Rehydroxylation of Fired‐Clay Ceramics: Factors Affecting Early‐Stage Mass Gain in Dating Experiments

To obtain accurate results in the RHX dating of ceramics, it is essential that the RHX measurements are continued until the rate of mass gain is constant with (time)^1/4. In this paper, we discuss how the initial stages of mass gain are affected by the specific surface area (SSA) of the ceramic material. The paper provides guidance on experimental protocols to avoid dating results being distorted by relatively early‐time mass gain data.     

The influence of temperature on rehydroxylation [RHX] kinetics in archaeological pottery

Almost all archaeological ceramics undergo slow, progressive rehydroxylation by chemical combination with environmental water. The reaction is accompanied by an expansion, and also by the small but measurable mass gain that provides the basis of the RHX dating method. The rate of the RHX reaction increases with increasing temperature. Here we describe comprehensively the effects of temperature on the RHX process in relation to the dating methodology. We deal in turn with the kinetic model of the RHX reaction, the temperature dependence of the RHX rate, and the influence of varying environmental temperature on the RHX mass gain. We define an effective lifetime temperature and show how this is calculated from an estimated lifetime temperature history. Historical meteorological temperature data are used to estimate the lifetime temperature history, and this can be adjusted for long-term climate variation. We show also how to allow for the effects of burial in archaeological sites on the temperature history. Finally we describe how the uncertainties in estimates of RHX age depend on the estimates of temperature history and effective lifetime temperature.     

Rehydroxylation dating of fired clays: an improved time-offset model to account for the effect of cooling on post-reheating mass gain

Dating of fired clay ceramics can be critical in building chronologies of archaeological sites. However, direct dating techniques such as luminescence, archaeomagnetism and radiocarbon dating are not always suitable or even possible. Rehydroxylation dating of fired clays is a developing method that has the potential to provide an alternative/complementary approach. Determination of a second stage, Stage II, mass gain rate in fired/reheated clay ceramics using a time^1/4-based model is central to this method. Experiments on modern brick samples demonstrate that the Stage II mass gain is not linear with t^1/4 as expected. The non-linearity observed can be taken account of through the introduction of a time-offset that represents the additional time required for mass gain at a specific aging temperature to equate to the additional mass gained above this aging temperature during cooling of the sample post-firing/reheating. Simulations of the mass gain curves expected during post-firing/reheating cooling highlight the significant effect of this mass gained during cooling and provide excellent agreement with experiment. Simulations also suggest that the two-stage structure in the mass gain curves observed might be better explained as largely the result of a single t^1/4-based process (Arrhenius in temperature dependence) across both stages; Stage I results from mass gained during cooling with Stage II resulting from mass gained during static temperature conditions. The overall implications for the dating method are that a non-linear Stage II mass gain (as a function of time^1/4) is expected and that an improved time-offset model provides a solution while removing issues of subjectivity in isolating the Stage II mass gain rate. This should lead to an improvement in the accuracy of RHX dates produced on archaeological material and further its development as a viable dating technique.     

Evidence for Complexities in the RHX Dating Method

The rehydroxylation (RHX) dating method applicable to virtually all baked clay fragments still requires testing and new developments. Here, we have obtained new weighing measurements from Syrian medieval ceramic fragments. In particular, they allow us to illustrate the scatter in RHX behaviour that may exist between different potsherds of the same age and found at the same archaeological site, thus having experienced the same effective lifetime temperature, and even between samples collected from the same artefact. Thanks to a so‐called time‐span analysis, we also report on some complexities in data sets previously obtained by Wilson et al. (2009, 2012, 2014).