Firing-induced transformations in Copper Age ceramics from NE Romania

Publisher： E. Schweizerbart'sche Verlagsbuchhandlung

ISSN： 0935-1221

Source： European Journal of Mineralogy, Vol.23, Iss.6, 2011-12, pp. : 937-958

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Abstract

The Copper Age Cucuteni-Tripolye (Trypillian) Cultural Complex extended from central and northern present-day Romania to the east, into Moldavia and Ukraine. Elegant-shaped light red to orange pots, painted with white, dark red and brown-black geometric patterns are characteristic of this culture. We studied 46 potsherds from Ruginoasa, 60 km west of Iaşi (Romania) in terms of mineralogy and petrography. Polarized-light microscopy shows a red, porous body, with fine to semifine microstructure and more or less oriented microtexture. The ceramic consists of a microcrystalline to amorphous/vitreous matrix and non-plastic clasts e.g. quartz, plagioclase, K-feldspar, muscovite, some chloritized biotite, heavy minerals and various lithoclasts, including calcareous ones.Backscattered electron (BSE) images revealed, in addition to large non-plastic clasts, a highly inhomogeneous and porous matrix, consisting of fine-grained clasts of quartz, feldspar and muscovite, some CaAl silicates and glass, all embedded in a fine-grained mass of clay minerals.Electron microprobe analyses (EMPA) indicate that several processes took place on a small scale during the firing. Upon softening and possible partial melting, quartz rims incorporate variable amounts of Al, K, Fe, Mg, Na, Ca and Ti. The alkali feldspar ranges from pure K-feldspar to albite, including anorthoclase. Plagioclases are rare. Several unusual compositions that plot in the feldspar immiscibility domain point to newly-formed phases. Frequently, former clasts of albite or plagioclase are replaced by K- or NaK-feldspars at the rims or inside the grains. Muscovite ranges from Al-rich composition to Si-rich phengite and is locally partially melted. The clay-rich part of the matrix is characterized by a loss of K and incorporation of Ca leading to compounds between K-illite and a hypothetical Ca-illite. The transformation processes, assisted by a fluid phase, indicate a temperature interval between 850 °C and 950 °C for the firing. Our results show that the thermal reactions did not reach equilibrium, most likely due to the short time of firing. At a small scale the resultant material consists of an inhomogeneous mix of various micro-domains.