# Weakly Bonded Water in Interstitial Sites: A Source of Inaccuracy in Rehydroxylation Dating
## 1 Introduction  
In archaeology and the study of the history of art, dating of ceramic artefacts and bricks is extremely important, supporting stylistic studies in determining the chronology of art pieces or the construction phases of historical buildings [1]. The current established technique is thermoluminescence (TL) [2], which is based on the luminescence properties of heated clays. Although it has been successfully used for dating in many cases [3, 4], it relies on many parameters that are not always available or easily extrapolated (e.g., the measurement of environmental gamma dose-rate or the water content over the centuries in the soil and the sample [2]). Another property of ceramic materials that can be exploited for dating is the rehydroxylation phenomenon [5], which can be examined in tandem with the correlation between mass increase and time. In fact, when clay is subjected to high-temperature heating, creating ceramic material, it undergoes a complete dehydroxylation, losing all the water molecules and OH groups. Subsequently, when the ceramic material is cooled at ambient temperature, it adsorbs water molecules (rehydration, RH) and OH groups (rehydroxylation, RHX), increasing its volume and mass as time passes. Specifically, the phenomenon consists mainly of two stages: in the first (Stage I), which has a very short duration compared to Stage II, the material adsorbs unbonded water by capillarity and weakly bonded water (interlayer) by diffusion. This step lasts until the pores and interlayer spaces are saturated. As diffusion continues, only the second process (Stage II)is involved: hydroxyl groups interact with the lattice structure to form strong bonds (rehydroxylation). The kinetics of this later, much slower, process form the basis of the RHX dating technique [6]. This method was first proposed following studies about the expansive properties of fired clay bricks [7] and has since undergone considerable investigation (for example, [8; 9; 10]). In [7], [PERSON] et al. studied the volume and mass increase as a function of time, finding that the mass gain of a ceramic sample as a function of time follows a power law with exponent 1/4 in Stage II. The application of RHX as a dating method resulted in a few promising results from ceramic artefacts, bricks and tiles of known ages on the order of many centuries [5]. Rehydroxylation (RHX) dating is a gravimetric technique: in principle, weighing a piece of ceramic under controlled environmental conditions, before and after reheating, its mass increases due to water adsorption. The mass the ceramic has gained since it was originally or last fired above 500 \({}^{\circ}\)C can be estimated. Then, the rehydration/rehydroxylation (mass gain) rate can be calculated by measuring the mass gain of this reheated sample as a function of time in an aging environment equivalent to that of the ceramic's burial or depositional position. Knowing both the rate of mass gain and the mass gained during its lifetime, the elapsed time since the ceramic was fired can be calculated. Since its first applications, the need for a precise evaluation of the mass-gain rate has been clear, as it depends on the material mineralogy and the initial firing conditions. Other problems come from the mentioned experimental steps. They will be concisely presented below. A more detailed discussion can be found in the papers by [PERSON] et al. [5; 6; 11]. The phenomenon begins when inorganic clays are fired at temperatures in the range 450-900 \({}^{\circ}\)C; under those conditions, hydroxyls of the clay minerals are converted to water and subsequently evaporate (a process accompanied by a collapse of the crystal structure of the clay). A state of complete dihydroxylation (DHX) should be reached. Following cooling, a reversal of this process takes place in the hardened ceramic; water from the environment is absorbed, first via the absorption of water molecules from the environment (rehydration, RH), then as OH groups fixed in the ceramic structure (rehydroxylation, RHX).