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The transformations induced in two clays differing in Ca content, by firing under reduced conditions up to 1080°C, were studied by X‐ray diffraction, scanning electron microscopy, Mossbauer, and magnetization measurements. In the calcareous clay, gehlenite forms at lower temperatures (900°C) and, in addition, wollastonite forms at higher temperatures (1080°C). Ferric iron persists even under strongly reducing conditions and its presence is attributed to trapping in gehlenite. Extensive vitrification is observed in the non‐calcareous clays. Ferrous iron, produced by dissociation of iron oxides, is partly dissolved into the vitreous matrix and partly incorporated into the spinel mineral hercynite. The key role of Ca in controlling the above transformations was verified by studying the clays after removal or addition of calcite. The interaction of Ca with the clay constituents and its progressive attack on the quartz grains forming wollastonite zones was observed with the electron microscope. The bulk magnetic properties of the samples depended principally on the amount of iron oxides present, which in turn were strongly affected by firing temperature and type of clay. Metallic iron was detected in strongly reducing atmospheres.

A formally FeIII7FeII complex, containing an inner Fe4O4-cubane and four peripheral Fe-centers, is derived from the one-electron reduction of its FeIII8 precursor. Spectroscopic analysis of the former reveals that the redox activity of this Fe8-system is confined within its cubane core. The resulting (Fe4O4)3+-cubane, which is valence-delocalized in the NMR, Mössbauer and IR, but valence-trapped in the X-ray photoelectron spectroscopy (XPS) timescale, is better described as a Robin-Day class-II system by the analysis of its near infrared (NIR) intervalence charge transfer (IVCT) band profile.

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Sixteen clays were studied by Moessbauer measurements in their unfired and fired forms. Thirteen of these clays were collected near ancient Greek sites and the remaining three in England and the United States. They were fired at 700°, 900°, and 108OOC to investigate the physicochemical transformations of the clay components in this temperature range. The results show distinctly different responses of the clays, according to their CaO content. Firing of the calcareous clays (>5 % CaO content) at 7OOOC produces small oxide particles (mainly hematite). This phase remains stable on further firing up to 1O8O0C, where it decreases slightly. Contrary to this behavior, the noncalcareous clays display a continuous increase of the amount and size of the oxide particles when fired up to 1O8O0C. This difference is attributed to stable calcium aluminosilicate phases which are formed in the calcareous clays as opposed to the continuous vitrification which occurs for the noncalcareous clays by firing up to 1O8O0C. The variation of the quadrupole splitting of the central doublet with firing temperature also differs for the two types of clays. The quadrupole splitting values among the clays span a range of 1.1 to 1.6 mm/s for a firing temperature of 700OC. Above this temperature, a distinct separation of the two groups occurs with the quadrupole splitting of calcareous clays lying higher.

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