# Weakly Bonded Water in Interstitial Sites: A Source of Inaccuracy in Rehydroxylation Dating
## 4 Discussion  
The proposed experimental sequence, with the initial TGA step, allowed for qualitative discrimination of the samples not suitable for analysis by the standard procedure. Figure 2 shows the TGA curves of the five samples, which can be clustered into two groups. In the first group are Brick_2, Brick_3 and Cer_1, which underwent limited weight loss in the TGA curves (mass fraction <1%) in the 100 \({}^{\circ}\)C dehydration range and a stepped behavior in the mass loss.  
However, the samples in the second group, Brick_1 and Cer_2 (for which we obtained an age overestimation after a 105 \({}^{\circ}\)C preheating) decreased in weight starting, as expected, at around 100 \({}^{\circ}\)C; it is remarkable that after this temperature, there were no discontinuity  
\begin{table}
\begin{tabular}{c c c c} \hline
**Sample ID** & **Expected Date** & \
\begin{tabular}{c} **Experimental Date** \\ **(100 \({}^{\circ}\)C Preheat)** \\ \end{tabular} &
\begin{tabular}{c} **Experimental Date** \\ **(300 \({}^{\circ}\)C Preheat)** \\ \end{tabular} \\ \hline Brick\_1 & 1030 \(\pm\) 80 CE & 1842 \(\pm\) 200 BCE & 1140 \(\pm\) 70 CE \\ \hline Cer\_2 & 1380 \(\pm\) 60 CE & 188 \(\pm\) 140 CE & 1460 \(\pm\) 90 CE \\ \hline \end{tabular}
\end{table}
Table 2: Results obtained from Brick_1 and Cer_2 samples preheated at different temperatures.  
points, but the mass continued to decrease at least until 400 \({}^{\circ}\)C, with the samples losing a considerable mass fraction (2-3%).  
Brick_1 and Cer_2 seem to retain a non-negligible fraction of water after low-temperature heating, as discussed below, leading to a wrong estimate of the adsorbed water content and, as a result, to a wrong estimate of the sample age, and it suggests that there is an incomplete dehydration of the weakly bonded water fraction with a low-temperature step.  
The most probable explanation for this behavior seemed to be the mass difference, which is calculated as follows:  
\[\Delta m=m_{2}-\ m_{4} \tag{4}\]  
Step 2 (100\({}^{\circ}\)C preheat) makes the physiosorbed molecular water in pores dehydrate, leaving on site the weakly bound adsorbed water  
The firing in Step 4 removes all water and hydroxyl groups from the sample, leaving only the ceramic mass. It is clear, then, that the relative amount of weakly bonded water in the structure is crucial for the reliability of the dating technique. In fact, the presence of a considerable amount of weakly bonded water molecules in the matrix can lead to an overestimation of the \(m_{2}\) mass. Raising the preheating temperature completely empties the adsorption channels, as well as the interstitial water sites, mimicking the initial firing conditions of dehydration and dehydroxylation of the material. Considering this experimental evidence, it is necessary to find the proper preheating temperature (see Step 2) to ensure a more reliable result or at least to pay particular attention to probable overaging effects. An increase in preheating temperature to about 300 \({}^{\circ}\)C for the two overaged samples in this study improved the accuracy of the dating process (see Table 2).  
As an example of the effect of the preheating temperature, the results obtained from the Brick_1 sample are reported in Figure 3: the mass difference decreases, and the age estimate is more accurate with preheating in a temperature range between 200 and 300 \({}^{\circ}\)C. The discrepancy between experimental and expected ages is due to the continuity of mass loss between 200 and 300 \({}^{\circ}\)C, as discussed before: the optimal temperature lies in this interval, but it is not easily determined.  
Finally, our attention focused on the reproducibility of the proposed procedure by testing the dating sequence on four subsamples of Cer_2: the results were in good agreement (Table 3 and Figure 4), even if only one sample was in perfect agreement with the expected age.  
Figure 4: Comparison of 100 \({}^{\circ}\)C and 300 \({}^{\circ}\)C preheating results for the Cer_2 sample.