Archaeometric analysis of prehistoric rupestrian paintings from the Toca do Estevo III site, Piauí, Brazil

Abstract

The “Toca do Estevo III” is an archaeological site located in the rural area of the municipality of João Costa, state of Piauí, Brazil. The sandstone walls of the shelter are decorated with prehistoric rupestrian inscriptions (zoomorphic and anthropomorphic motifs over a background figuring a large white ounce), painted mainly in light and dark red hues, but also in white and in yellow. The chemical-mineralogical analyses of three samples containing pigments of the prehistoric rupestrian paintings, along with the material of a saline efflorescence, were made in the laboratory by (i) energy dispersive X-ray fluorescence (EDXRF); (ii) 57Fe Mössbauer spectroscopy at 298K, 110K and 21K; (iii) Fourier-transform infrared spectroscopy (FTIR); (iv) scanning electron microscopy (SEM); (v) energy dispersive spectroscopy (EDS) and (vi) powder X-ray diffractometry (XRD). Results indicate that the pigments of the reddish paintings were essentially prepared with a clayey material, which was rich in hematite (α-Fe2O3). Goethite (α-FeOOH), but also some hematite, was found in the yellowish-red painting. From the Mössbauer data, part of the very fine particles of these iron-bearing minerals behaves superparamagnetically; the magnetic relaxation appears to be significantly blocked at ~100K. The saline efflorescence was found to consist mainly of a complex mixture of salts: K2SO4·7KHSO4·H2O; 5ZnO·Al2O3·ZnSO4·15H2O; Ca2Al4Si4O15(OH)2·4H2O and Ca2Al4Si8O24·12H2O. The 57Fe Mössbauer spectroscopy is reaffirmed to be a central technique on the chemical and mineralogical analyses of iron-rich materials from the archaeological heritage, as pigments of prehistoric rupestrian paintings.