NMR Studies of the Rehydroxylation of Ceramic Materials with Respect to Rehydroxylation Dating

Monitoring the rehydroxylation (RHX) in ceramic samples after firing was introduced in archeology as the RHX dating in order to determine the time elapsed since the firing of a ceramic artifact. But the mechanisms of hydroxyl transport during the RHX mechanism are still largely unclear. The present study investigates hydroxyl groups of three at 800 degrees C fired ceramics upon hydrothermal treatment of the samples. The rehydroxylation of the ceramics between 600 and 200 C-degrees results in an increase of less than a factor of 1.5 for most samples, whereas early studies showed that the decrease of the hydroxyl concentration is a factor of about 10 for the basic powder material in the dehydroxylation range 200-800 C-degrees. A sieve fraction of 100-140 mu m of ceramic materials crushed in a mortar was used for the magic-angle spinning (MAS) NMR experiments because larger particles and of course the fired cylinders (10 mm diameter) of the ceramic materials do not rotate at higher frequencies. The less accurate static NMR experiments (without MAS) on the ceramic pieces with 1 cm diameter did not show significant differences because the ceramic is porous. H/D tracer exchange experiments show that H/D exchange time constants do not significantly depend on temperature. Therefore, the hydroxyl transport mechanism cannot be described by an activation energy like for normal diffusion. We explain the transport of hydroxylation by a quantum Brownian diffusion of hydrogen atoms between hydroxyl groups and (opening of) constrained bonds in the near neighborhood.

Automated summary

This study investigates the hydroxyl groups of ceramics fired at 800 degrees C using hydrothermal treatment. The rehydroxylation process between 600 and 200 C-degrees results in an increase of less than 1.5 times in the hydroxyl concentration. The findings contribute to a better understanding of the time elapsed since firing of ceramics.