THE EFFECT OF FE-OXIDES ON THERMOREMANENT MAGNETIZATION OF POTTERY: PRELIMINARY RESULTS

Previous archaeometric studies on potsherds belonging to Palma Campania facies excavated in a Bronze Age village close to the town of Afragola (Naples, Italy), showed that TRM analysis was useful to detect possible heating in a range between 150 and 680°C, and that the magnetic behavior of potsherds is not only determined by the raw materials, but also by their processing and firing. A selection of 8 potsherds with strong differences in TRM was analysed with 57Fe Mössbauer spectroscopy to understand the role played by Fe-oxides formed during firing in the magnetic behavior of the ceramic body. All the pottery samples were made using a fine non-calcareous clay tempered with grog and fired between 600-800°C in variable redox conditions. The magnetic behavior of the potsherds was not explainable with the presence of magnetite and/or hematite already existing in the clay or formed during firing. 57Fe Mössbauer spectroscopy was performed on portion of potsherds of different color in order to understand the distribution of Fe species in the mineralogical assemblage. The Fe(II) over Fe(III) ratio was estimated and correlated to local variation in firing/heating atmosphere. RT measurements suggest the presence of microcrystalline (dimension > 10 nm) maghemite, not detected with XRPD, formed at the expense of Fe-hydroxides originally present in the clay which account for the heterogeneous magnetic behavior of the samples. Low temperature measurements will investigate on the presence of nanosized Fe(III) oxides, which existence can be related to competition between nucleation and growth processes during the firing. Our preliminary results point to an important evolution of the magnetic behavior of the ceramic body as consequence of phases formed during firing in the reaction path from Fe-hydroxides to hematite or magnetite. Low crystallinity and changing Fe(II)/Fe(III) ratio due to the redox conditions during firing, make hardly detectable these new-formed phases, but able to affect the magnetic behavior and the colour of the ceramic body.