Gianluca Catanzariti

High coercivity remanence in baked clay materials used in archeomagnetism

A study of the high coercivity remanence in archeological baked clays has been carried out. More than 150 specimens from 46 sites across Europe have been analyzed, selected on the basis of the presence of a fraction of their natural remanence that was resistant to alternating field demagnetization to 100 mT. The study was based on the stability of isothermal remanence to alternating field and thermal demagnetization and its variation on cooling to liquid nitrogen temperature. Results indicate that the high coercivity remanence may be carried by magnetite, hematite, and in isolated cases partially oxidized magnetite and goethite. In addition, a high coercivity, thermally stable, low unblocking temperature phase has been identified. The unblocking temperatures of both the isothermal remanence and the alternating field resistant natural remanence exhibit similar unblocking temperatures, suggesting that the same phases carry both signals. The high coercivity, low unblocking temperature phase contributes to the natural remanence, sometimes carrying a stable direction and behaving ideally during palaeointensity experiments and sometimes not. An unambiguous mineralogical identification of this phase is lacking, although likely candidates include hemoilmentite, related to clay source lithology, and substituted hematite or magnetic ferri-cristabolite, both possible products of thermal transformation of iron-bearing clays.

Roman amber identified as Siegburgite

A small amber fragment from the period of Roman occupation of Numantia (Garray, Spain) was characterised by Fourier transformed infrared spectroscopy and optical microscopy. The material was found to be a polystyrene-like material, which is a very rare fossil resin known as Siegburgite or class III amber. Until now, this material was found in Europe only at two sites, both localised in Germany, and it is the first time that this type of amber was identified in wrought archaeological artefacts. The discussion includes the applicability of the method to distinguish false from true and amber of different origins, its use and trading as well as a hypothesis for a transport route. The study highlights the need for a more systematic study of archaeological amber remains. The finding also proves the exploitation and trading of local, non-Baltic amber deposits throughout the Roman Empire.