M?ssbauer and XRD studies of Roman amphorae buried in the sea for two millennia

摘要

During the years 2004-2007 many Roman amphora sherds were found in the sea near Corti?ais, off the southern coast of the Peniche peninsula on the Atlantic coast of Portugal. The amphorae are of the Haltern 70 type and stem from a shipwreck that has been dated to the time of the emperor Augustus, between about 15 BC and 15 AD. They were produced in the Roman Province of Baetica in the south of Spain and used to transport wine and other staple foods by sea to other Roman settlements.We have studied several fragments of these amphorae by ~(57)Fe M?ssbauer spectroscopy and X-ray diffraction in order to look for changes in the ceramic material caused by two millennia of exposure to sea water. For comparison M?ssbauer data on a Haltern 70 type amphora handle excavated on land at the site of Castro do Vieito in the north of Portugal were used. The fragments on which we report here are a body sherd and two handles. The sherds show a visible layer structure. The different layers were studied separately. The M?ssbauer spectra of the buff surface layers indicate that up to about 60 % of the iron is present as very fine goethite particles, which are superparamagnetic at RT but exhibit magnetically split spectra at 4.2 K. Their blocking temperature is around or even below 50 K. The goethite is too fine grained to be detected by X-ray diffraction. Re-firing experiments confirm the presence of goethite, which is found to convert to hematite between 300 and 600 °C. The results show that the iron in the silicate matrix of the ceramic material converts to goethite under the prolonged influence of the sea water, mainly in the outermost several millimetres and apparently depending on the nature of the ceramic material.