THE CHEMISTRY OF SIMPLE CERAMIC GLAZES .1. MOSSBAUER STUDIES OF IRON-CONTAINING GLAZES AND GLASSES

A simple feldspathic glaze which matures at 1280°C and which contains iron as a colouring agent was examined by 57Fe Mössbauer spectroscopy. When fired in air, oxidation of the iron (added as iron(II) oxalate) was complete and the iron(III) occupied a distorted octahedral site. By contrast a soda-lime-silica glass, fired under the same conditions, showed two iron sites, one octahedral [iron(III)], the other tetrahedral, although the oxidation state is uncertain. Firing under dinitrogen produced glazes containing iron(II) and iron(III); the iron was added as iron(II) oxalate or as a mixture of iron and Fe2O3. The chemical environment of the iron in the matured glaze was shown to be a function of the way in which the element is introduced into the raw batch, to the firing conditions, and may also be influenced by the replacement (gradual or complete) of CaCO3 with SrCO3 or BaCO3. Using this simple glaze, good reproduction of Chinese "mirror black" and celadon were achieved by firing under dinitrogen and manipulating the form in which iron was added. Preliminary studies of iron-vanadium glazes are reported. Vanadium(IV), or VO2+, was identified in the matured feldspathic glaze by ESR. The addition of iron (as metal) produced glazes, the ESR spectra of which were dominated by contributions from iron(III). The Mössbauer spectrum of a glaze containing 6% iron confirmed the presence of iron(III) but also revealed the presence of another iron site. The data are discussed in terms of an "FeO2+" species. © 1990.