# Weakly Bonded Water in Interstitial Sites: A Source of Inaccuracy in Rehydroxylation Dating
## 1 Introduction
[PERSON] et al., in 1991 [12], reported some IR spectra that evidenced very different behavior in the release of water molecules and OH groups as a function of temperature: after heating to a series of temperatures, from 110 \({}^{\circ}\)C to 550 \({}^{\circ}\)C, for 24 h, the movements of water molecules and OH groups were measured, yielding very different results for samples coming from the same place. While the results obtained in quite a few experiments indicate that ancient ceramic artefacts can be dated by the RHX technique [11], some experimental groups have declared that RHX cannot generally give reliable results [10; 20] due to various causes that must be separately analyzed. In [10], the authors claim that the duration of heating at both 105 \({}^{\circ}\)C and 500 \({}^{\circ}\)C is a critical parameter for the RHX dating method. In some samples, they noted a systematic lack of stabilization of the mass of the samples after their heating at 105 \({}^{\circ}\)C, even after several weeks of monitoring. In such cases, they observed that mass-gain data do not allow us to conclude that the (re)hydroxylation process acting in archaeological or industrial fired-clay materials systematically and accurately follows a \(t^{1/4}\) power law [20]. In a recent work [21], we have shown that in a specific case, treatment in the first step with preheating to temperatures increasingly higher than 105 \({}^{\circ}\)C could yield correct dating results. Specifically, the dating of this sample yielded an age much greater than the expected value after heating to 105 \({}^{\circ}\)C, as is usual in the first heating step, while the dating was correct when the first heating step was carried out at 300 \({}^{\circ}\)C. The validity of this procedure will be discussed in this work, testing it on well-dated archaeological and historical materials such as building bricks and pottery remains (see Section 2.1 for details). The Lambda laboratory in Milano\(-\)Bicocca University started studying RH/RHX in well-dated samples in 2016; about 50 RHX dating measurements have been carried out. The dating process proved successful in about half of the attempts. Following the obtaining of the above-mentioned results [21], it was proposed that in some types of ceramics, a material parameter possibly dependent on mineralogical composition could be critical for the precision of the analysis. This parameter was identified, in the samples examined in the present work, to be associated with the partial removal of weakly bonded water molecules during the first preheating step, which may lead to an incorrect evaluation of the mass analysis. Here, we propose an experimental assessment of the preheating temperatures and the use of thermogravimetric analysis (TGA) to identify the temperature at which the differently absorbed molecules are released. As is well known, TGA is a method used to characterize the control of the reaction processes and the properties of the materials. The proposed procedure has been applied to date to five samples whose ages had already been independently determined [22; 23], matching experimental and historical data. Gaps remain in our understanding of the fundamental processes that drive hydroxylation in ceramics, a phenomenon crucial to RHX dating. As mentioned above, this study is designed to address these inconsistencies. In particular, the focus was on the presence of interstitial water molecules not eliminated in the lower-temperature heating step because these molecules can contribute to the RHX mechanism, possibly giving unreliable dating results.