Oxide Transformation During Preparation of Black Pottery in Hungary

Abstract

Traditional black pottery produced in Nadudvar, E-Hungary, was studied by 57Fe Mossbauer spectroscopy, X-ray diffractometry and microscopy. Quartz, feldspar, clay minerals (kaolinite, smeetite, illite) and calcite were identified in the basic clay material by X-ray diffractometry (XRD). Mossbauer spectroscopy (MS) of the original clay revealed that about 35% of iron compounds were present in goethite while the rest in clay minerals (illite and smectite). After firing the clay in air using an electric furnace (red pottery is prepared in the same way), the Mossbauer spectra showed hematite as the only iron oxide or hydroxide phase, being in good agreement with X-ray diffractometry. In the black product itself, fired in the traditional open-flame furnace, the Mossbauer spectra reflected the presence of iron in magnetite and in sheet silicates with approximately the same relative ratio of oxides and silicates as in the starting material. This can be interpreted as a result of the transformation of goethite to hematite in the first step of firing (in air), and as a reduction of hematite to magnetite in the second step of firing (closed from air). A significant difference was found in the distribution of iron at the Fe2+ and Fe3+ cation sites in the black surface (more Fe2+) and at the dark gray bulk of the fired pottery (less Fe2+), showing that the reduction of Fe3+ occurs in the silicates instead of further reduction of the magnetite (e.g., to wustite).