The purpose of this study is to examine types and characteristics of alkali glasses and to review distribution period and regional characteristics of each, by analyzing microstructure and chemical composition of 491 glass beads excavated from 12 sites...
The purpose of this study is to examine types and characteristics of alkali glasses and to review distribution period and regional characteristics of each, by analyzing microstructure and chemical composition of 491 glass beads excavated from 12 sites of the Three Kingdoms period and by applying new classification of chemical composition on 847 glass beads excavated from 29 sites after summarization of analysis data presented until now. In addition, types of classes and distribution aspects that differ according to color are to be compared. Trading relationship of alkali glasses during the Three Kingdoms period and domestic glass production possibility will be discussed by reviewing the correlation between forming technique and chemical composition.
As analysis methods used in this study, forming technique was measured by microstructure observation using stereoscopic and electron microscopes. Chemical composition was measured using SEM-EDS, trace element of gold foil glass beads using LA-ICP-MS, lead isotopic ratio of high alumina glass of yellow and green using TIMS, and sintering temperature of cast glass beads using high temperature microscope.
According to the study results for each site, chemical composition of glass beads differs depending on the color of glass beads as well as period and regional characteristics of the excavated site. While sites of Baekje regions where blue beads are mainly excavated show dominance of potash glass prior to 3rd century and natron glass after 3rd century, Mahan sites with different colors of glass beads generally show dominance of high alumina glass. On one hand, sites of Shilla and Gaya have monotonous blue color for glass beads compared to Baekje regions. These sites show dominance of potash glass until 2nd century and diverse soda glasses since 3rd century.
Chemical composition of alkali glasses is classified into potash glass I and II types, high alumina glass, natron glass, plant ash glass, and mixed alkali glass. Potash glass I type appeared during 1st century B.C., potash glass II type during 1st century A.D., plant ash glass during 1st~2nd centuries, high alumina glass and natron glass during 2nd century, and mixed alkali glass during 3rd century to show sustained distribution. However, potash glass II type was not found after 4th century. Number of entities for potash glass I type simultaneously increased with appearance, and number of high alumina glasses and natron glasses increased during 2nd and 3rd centuries to become the new main types. Considering the fact that mixed alkali glass appeared later and had smaller share than potash glass and soda glass, as well as the difference in material characteristics from mixed alkali glasses of Goryeo and Joseon periods, mixed alkali glass during this time seems to be a glass composition temporarily created by mixture of potash glass and soda glass.
In terms of characteristics of each color, greenish blue and bluish green show diverse compositions including potash glass I and II types, high alumina glass, and plant ash glass. Purple blue also shows diverse compositions such as potash glass I type, high alumina glass, natron glass, plant ash glass, and mixed alkali glass. On the contrary, red is shown in high alumina glass, natron glass, and plant ash glass. Yellow and green are shown in high alumina glass, showing a close relationship with soda glass culture. As such, chemical composition of glasses differed according to color, and differences were found in the period of appearance, distribution and aspect of trend among different colors of the same composition. Main chemical composition of greenish blue and bluish green glasses changes from potash glass I and II types to high alumina glass during 2nd and 3rd centuries. Main composition of purple blue glass changes from potash glass I type to natron glass during the same period. For red, yellow and green glasses that appear after 2nd and 3rd centuries, high alumina glass becomes the trend of these colors concurrently with their appearance. Yellow and green high alumina glasses containing lead as opaque materials were analyzed for lead isotopic ratio, which shows that most of lead were produced in Thailand. This result supports existing study results that high alumina glass is related to Southeast Asia.
Forming techniques for glass beads during the Three Kingdoms period include drawing technique, segmental technique, casting technique, winding technique and folding technique. Most of glass beads were formed by drawing technique. Gold foil glass beads and monochromatic segmented beads formed by segmental technique were verified as natron glass or plant ash glass predicted as Western glasses such as glass ware. Therefore, they were probably introduced through identical origin and distribution process as glass ware. Especially, gold foil glass beads are classified into three types including gold foil and natron glass, gold foil and plant ash glass, and silver foil and plant ash glass depending on the material of foil and chemical composition of glass. While first type was frequently used in wide range of time and space between 2nd and 7th centuries when gold foil glass beads appeared, third type was introduced to the Korean peninsula around 5th century and used during 6th century. Glass beads formed by casting technique seem to have been manufactured by sintering glass powders from damaged glasses at 780~800℃. Such technique is sufficient to produce beads using damaged glasses and mold without workshop facility or raw material. Thus, secondary processing of glass in Korea was probably carried out since 3rd century, when such glass beads were excavated.