RHX Dating: measurement of the Activation Energy of Rehydroxylation for Fired-Clay Ceramics (original) (raw)
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Archaeometry, 2013
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.
Dating Fired-Clay Ceramics Using Long-Term Power Law Rehydroxylation Kinetics
… of the Royal …, 2009
Fired-clay materials such as brick, tile and ceramic artefacts are found widely in archaeological deposits. The slow progressive chemical recombination of ceramics with environmental moisture (rehydroxylation) provides the basis for archaeological dating. Rehydroxylation rates are described by a (time) 1/4 power law. A ceramic sample may be dated by first heating it to determine its lifetime water mass gain, and then exposing it to water vapour to measure its mass gain rate and hence its individual rehydroxylation kinetic constant. The kinetic constant depends on temperature. Mean lifetime temperatures are estimated from historical meteorological data. Calculated ages of samples of established provenance from Roman to modern dates agree excellently with assigned (known) ages. This agreement shows that the power law holds precisely on millennial time scales. The power law exponent is accurately 1 / 4 , consistent with the theory of fractional (anomalous) 'single-file' diffusion.
A Review of Rehydroxylation in Fired-Clay Ceramics
Journal of the American Ceramic Society, 2012
Understanding the hygral reactivity of ceramic materials is essential to understanding the long-term behavior of building materials and of archeological pottery, especially in relation to dating. We explore the literature on the expansion of fired-clay ceramics, reviewing strain and mass measurements at the bulk scale, and rehydroxylation (RHX) and dehydroxylation (DHX) processes in clay minerals at the molecular level. We present open questions on the nature of ceramic rehydroxylation and its apparent adherence to a sub-diffusive kinetic (time) 1/4 power law. We discuss measurement of the RHX process through mass gain in relation to a proposed new dating method for archaeological ceramics.
Rehydration/Rehydroxylation Kinetics of Reheated XIX-Century Davenport (Utah) Ceramic
Journal of the American Ceramic Society, 2011
Rehydroxylation Dating (RHX dating) has recently been proposed as a new chronometric dating tool for use on archaeological fired-clay ceramics. The technique relies upon the wellknown characteristic of reheated porous ceramic vessels to regain water, the kinetics of which has been shown to follow a (time) 1/4 power law at temperatures of 13-50°C . In this study, experiments were conducted in which the mass measurements taken from 19th-century ceramic artifacts revealed a deviation from the (time) 1/4 power law over a wide range of temperatures. This finding has led to the formulation of an empirical equation which describes the observed ceramic's rehydration and rehydroxylation behavior as an additive process in which the longterm mass gain is dictated by a (time) 1/n power law. As part of this study, the mineralogy of the ceramics and their thermal properties have been evaluated. The instantaneous effect of humidity on mass measurements was demonstrated to be the principal source of error.
The influence of temperature on rehydroxylation [RHX] kinetics in archaeological pottery
Journal of Archaeological Science, 2013
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.
Journal of Archaeological Science, 2017
Rehydroxylation (RHX) dating was recently suggested as a simple, cheap, and accurate method for dating ceramics. It depends on the constant rate of rehydroxylation (the slow reintroduction of OH) of clays after they are fired and dehydroxylated (purged of OH) during the production of pots, bricks, or other ceramics. The original firing of the ceramic artifact should set the dating clock to zero by driving all hydroxyls out of the clay chemical structure. To examine whether this assumption holds, especially for pot firings of short duration and low intensity, as those in small-scale traditional settings, we performed thermogravimetric analysis of clay samples of known mineralogy at temperatures and for durations reported from traditional sub-Saharan, American, and South Asian pottery firings. Results demonstrate that in the majority of samples, complete dehydroxylation (DHX) did not occur within, or even beyond, the conditions common in traditional firings. Consequently, between 0.01 and 1.5% of a sample's mass in residual OH may remain after firings analogous to those observed in the ethnographic record. Lack of complete DHX at the scales we have observed can result in the over-estimation of ceramic ages by decades to tens of thousands of years, depending largely on the age of the sample, and the amount of residual OH present. Thus, in many cases, a key assumption underlying current RHX dating methods is unlikely to have been met, introducing considerable error in dates.
