EMU Notes in Mineralogy, Vol. 20 (2019), Chapter 6, 233–281 The struggle between thermodynamics and kinetics: Phase evolution of ancient and historical ceramics 1 2 ROBERT B. HEIMANN and MARINO MAGGETTI 1Am Stadtpark 2A, D-02826 Go¨rlitz, Germany
[email protected] 2University of Fribourg, Department of Geosciences, Earth Sciences, Chemin du Muse´e 6, CH-1700 Fribourg, Switzerland
[email protected] This contribution is dedicated to the memory of Professor Ursula Martius Franklin, a true pioneer of archaeometric research, who passed away at her home in Toronto on July 22, 2016, at the age of 94. Making ceramics by firing of clay is essentially a reversal of the natural weathering process of rocks. Millennia ago, potters invented simple pyrotechnologies to recombine the chemical compounds once separated by weathering in order to obtain what is more or less a rock-like product shaped and decorated according to need and preference. Whereas Nature reconsolidates clays by long-term diagenetic or metamorphic transformation processes, potters exploit a ‘short-cut’ of these processes that affects the state of equilibrium of the system being transformed thermally. This ‘short-cut’ is thought to be akin to the development of mineral-reaction textures resulting from disequilibria established during rapidly heated pyrometamorphic events (Grapes, 2006) involving contact aureoles or reactions with xenoliths. In contrast to most naturally consolidated clays, the solidified rock-like ceramic material inherits non-equilibrium and statistical states best described as ‘frozen-in’. The more or less high temperatures applied to clays during ceramic firing result in a distinct state of sintering that is dependent on the firing temperature, the duration of firing, the firing atmosphere, and the composition and grain-size distribution of the clay.