The Naedeok-ri site in Gimhae contains remains in the Jangyu area from the late Geumgwan Gaya period. The architectural style of this ancient tomb is estimated to date from the mid-5th century to the early 6th century. This period corresponds to the period when Geumgwan Gaya began to decline after the historical event of Goguryeo’s expedition in 400 CE. Nevertheless, the architectural style may be related to the phenomenon followed by the dissolution of the social structure in the early period of Geumgwan Gaya when this style was displayed in the Daeseong-dong and Yangdong-ri tombs when it reached its peak. The wooden coffin tomb No. 19, which may date back to the Proto-Three Kingdoms period, at the Naedeok-ri site in Gimhae was identified as a chieftain-level tomb, similar to the Daeseong-dong and Yangdong-ri tombs. Bronze mirrors and swords were excavated from it, along with glass necklaces. In the case of buried glass products, 87 pieces of glass beads were found in the wooden coffin tomb No. 19, and a small number of glass pieces were excavated from tombs formed during the Three Kingdoms period after the 4th century. Later, typical brown beads began to appear during the Joseon Dynasty. Thus, the Naedeok-ri site in Gimhae consists of a tomb that shows the style of the late Geumgwan Gaya period. Additionally, it may further describe the process of the decline of Geumgwan Gaya during its early period, as well as the period of the arrival of Silla troops. Through a scientific analysis of glass beads, this study intends to determine the temporal precedence of these events based on these remains.

The remains subjected to analysis consist of two pieces from the Proto-Three Kingdoms, five pieces from the Three Kingdoms period, and one piece from the Joseon Dynasty. In the case of the Proto-Three Kingdoms and the Three Kingdoms period samples, the creation of the remains may date back to between the 2nd and 5th centuries. The colors of these beads are mainly blue, including cobalt blue and copper blue. In the case of the Joseon Dynasty, the bead colors are identified as brown and light yellow (buff). Forty-eight beads were subjected to analysis, consisting of 10 beads for SEM-EDS analysis and 38 beads for ED-XRF analysis (Table 1).

Cross-sectional samples of the glass beads are prepared for analysis of the beads’ microstructures and internal compositions. These selected samples are fabricated to obtain samples most suitable for the selected analysis methods using the pretreatment process described as follows.

A cross-sectional sample is prepared with epoxy resin, and then ground and polished with sandpapers (Nos. 400, 600, 800, 1200, 2400, and 4000) and diamond abrasives (6, and 1 μm) to flatten the cross-section subject for analysis. Subsequently, to prevent contamination of the sample cross-section, the sample is cleaned three times for 5 minutes at each polishing stage using an ultrasonic cleaner.

Because ancient glass beads are small artifacts, it is difficult to identify their visible characteristics through typical naked-eye observation. This study intends to determine the characteristics observed on the surfaces of round glass beads using a microscope and an EDS spectrometer.

The observed cross-sectional characteristics of 10 glass beads from Naedeok-ri, Gimhae, are summarized as follows (Table 4). All these glass beads are cobalt blue. Wooden coffin tomb No. 19, from the Proto-Three Kingdoms period, and wooden coffin tomb No. 92, from the Three Kingdoms period, are matched with ultramarine and Wedgewood blue using the Munsell Color System. The same results are identified as cobalt blue, as in Yangdong-ri and Daeseong-dong in Gimhae (Kim, 2023). The production technique is determined through bubble arrangement to be a drawn technique, and severe weathering was identified on the outer surface of the beads based on the measurement of the weathering rate.

