NMR Studies of the Dehydroxylation and Rehydroxylation (RHX) of Clays with Respect to the RHX Dating of Ceramic Materials

Abstract

Monitoring the recombination of OH groups in a ceramic sample after firing, also known as rehydroxylation (RHX), was proposed as a way to determine the time elapsed since the firing of a ceramic material, thus providing archeologists with the only up-to-date known method for determining the age of fired ceramics directly. A nuclear magnetic resonance (NMR) study was performed in order to understand the RHX dating of ceramic materials in archeology. We perform MAS NMR investigations on four pure clay minerals and one mixed ceramic. We point out a large discrepancy between NMR measurements and TG in the obtained total concentration of hydrogen. We are able to differentiate and investigate the dynamics (by monitoring H/D exchange) of the three types of hydrogen species present in the samples: T0 (physisorbed), T1 (interlayer), and T2 (chemisorbed) water. We use H/D tracer exchange to monitor the mobility of hydrogen species and obtain the exchange time constants of T2 water, which is in the order of a few to 100 days. Interestingly, we find that H/D exchange time constants do not significantly depend on temperature. The slow exchange times of T2 water, in the order of days, can be compared with the diffusion time scales of T1 water (in the order of 100 s) obtained with tracer desorption and with T0 water (order of 100 ms) obtained by PFG MAS NMR measurements.