Moessbauer Study of the Effect of Calcium Content on Iron Oxide Transformations in Fired Clays

Abstract

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.

Archaeological ceramic amphorae from underwater marine environments: Influence of firing temperature on salt crystallization decay

“…25 However, it must be taken into account that calcareous lumps in calcium-rich clay prevent iron fixation in the network of neoformed calcareous silicate and aluminosilicate lattices and, consequently, inhibit the nucleation of new hematite crystals. 39 The main difference between the two Iberian amphorae samples (Fig. 4a-b) lays in the absence in sample A-Ibe-1 of illite, biotite and calcite and the presence of the gehlenite (Ca 2 Al 2 SiO 7 )-akermanite (Ca 2 MgSiO 7 ) series of minerals, along with a higher hematite content (Fig.…”

Alteration and Contamination of Archaeological Ceramics: The Perturbation Problem

“…This evolution is supported by the high temperature experiment conducted on individual 25 (Figure 6), which exemplifies mineralogical category C1 in Figure 5, and by the results from SEM on the individuals ARS and the experiments of refiring (Buxeda, 1995a;Buxeda & Cau, 1995a). The equivalent firing temperatures were estimated from the data obtained by XRD and SEM, and can be divided into four groups: below 800-850ºC for C1 (low fired), around 950ºC for C2 (well fired) (crystallisation of gehlenite, pyroxene and plagioclase), between 1000 and 1050ºC for C3 (overfired) (decomposition of illitemuscovite and gehlenite, further development of pyroxene and plagioclase) and over 1100-1150ºC for C4 (severely overfired) (partial decomposition of quartz, crystallisation of leucite) (Maggetti, 1981;Maniatis et al, 1981). It is also important to notice that these results are in good agreement with such indications as the matrix colour (C1, brown or orange depending on the crystallisation of hematite; C2, light orange; C3, yellowish; C4, greenish), and the deformation of some individuals in category C4, because of the collapse due to the advanced liquid phase produced during firing (Picon, 1973).…”

Construction and Building MATERIALS

“…Contrary to what occurs in the bricks without additives, in these specimens, the presence of gehlenite can be related to that of haematite at very low concentrations. This takes place because, according to the observations of Maniatis et al [19], the Fe 3þ present in the specimens remains ''trapped'' in the structure of calcium silicates and aluminosilicates, as is the case of gehlenite, thus inhibiting the formation of haematite. The feldspar evolves into anorthite and the values of m are lower than in the bricks with additives.…”