## 2 Materials and Methods
### Methods  
The contents of elements in samples were analyzed and classified as major, minor, and trace elements using SEM-EDS (with the contents of \(\geq\)0.10 wt%), while PIXE confirmed and focused on trace elements in ppm. A QUANTA 450 (FEI Company s.r.o., Brno, Czech Republic) scanning electron microscope (SEM) coupled with X-Max EDS System (Oxford Instruments, Oxford, UK) energy-dispersive X-ray spectrometer (EDS) at Science Equipment Center of Faculty of Science (Kasetsart University, Bangkok, Thailand) was carried out to characterize the microstructure and elemental composition of samples. The SEM was operated at 15 kV with magnification ranging from 25x to 500x. Semi-quantitative analysis of EDS spectra was carried out using INCA software (Oxford Instruments, Oxford, UK). X-ray spectra were recorded from three different positions for each sample: the outer layer; core; and inner layer as shown in Figure 3.  
Proton-induced X-ray emission spectroscopy (PIXE) (operated at Plasma and Beam Physics Research Facility of Chiang Mai University, Chiang Mai, Thailand) was applied to identify and analyze contents of elements, especially the trace ones. The analysis was based on 2-MeV-proton beam produced by 1.7 MV tandem Tandetron accelerator (Model 4117, High Voltage Engineering Europe Corp., Utrecht, the Netherlands). The proton beam was collimated with a diaphragm of 1 mm in diameter. The beam current on sample was controlled to be of 10 nA. The sample was probed at the same position of SEM-EDS using a signal detector of Si(Li) with 6.2 mm in diameter and 3 mm thick (Model SL30170, Canberra Industries Inc., Connecticut, USA). Silicon with known concentration was used as a standard reference for PIXE analysis as conducted in previous research [27; 28; 29; 30; 31; 32; 33]. Quantitative analysis of chemical composition (for \(Z\geq 13\)) was performed using GUPIXWIN code (GUPIX software for Windows version, Department of Physics, University of Guelph, Ontario, Canada) [34; 35; 36]. The quantitative calibration includes normalization at (100%-x) of the total amount of oxides, where the value of x is a sum of Na\({}_{2}\)O and MgO determined by EDS. The subtraction of x was due to the accuracy of detection of Na and Mg by PIXE was not good when compared to other elements with Z \(\geq\) 13. Figure 2: Plan of jar burial sample pits at Ban Muang Bua. Adapted with permission from ref. [24] (2010) (Fine Arts Department: Ubol Ratchathani, Thailand). Optical glasses with known chemical compositions were used for calibration of PIXE and EDS results. It was found that the relative errors of results of two techniques were less than 1%. Figure 3: Top view (**left**) and cross-sectional (**right**) images of MB051, MB052, MB054, MB055, MB056, MB058, MB059, and MB060. than 10%. PIXE results were therefore compared to EDS and the average content of each element from two techniques was evaluated [36]. Synchrotron radiation X-ray tomographic microscopy (SR XTM) was operated at XTM beamline (BL1.2W), SLRI (Nakhon Ratchasima, Thailand). For a complete dataset, X-ray projectors of each specimen were collected for 180\({}^{\circ}\) with 0.1\({}^{\circ}\) angular increment. In order to minimize the scattering and beam hardening artifacts, polychromatic X-rays were attenuated with 400 \(\upmu\)m thick aluminum foil to allow penetration of the X-ray with mean energy of 11.5 keV for X-ray imaging. The X-ray images were taken by a 2560 \(\times\) 2160 pixels CMOS camera (PCO.EDGE 5.5, Optique Peter, Lentilly, France) with a 0.7 micron resolution. The data were pre-processed and reconstructed in 3D based on a filtered-back projection algorithm using Octopus Reconstruction software (Octopus 8.9.4, TESCAN-XRE, Ghent, Belgium). The 3D projection and analysis of pores and grains were determined using Octopus Analysis software (Octopus 8.9.4, TESCAN-XRE, Ghent, Belgium). This technique was applied to examine microstructures of materials in our previous works [20; 37].