To determine the cross-sectional composition of 10 glass beads from Naedeok-ri, Gimhae, the microstructure variables (color, weathering state, and weathering rate) are measured three times and summarized with averages and standard deviations. The results are diagrammed according to the flux and stabilizer components to evaluate the composition characteristics of the glass beads (Table 5, Figure 3). The flux characteristics of the glass beads from the Naedeok-ri site, Gimhae are represented by the potash glass groups, including soda (Na₂O) of 0.31–1.02% and potash (K₂O) of 19.23–21.62%. The stabilizer characteristics correspond to potash type I in that both lime (CaO) and alumina (Al₂O₃) are less than 3%. The colorant is identified as cobalt (Co) based on a high composition of manganese oxide (MnO) and iron oxide (Fe₂O₃). Cobalt has a high color development power, enough to produce blue in small amounts of less than 0.1%. Because it is contained in trace amounts in ancient glass, it is not detected during EDS analysis. Since cobalt is produced together with manganese when extracted from a mine, cobalt can be inferred based on the manganese content (Kyriaki P et al., 2011).

All ten glass beads from the Naedeok-ri site correspond to potash type I. Considering the analysis results of glass beads excavated from the Yangdong-ri and Daeseong-dong tombs in Gimhae, despite the influx of low-soda alumina in the early 3rd century, potash type I glass beads had been in continuous use throughout the entire Geumgwan Gaya period. Because these results are obtained from broken beads, the possibility exists that other materials may have been introduced into the glass. Therefore, this study proceeded to compare the results of the surface composition analysis by increasing the number of analysis samples, mainly of the round beads (Figure 4).

The surface characteristics of 38 round glass beads excavated from Naedeok-ri, Gimhae, are investigated, and shape, color, heat treatment, surface conditions, size, and production technique data are summarized as follows (Table 6).

The shape of the beads is classified based on the outer diameter of the bead divided by its length. There are 20 round shapes, 13 donut shapes, 2 tubular shapes, and 2 square shapes (Figure 5).

The eight color types of the beads are identified and classified as consisting of 19 cobalt blue beads, 6 copper blue beads, 1 greenish blue bead, 1 Blueeish green bead, 1 green bead, 6 brown beads, 2 lemon yellow beads, and 1 gray bead. The bead colors vary depending on the period: blue colors, such as cobalt blue and copper blue, correspond to the Proto-Three Kingdoms and Three Kingdoms periods, while brown and lemon yellow beads correspond to the Joseon Dynasty (Figure 6). The gray color is determined to be a mineral bead.

The heat treatment technique is used to verify whether the beads had been appropriately molded after fabrication, which appears to generally be the case. Traces of polishing are identified in the long tubular beads (KN-19N-01) and mineral beads (KN-67N-02). Although the surface conditions of the beads may have been generally good, some beads showed surface weathering or cracks; thus, limited data is expected from surface analysis. The production technique is judged to be a stretching technique if any stripe or bubble arrangement matches the direction of the inner diameter, a drawn technique if there are stripes in the vertical direction of the inner diameter, and a winding technique if there are stripes parallel to the inner diameter. According to the classification results, the glass bead production techniques used in Naedeok-ri Gimhae varied depending on the period. Folding and drawing, winding techniques are found in remains created during the Proto-Three Kingdoms period, the Three Kingdoms period, and the Joseon Dynasty, respectively. The bead sizes are compared by referring to the previously determined size classification data of 129 pieces from the Geumgwan Gaya region. The sizes of Geumgwan Gaya glass beads are divided into three categories based on the outer diameter: Gimhae small-sized, 2.9±0.4 mm, Gimhae medium-sized, 4.8±0.6 mm, and Gimhae large-sized, 7.0±0.7 mm (Kim et al., 2021). The sizes of glass beads from Naedeok-ri, Gimhae, are identified as 21 small-sized, 8 medium-sized, and 1 large-sized, with 7 unclassified between small- and medium-sized beads. Depending on the period, glass beads from Naedeok-ri, Gimhae are divided into small- and large-sized beads from the Proto-Three Kingdoms and Three Kingdoms periods, and medium-sized beads from the Joseon Dynasty. Considering all Gaya remains, small-sized beads were popular during the Proto-Three Kingdoms and Early Gaya periods, while medium-sized beads became widely accepted during the later Gaya period. Small-sized beads appear to have been popular in Naedeok-ri, Gimhae, until the late Gaya period, and the size of one tubular-shaped bead (KN-19N-01) is confirmed to equal that of the beads from the Proto-Three Kingdoms period (Kim, 2023).

