Thermally triggered two-stage reaction of carbonates and clay during ceramic firing — A case study on Bronze Age Cypriot ceramics

Cypriot Bronze-age Plain White Wheelmade ware samples were investigated in order to examine their chemical and mineralogical features, their manufacturing procedures and associated firing temperature controlled mineral reactions. Beside the geochemical characterisation of the ware group, the main focus of the study was to determine phase transformations (especially the reaction of calcareous inclusions in phyllosilicatic matrices) via petrographical, mineralogical, microtextural and phase-geochemical methods. Three groups of pottery, fired at increasing temperatures (group I: < 600 °C, group II: 650–800 °C and group III: 850–1000 °C), could be distinguished taking their individual states of carbonate decomposition together with accompanying mineral reactions into account. In group I, no mineral transformations occur, whereas in group II and III, two conspicuous and temperature-indicative stages of carbonate reactions are observable. The first reaction stage takes place in group II samples and is characterized by the development of spherical reaction domains around paste matrix included calcareous grains by means of Ca-dispersion from the grains into the surrounding clayish matrix. This process provides the initial position for partial melting and blastesis of new mineral phases that follow. In the second-stage reaction, designative for group III, advanced carbonate decomposition takes place. Temperature triggered melts evolve from breakdown products of sufficiently heated clay minerals and build up glassy Al–Si rims at the interface of carbonates and the surrounding clay paste. At the same time, Ca-silicates grow in the domain of the Ca-affected matrix that indeed already evolved at lower temperatures (first-stage reaction) but only now reaches the sufficient heat for mineral blastesis. We assume that the carbonate-tempered ceramics were deliberately fired up to temperatures that are represented by group II but due to heterogeneous firing conditions parts of the vessels were unintentionally under- and overfired respectively.