This post, second in a series of two (click for part one), discusses our communication to the Journal of the American Ceramic Society, entitled “Effect of humidity instability on rehydroxylation in fired clay ceramics” by J. Drelich, P. K. Bowen, and T. J. Scarlett, which appeared in vol. 96, issue 4, pages 1047-1050. The abstract is as follows:
Several samples of the XIX-century Davenport pottery and XX-century structural masonry were reheated at 500°C and then exposed to a humid gas of controlled relative humidity. Changes in the sample masses were recorded in response to both systematic and transient step changes in humidity. In addition, a reheated masonry sample underwent a sequence of soaking and drying and hundreds of hours of interactions with humid air in between these treatments to examine long-term effects of extreme humidity fluctuations. All experimental results indicate that instantaneous humidity and the sample's hygral history have a negligible effect on the long-term kinetics of mass gain. This important finding provides strong experimental support for the newly developed rehydroxylation (RHX) ceramic dating technique by proving that humidity affects physically bonded water in the ceramics, but has a negligible effect on chemically bonded water.
In plain English: we messed with the humidity during rehydroxylation of fired clay ceramics, and it had no effect on the kinetics of mass gain. Therefore, there is no long-term effect on the rehydroxylation (RHX) behavior that would skew projected dates.
It was stated in earlier work on RHX dating that humidity did not seem to have any serious effect on the performance of rehydroxylation ceramic dating, but definitive evidence was not provided. This communication works to fill this void, and provides some definitive results for a certain sample type that shows that the original assertions were correct. To prove this, the humidity was modulated both transiently (a brief burst of high humidity) and systematically (a change from low to high humidity partway through the test).
In the first (transient) case, the slope of the time1/4 trendline was shown to be the same before and after the humidity was increased and again decreased, indicating that the rehydroxylation kinetics did not change. Even when the sample was severely abused by soaking in water (i.e. raining) and being dried near the boiling point (i.e. baked in the sunshine), there were no indications that the long-term behavior experienced any changes (see image below). This is good news!
The second (systematic) case showed the same thing: no changes in the trend were observed with a single jump in the humidity level. This shows that a systematic change in humidity, like would be experienced season-to-season (such as winter-to-spring or monsoon-to-dry season), does not have an impact on the rate of water uptake. (The same cannot be said for temperature, of course!)