The non-destructive analysis results of 38 round glass beads excavated from Naedeok-ri, Gimhae, and their classification results according to composition are described as follows (Table 7, Figure 8).

According to the classification results of the flux characteristics of Naedeok-ri glass beads, there are 3, 28, 5, 1, and 1 beads consisting of lead glass, potash glass, soda glass, mineral, and unclassified groups, respectively.

The components of the lead glass group vary depending on the era. These include one lead-barium glass bead (KN-19N-01) in which barium (Ba) is detected, which was excavated among remains from the 2nd century, and two lead glass beads in which only lead (Pb) is detected. They were excavated from the remains of the Joseon Dynasty (KN-98N-07, 08). Regarding the stabilizer characteristics, lead-barium glass beads are found to contain 0.97% lime (CaO) and 2.66% alumina (Al₂O₃). Therefore, with lime being less than 2% corresponding to lead-barium type II, these are identified as from before the 3rd century CE. Converting the detected elements into the ratio of silicon oxide (SiO₂) and lead oxide (PbO), it is estimated that the two lead glass beads from the Joseon Dynasty were made with an average ratio of silicon to lead of 60:40. In the case of leaded glass, it is made of 60:40 and exists as a composition value (Kim, 2023).

The potash glass group is divided based on the type of added stabilizers into potash type I with 13 beads, potash type II with 9 beads, and potash type III with 6 beads. The stabilizer composition of potash type I is lime (CaO) and alumina (Al₂O₃) of less than 3%, of which all the beads are cobalt blue and are excavated from wooden coffin tomb No. 19, which was constructed in the 2nd century. The sample analysis showed the same results, indicating that potash type I had been used until the 4th century. Five cobalt blue and four copper blue beads corresponded to the potash type II group, showing a composition of 3% or more of alumina (Al₂O₃). Emerald blue beads showed a visible characteristic of a large amount of internal bubbles, which appears to result from the addition of alumina at the time of production. The cross section analysis identified all the beads from Tomb No. 92 as potash type I, while the surface analysis classified all except one as potash type II. During the 4th century, the stabilizer components of the cobalt blue beads varied. Potash Type III was found in remains No. 98 from the Joseon Dynasty, with stabilizer components of lime (CaO) and alumina (Al₂O₃) of 5% or more. This glass composition is typically found in beads from the Goryeo and Joseon Dynasties. Although this investigation separated the samples into dark brown and brown, there is only a difference in the content of iron oxide (Fe₂O₃), a glass colorant, and the remaining components are the same.

The soda glass group consists of four beads from No. 62 and one bead from No. 67, all created in the 4th century. These exist in various colors, such as copper blue, green, and turquoise. The stabilizer component is determined to be soda alumina with an alumina content of 5% or more.

The mineral beads are gray and are excavated from 5th-century remains. They contain 55.88% of silicon oxide (SiO₂) and 10.63% of magnesium oxide (MgO), which is considered to be talc. The chemical formula of talc is Mg₃Si₄O₁₀(OH)₂ and it have the lowest hardness of 1 on the Mohs scale of mineral hardness. Its softness allows drilling and facilitates the production of beads.

This study examines the characteristics of remains by comparing surface analysis results of glass beads from Naedeok-ri, Gimhae, with the chemical compositions of beads from the Gimhae region (Figure 9). In Naedeok-ri, Gimhae, lead-barium glass beads appear during the Proto-Three Kingdoms period. In Naedeok-ri, Gimhae, only potash glass beads are identified as originating during the early Gaya period, and soda glass beads are found from the later Gaya period. However, compared with other remains in Gimhae, the absence of potash glass and the high proportion of soda suggest that Naedeok-ri, Gimhae, is influenced by Silla rather than Gaya in the 5th century. As Naedeok-ri in Gimhae entered the Joseon Dynasty, its glass beads exhibit different compositional characteristics from those of the previous Proto-Three Kingdoms and Three Kingdoms periods.