Modeling Rehydration/Rehydroxylation Mass‐Gain Curves from Davenport Ceramics

Abstract

Rehydroxylation ceramic dating, a new technique that has shown promise as an archeometric breakthrough, was applied to XIX‐century samples of Davenport ceramics from Parowan, Utah in the United States. The samples were dried at 500°C to remove both physically and chemically bonded water and then exposed to a 20% relative humidity air to record the progression of rehydration/rehydroxylation over a period of 40 d. Both time1/4 and time1/n analyses were applied to the experimental mass gain versus time results in an attempt to find the most appropriate treatment for the data. The time1/4 analysis yielded poor reproducibility and nonideal fitting results to the Stage II mass gain, in which water reacts with meta‐clays. Application of the time1/n model, where “n” is the rehydroxylation exponent, improved the apparent linearity of Stage II mass gain in small samples. However, the time1/n treatment still provided a poor fit to data from larger specimens, indicating that some secondary effects related to sample size and water transport may exist. To examine the effects of porosity and macrostructure on rehydration/rehydroxylation processes, a pulverized sample of the same material was analyzed, resulting in improved sample‐to‐sample agreement in time1/n irrespective of mass variations. These findings have implications on the application of rehydroxylation ceramic dating.

Experimental Variability in Kinetics of Moisture Expansion and Mass Gain in Ceramics

“…Since the seminal article of Wilson et al ., other groups around the world have tried to reproduce this method, but without success [e.g., Ref. )]. These studies highlighted several complexities with the method and in particular, the foundation of the RHX dating method itself, that is, the universality of the t ¼ time power law …”

Effect of Humidity Instability on Rehydroxylation in Fired Clay Ceramics

“…(a)] is of the typical shape reported in previous studies for various ceramic artifacts (either as mass versus time, mass gain versus time, or fractional mass gain versus time curves) and reflects the kinetics of water pickup by the ceramic from the environment . The condensation and adsorption of physical water within a porous structure of ceramic is a spontaneous process and dominates for 5–17 h for the Davenport pottery after initial exposure to moisture . This time is much longer, ~ 175–190 h, for the modern masonry samples, as discussed later.…”

“…), which is an example of a fired clay ceramic artifact that one may recover in the field. Kiln wasters of the Davenport type have been thoroughly characterized and their rehydration/RHX behaviors examined in earlier reports . A second set comprised pieces of broken brick recovered from the parking deck in Houghton, MI (~1965–1970 C.E.)…”

“…Then the samples were placed in a constant‐humidity glove box (Coy Laboratory Products; Grass Lake, MI) equipped with a Citizen's CM‐11 microbalance (Citizen Scale Inc., Edison, NJ) having 1 μg precision and operated in a room with controlled temperature of 22°C ± 1°C. The glove box was equipped with a desiccation circuit, dry N 2 injection, and humidification apparatus and was presented in our previous contribution . The samples remained in the glove box's controlled humidity environment at levels of 20%, 28%, or 40% relative humidity for the duration of the experiment.…”

Reproducibility in Rehydroxylation of Ceramic Artifacts

“…In our previous contributions, we have used a generalized power law ( 1/n ) with a variable exponent ( n ), to describe the RHX behavior of ceramics without identifying their ages . The ∆ m versus 1/n curves produced for several samples demonstrated improved linearity in RHX Stage II …”