundance of sites displaying rock paintings. The Victoria valley at the northeastern part of the state has around 60 registered sites with these types of representations. Among them, one called La Sobrepiedra was explored with the aim to analyze the pigments employed in its rock painting motifs. La Sobrepiedra was chosen based on its iconographic variety and the deterioration it presents due to water filtration and weathering. Other nearby locations with rock paintings were studied as well: Arroyo Seco and La Cueva del Cuervo. These sites present different type of motifs, and in contrast to La Sobrepiedra, they are well preserved. A rich variety of pigment colors was found in La Sobrepiedra where red, yellow and black were observed, whereas for the other sites the colors were mainly red and some black. Samples carefully extracted from selected motifs were analyzed with Raman spectroscopy. A detached rock containing motifs was additionally recovered from Arroyo Seco and analyzed at the laboratory. In addition to the paintings, small mineral clusters were found close to La Sobrepiedra and Arroyo Seco and analyzed as well, since they could have been used to produce the paintings. The Raman spectra of the examined pigments identified them as hematite, goethite and carbon from organic origin. Additionally, minerals from the support rocks as well as deterioration products were identified. Complementary analytical techniques, including SEM-EDS and infrared spectroscopy were used to confirm the Raman findings. It was also possible to perform a 14C dating for one black anthropomorphic figure from La Sobrepiedra. The possible provenance of the mineral pigments and relevance of the present findings are discussed.