Oxidation states of iron as an indicator of the techniques used to burn clays and handcraft archaeological Tupiguarani ceramics by ancient human groups in Minas Gerais, Brazil

摘要

Ceramics of the Tupiguarani Tradition typically have in common the burning characteristics, their forms and decoration motifs. Dating such ceramic pieces with the radiocarbon method indicate that these artifacts were probably handcrafted between 1,500 and 500 years before the present. Fragments ceramic utensils were collected in the archaeological site of Beltr?o, in the municipality of Corinto, state of Minas Gerais, Brazil. A fragment of about 50 mm in diameter and 15 mm thick, with a color gradation across the ceramic wall ranging from red, on one side, grayish, in the middle, and orange, on the opposite side, was selected for a more detailed examination. The fragment was transversely cut and a series of subsamples were separated from different points along the piece wall, in layer segments of ～3 mm. All subsamples were analyzed with M?ssbauer spectroscopy at room temperature and 80 K. Results reveal that hematite is the magnetically ordered phase. A Fe~(2+) component (relative spectral area, 50 %) appears for the grayish subsample. According to these first results, the red subsample seems to be the side that had direct contact with fire used to burn the precursor clay in air. The grayish middle layer is probably due to the clay mixed with some ashes. M?ssbauer data reveal that the orange layer, corresponding to the opposite side of the ceramic relatively to the direct fire, does contain about the same Fe~(2+):Fe~(3+) ratio but much lower proportion of α-Fe_2O_3 than the red layer.