AN EXPERIMENTAL STUDY OF HYDROGEN ISOTOPE FRACTIONATION DURING CERAMIC TILE FIRING

Dehydration processes associated with annealing of pottery during firing are investigated using hydrogen isotopes and the documentation of mineralogical changes across a wide range of temperatures and time intervals. Pottery was modeled using a mixture of four parts Molding Clay Red, one part quartz temper, and one drop of distilled water for each gram of clay. Quartz had a grain size no larger than 0.35 mm. The clay is a mixture of halloysite (50%), kaolinite (28%) and talc (22%) and was determined to have 7.7 wt.% H₂O. Mineralogical changes were monitored using X-ray diffractometry and infrared spectroscopy (PIMA). Two sets of experiments were performed: (1) At 800°C and for variable times up to 12 hours to investigate the kinetics of dehydration. (2) At variable temperature (300°C to 1100°C) for two hours to determine mineralogical changes during firing. Rapid reaction kinetics is indicated by the breakdown of both halloysite and kaolinite within two hours at 800°C, leaving talc as the only stable hydrous mineral at that temperature. The breakdown of talc to enstatite+quartz+H₂O occurs at approximately 800°C, but evidence of this reaction doesn't appear until after six hours of firing at this temperature. However, reaction kinetics was sufficiently fast for the formation of amorphous quartz during this reaction. Kaolinite and halloysite break down to aluminosilicate and quartz between 450° and 500°C. This is accompanied by an abrupt decrease in H₂O content from 7.7 wt.% to 2.0 wt.%. The gradual breakdown of talc results in a gradual decrease in wt.% H₂O from 2.0 wt.% to ~0.3 wt.% between 600° and 1100°C. A systematic decrease in δD values from –80‰ to –120‰ from the starting point up to 1100°C indicates Rayleigh fractionation during dehydration of clay mineral. The 800°C runs produced homogeneous δD values (–100±5‰), indicating rapid equilibration of D/H ratios during the experimental runs. These results suggest that ancient peoples only needed to fire their clay/temper mixtures only a short time during the production of pottery. This is the first study that utilizes hydrogen isotope fractionation patterns to recreate ancient technologies and methodologies. Continuing experiments are exploring the effects of diversity in temper materials and firing duration and temperature on ancient pottery manufacturing.