Tem observations of goethite dehydration: application to archaeological samples

Abstract

Red and black were the two colours around which Palaeolithic art was organised. Manganese oxides and charcoal were the black pigments and hematite (α-Fe 2O 3) the red one. The latter mineral is naturally abundant, but archaeological observations nevertheless suggest that the well-known colour change accompanying the dehydration of yellow goethite (α-FeOOH) to red hematite may have been employed by Prehistoric artists to obtain red pigment. In order to confirm this assumption, a study was carried out on synthetic goethite samples using XRD and TEM. In particular, the goethite-to-hematite transformation was observed in situ and provided useful information about both dehydration and recrystallisation processes. The existence of water up to high heating temperatures was found to be coherent with the typical porous microstructure accompanying the phase transformation. Similar studies were carried out on archaeological hematites coming from Troubat, a French Pyrenean Palaeolithic site. Characteristic features of previous heating were identified, such as pores or small amounts of maghemite (α-Fe 2O 3), which confirms that Prehistorics had acquired this technical know-how.