## 3 Results and Discussion
The low contents of Ca and K for both outer and inner layers indicated low firing temperatures for the two layers of samples (\(\sim\)600-900 \({}^{\circ}\)C) in open-air kilns [38; 39]. Ca was added to the clay paste as temper [40]. It has been reported that the source of phosphorous (P) contamination in ancient potteries was different, i.e., minerals, bone ashes that were added into the clay paste, and burial environment [41; 42; 43]. P contained in all samples, especially in the inner layer, was presumed to be come from the burial environment. Trace elements such as Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, and Zr were detected using PIXE due to their contents beyond limit of detection of SEM-EDS. The contents (ppm) of Sc (3.1-77.7), V (3.8-19.5), Cr (4.9-9.1), Mn (1.8-73.0), Co (19.0-28.6), Ni (6.0-11.3), Cu (6.7-9.9), Zn (6.5-60.3), Ga (7.2-22.8), and Zr (89.1) are shown in Table 3. Mn (MB056), Zn (MB056 and MB058), and Ga (MB055) were below detection limit in some samples. Zr was only found in MB051, while Cr (MB056, MB058, and MB060), Co (MB052, MB054, and MB055), Ni (MB052, MB056, MB058, and MB060), and Cu (MB052, MB055, MB058, and MB060) were detected in some samples. Sc (MB052, MB054, MB056, and MB059) and V (MB052, MB055, MB058, MB059, and MB060) were present in some samples. These elements were trace elements in soil and clastic sedimentary rocks found in northeastern Thailand [44]. The variation of elemental composition indicated different characteristics of raw materials. Geological data indicated that these raw materials were sandy soil derived from marine sands, clays, and salt deposits that were mostly iron-rich-kaolin clay [44]. This archaeological evidence reflected iron smelting and salt manufacturing in this area [45; 46]. Raw materials with a high proportion of Si possessed low mineral contaminations. The possession resulted in lower plasticity, unfired strength, and drying shrinkage than one with the high mineral contents. However, large amount of Al was also able to reduce the plasticity [39; 47]. SEM micrographs of samples MB051, MB052, MB054, MB055, MB056, MB058, MB059, and MB060 are shown in Figure 6. The figure shows the different microstructure and morphology between core, and outer and inner layers for each sample. It is obvious that the core contained heterogeneous morphologies of composites in irregular plate-like particles with a wide range of size distribution while the outer and inner layers possessed slipped microstructure of glazes. The core showed none or only a small amount of the glass phase. This difference was due to the phase transformation of raw material compounds during the firing process. Figure 7 shows the SR XTM 3D reconstructive results which provided information about the porosity of the samples. The results in Table 4 exhibit some variation of porosity between a core, and outer and inner layers of four selected samples of MB051, MB055, MB056, and MB059. The total porosity of a core tended to be higher than the outer and inner layers. Some samples possessed high porosity in outer and inner layers due to possession of thin layers in which porosity from some parts of core were combined. The results corresponded to SEM in which outer and inner layers possessed dense microstructure of glazes, while the core contained a small amount of the dense glass phase. Even though the pottery samples in this work were grave goods excavated from the location nearby our previous work which was the pottery used in daily life [20], the results in Table 5 show that their elemental compositions were slightly different. It is proposed that their raw materials originated from the same source. The potteries in our previous and present works might be produced with different kilns and amounts of raw materials but fired with open-air kilns at low temperatures (<900 \({}^{\circ}\)C). The small difference in composition resulted from variations of the production or the use of raw materials from different batches as well as weathering and surface contamination.