Raman spectroscopy applied to iron oxide pigments from waste materials and earthenware archaeological objects

Abstract

Raman spectroscopy is a vibrational spectroscopic technique. It gives a unique combination of non-destructive analysis, high spatial resolution and phase characterisation. In the current study Raman spectroscopy was used as the primary technique during the study of chemical components in archaeological earthenware samples (i.e. low temperature fired clay pottery) of South African and Chinese origin, and characterisation of iron oxides derived from mill scale. One shard from each of the South African archaeological sites (Rooiwal, Lydenburg, Makahane and Graskop) was analysed by Raman spectroscopy, FT-IR spectroscopy, X-ray fluorescence (XRF) spectroscopy and X-ray diffractometry (XRD). The common features observed were montmorillonite (Mg3(Si,Al)4(OH)2.4.5H2O[Mg]0.35), kaolin (Al2Si2O5(OH)5), quartz (α-SiO2), feldspar (K- and NaAlSi3O8), hematite (α-Fe2O3), calcium silicate (CaSiO3) and illite (KAl4(Si7AlO20)(OH)4). Gypsum (CaSO4.2H2O) and calcium carbonates (CaCO3) were detected in Lydenburg, Makahane and Graskop shards. Amorphous carbon was observed in Lydenburg and Makahane shards while rutile appeared only in Makahane shard. The Lydenburg and Rooiwal shards showed the presence of anhydrite (CaSO4). The Chinese clay samples investigated by Raman spectroscopy were from the J A Van Tilburg museum at the University of Pretoria. The large red shard was recovered from the 1552 Portuguese shipwreck, São João, found around Port Edward, South Africa. Four other shards (two red and two gray) were recovered from the 1622 Portuguese shipwreck, the São João Baptista, found around Kenton-on-Sea off the South African coast. A 19th century Chinese teapot was also analysed. Hematite, kaolin, quartz, amorphous carbon and aluminosilicates were observed in all three red shards. All these components, except quartz, were also observed in the teapot. The gray shards showed the presence of quartz, kaolin, amorphous carbon and aluminosilicates. The pigments identified were hematite (in red samples) and black amorphous carbon (in all samples). Magnetite and goethite were precipitated from mill scale-derived precursors in aqueous media. Hematite was then prepared from the calcination of goethite at 750°C and maghemite from the thermal treatment of magnetite at 200°C. The iron oxides were characterised by Raman spectroscopy, XRD, surface area determination and scanning electron microscopy (SEM). They were generally composed of very small sized particles showing high surface area values.