A methodological study of a simplified rehydroxylation dating procedure
Rehydroxylation is a developing method of dating fired materials that was introduced to fired brick in 2009 and archaeological pottery in 2012. This technique is based upon dating the Stage II kinetics of the rehydroxylation process using a (time)1/4 power model. The original rehydroxylation method utilised very expensive equipment so this experiment proposes a different measurement protocol that most university laboratories can implement easily. Some scholars have noticed flaws in this original formula and therefore this experiment will test an amalgamation of their proposed alternative models. Thermogravimetric analysis complements the rehydroxylation research in understanding the influence of carbonates in the rehydroxylation rate. The chronological limits are tested using excavated material from Iron Age, Jordan while known age brick samples are used to explore the influence of extreme temperatures on the rehydroxylation rate. The reaction of the mass gain of samples in extreme thermal environments demonstrates the need for methodological precision as well as a uniform physical sample state. Different levels of humidity have had a significant effect on mass gain, contrary to previous literature. Mathematical correction for temperature cannot compensate for imprecise methodology. Excavated materials prove difficult to date because of the different thermal environments of different loci. Measuring temperatures of different depths in the field should be explored to counteract this limitation. The original (time)1/4 model needs to be further developed and the more complex amalgamation model should be focused on in future research. TGA analysis, accompanied with x-ray diffraction, should complement all rehydroxylation to better understand the structure of samples and the potential influence of carbonates. To further explore this accessible rehydroxylation measurement protocol researchers need to use controlled environmental conditions.
Rehydroxylation (RHX) dating of archaeological pottery
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012
We show that the rehydroxylation (RHX) method can be used to date archaeological pottery, and give the first RHX dates for three disparate items of excavated material. These are in agreement with independently assigned dates. We define precisely the mass components of the ceramic material before, during and after dehydroxylation. These include the masses of three types of water present in the sample: capillary water, weakly chemisorbed molecular water and chemically combined RHX water. We describe the main steps of the RHX dating process: sample preparation, drying, conditioning, reheating and measurement of RHX mass gain. We propose a statistical criterion for isolating the RHX component of the measured mass gain data after reheating and demonstrate how to calculate the RHX age. An effective lifetime temperature (ELT) is defined, and we show how this is related to the temperature history of a sample. The ELT is used to adjust the RHX rate constant obtained at the measurement temper...
Journal of the American Ceramic Society, 2021
Determining absolute ages of archaeological ceramics is crucial for understanding past societies and reconstructing their accurate chronologies. The amount of OH hydroxyl chemically combined with ceramic material has been claimed to provide an ‘internal clock’ that can be read via RHX dating to determine the elapsed time since it was fired. The hydroxylation reaction, controlled by the slow diffusion of water molecules within the structure of clay minerals, has been described by a quartic root (time)1/4 power law dependence. However, previous attempts of RHX dating by gravimetric methods have not been successful, since the mass gain due to OH hydroxylation or H2O hydration could not be distinguished. We carried out a preliminary study of RHX dating via Infrared (IR) and Nuclear Magnetic Resonance (NMR) spectroscopy of three pure clay minerals, beidellite, illite and muscovite, as analogues for components of archaeological materials. Our study of RHX kinetics via IR microscopy gives important evidence regarding the quartic root time power law dependence. Furthermore, NMR allows us to study the structural as well as dynamic features of clays. Through observing the H/D exchange, we obtain access to the relevant activation energies and diffusion coefficients. We show that IR and NMR methods hold significant potential to refine the RHX dating method by understanding the elementary processes of mass transfer and hydroxylation in pure clays.
Modeling Rehydration/Rehydroxylation Mass-Gain Curves from Davenport Ceramics
Journal of the American Ceramic Society, 2013
Rehydroxylation ceramic dating, a new technique that has shown promise as an archaeometric 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 days. Both time 1/4 and time 1/n analyses were applied to the experimental mass gain vs. time results in an attempt to find the most appropriate treatment for the data. The time 1/4 analysis yielded poor reproducibility and non-ideal fitting results to the Stage II mass gain, in which water reacts with meta-clays. Application of the time 1/n model, where 'n' is the rehydroxylation exponent, improved the apparent linearity of Stage II mass gain in small samples. However, the time 1/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 time 1/n irrespective of mass variations.