Rehydration/Rehydroxylation Kinetics of Reheated XIX-Century Davenport (Utah) Ceramic (original) (raw)
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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.
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.
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.
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...
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.
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.
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.
RHX Dating: measurement of the Activation Energy of Rehydroxylation for Fired-Clay Ceramics
Archaeometry, 2014
In rehydroxylation (RHX) dating, the activation energy of the rehydroxylation reaction is required first in the estimate of a material's effective lifetime temperature (ELT), and second to correct the RHX rate constant obtained at a given measurement temperature to that at the ELT. Measurement of the activation energy is thus integral to the RHX methodology. In this paper, we report a temperature-step method for the measurement of activation energy and develop fully the underlying theoretical basis. In contrast to obtaining the activation energy from a series of separate experiments (each of which requires the sample to be dehydroxylated prior to measuring the RHX rate constant), the temperature-step method not only requires a single dehydroxylation at 500°C but also eliminates repeated acquisition of Stage I data, which are not required for dating purposes. Since the first temperature step is set to correspond to the temperature at which a dating determination is carried out, the measurement of rate constants at higher temperatures simply becomes an extension of dating. Consequently, the logistics of obtaining the activation energy of rehydroxylation are greatly simplified.
Ageing of low-firing prehistoric ceramics in hydrothermal conditions
Processing and Application of Ceramics, 2012
Remains of a prehistoric ceramic object, a moon-shaped idol from the Bronze Age found in archaeological site Zdiby near Prague in the Czech Republic, were studied especially in terms of the firing temperature. Archaeological ceramics was usually fired at temperatures below 1000?C. It contained unstable non-crystalline products, residua after calcination of clay components of a ceramic material. These products as metakaolinite can undergo a reverse rehydration to a structure close to kaolinite. The aim of this work was to prove whether the identified kaolinite in archaeological ceramics is a product of rehydration. The model compound containing high amount of kaolinite was prepared in order to follow its changes during calcination and hydrothermal treatment. Archaeological ceramics and the model compound were treated by hydrothermal ageing and studied by XRF, XRD and IR analyses. It was proved that the presence of kaolinite in the border-parts of the archaeological object was not a p...
Effect of Humidity Instability on Rehydroxylation in Fired Clay Ceramics
Journal of the American Ceramic Society, 2013
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. Additionally, a reheated masonry sample underwent a sequence of soaking and drying and hundreds of hours of interactions with humid air in between these treatments in order 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 longterm kinetics of mass gain. This important finding provides strong experimental support for the newly developed rehydroxylation ceramic dating technique by proving that humidity affects physically bonded water in the ceramics, but has a negligible effect on chemically bonded water.