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Traditions and transitions in Korean bronze technology
Journal of Archaeological Science 34 (2007) 1991e2002 http://www.elsevier.com/locate/jas
Traditions and transitions in Korean bronze technology Jang-Sik Park a,*, Robert B. Gordon b,1 a
Department of Metallurgical Engineering, Hongik University, Chochiwon, Choongnam 339-701, Republic of Korea b Department of Geology and Geophysics, 207 KGL, Yale University, New Haven, CT 06520-8109, USA Received 5 January 2006; received in revised form 17 January 2007; accepted 23 January 2007
Abstract Metallurgical examination of Korean bronze artifacts shows that a technical tradition based on casting and use of leaded high-tin alloys was established in Korea at the early stages of bronze use. After the subsequent discovery of quenching methods that suppress formation of the brittle d phase, new thermo-mechanical techniques were introduced between the 7th and 10th centuries AD. Lead-free alloys were used, and tin contents near that of the peritectic point in the CueSn phase diagram were chosen. Leaded high tin alloys continued in use, but only in cast objects, and with significant composition variation. The unique conditions during the time of innovation suggest that the transition to new metallurgical techniques was gradually achieved through domestic technical innovation inspired by external influences. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Korea; Bronze technology; Alloy composition; Casting; Hot forging; Quenching
1. Introduction It is believed that bronze objects were first used in Korea around the turn of the 1st millennium BC. It is yet to be determined if they were imported or locally produced. Local bronze production is estimated from archaeological evidence to have started no later than the 6th century BC (Kim, 1998). The Korean bronze culture at the early stage is characterized by the two prominent elements, daggers and mirrors. The early Liaoning style daggers were soon replaced by the so-called Korean-style daggers with a substantial modification in shape (Kim, 1986). The mirrors are famous for the fine, cast-in decoration on their back faces based on a linear geometric pattern depicting arrays of hatched triangles. Attempts have been made to characterize the early Korean bronze culture on these typological grounds (Barnes, 1999; Kim, 1986). The results show that this culture received a significant influence from the arts of the Ordos and Siberian steppe, which are distinguished from the * Corresponding author. Tel.: þ82 41 860 2562; fax: þ82 41 866 8493. E-mail addresses: [email protected] (J.-S. Park), robert. [email protected] (R.B. Gordon). 1 Tel.: þ1 203 432 3125; fax: þ1 203 432 3134. 0305-4403/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2007.01.010
style of Mainland China. The unique transitions found thereafter in both the shape of daggers and the decoration patterns of mirrors, which may be traced in a chronological order, are interpreted as a sign of localized development. Later, iron was introduced and spread throughout the Korean peninsula, and new articles for varying purposes such as vessels, bowls, spoons, ornaments and horse trappings (Kim, 1986, 1998) were made while the production of bronze daggers and tools was abandoned. The changes apparent in artifact typologies raise a question concerning the transitions in technology. Recent research shows that the important feature of the early Korean bronze is the use of casting as the major fabrication technique and the use of relatively high tin alloys with frequent lead additions (Choi, 1996; Lee et al., 2001). For the later development, Voce (1951) examined the microstructures of two Korean bronze bowls with an unknown provenance dated to the 12th to 14th century AD, and found the application of hot forging and quenching on alloys of about 20% tin. His work may be the earliest example showing Korean high tin bronzes consisting of the twinned a phase in the matrix of the quenched b phase. Recent metallurgical work by Choi and Park (2004) on objects from this period showed that similar
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technique and alloys were employed for kitchenware including spoons, chopsticks and large vessels. Lee and Lee (2002) reported that the traditional musical instruments are made in Korea today by the same technique. In their work on the artifacts from a historic site of the 7th to 10th century AD, Jung and Park (2005) found evidence of a gradual technical transition in high tin bronzes involving forging or quenching. Kang et al. (2000) found that bronze objects from a Korean temple site in use from the 10th to 14th century AD were either cast or forged. The forged objects contained 20% tin but no lead, and were finished by quenching. The cast objects contained approximately 10% tin and up to 20% lead. It is apparent that the early Korean bronze technology underwent a significant transition with the introduction of thermo-mechanical treatments. These treatments, applied subsequently after the initial casting operation, must have required a closer adjustment of alloy composition than in earlier practice. This study interprets the microstructures of Korean bronze objects spanning approximately 2000 years, with a special emphasis on tracing the particular sequence of thermal and mechanical treatments chosen for a given alloy composition. Trends were found in the temporal variation in alloy compositions and the associated fabrication techniques. These give us a better understanding of the process of technical development between the 7th and 10th century AD in Korea.
2. Comments on artifacts and history A hundred and five artifacts from the southern part of the Korean peninsula were examined. Map 1 locates some of the major sites from which they were excavated. The dates estimated on typological grounds extend from the 3rd century BC to 18th century AD. Table 1 summarizes the number of objects examined, the artifact types and dates. Those from the same site are expected to have a similar age, but quite a few artifacts were excavated from temple sites, residential areas and military fortresses used by more than one kingdom. They were placed in Table 1 in such a way as to span the entire probable period. Kitchenware was the item excavated most frequently, and consisted primarily of bowls, vessels and spoons. Spoons had special significance in their quantity and microstructures, and were placed separately. The weapon included five daggers and two spearheads and an arrowhead, and the tool included four axes and two chisels. The weapons and tools were distinguished from the others in that none of them were recovered from the sites of the Korean Three Kingdoms period (ca. 300 ADe668 AD) and afterwards. The character next to each number in Table 1 indicates the method of fabrication as observed in the microstructure examination. Objects made primarily by casting with no other noticeable treatments applied subsequently are designated c. The letter f indicates items finished by hot forging with no quenching,
Map. 1. Map of the Korean peninsula locating some of the major historical sites from which the present bronze artifacts were excavated.
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Table 1 Number of Korean bronze objects examined versus artifact type and date Date
BC 3eBC 1
AD 4e7
AD 7e10
AD 10e14
AD 14e18
Type of artifact
Early Iron Age
Three Kingdoms
Unified Silla
Koryo
Choseon
Weapon Tool Mirror
8c* 6c 3c*
1c
2c*
Total
1c
1c
8c 6c 9c
10fq*
6c* þ 6fq*
6c þ 1q 19c þ 1f þ 6q þ 19fq
7fq*
10fq
1f þ 21fq
5c þ 1f 6c þ 1f þ 17fq
1c 8c þ 16fq
7c þ 1f 105 (55c þ 3f þ 7q þ 40fq)
1c Ornament Kitchenware
2c 4c* þ 1f*
4c* þ 1q 3c þ 4q* 6c þ 2q þ 3fq
Spoon 1f* þ 4fq Others Total
1c 8c þ 1f
17c
9c þ 5q 7c þ 1f þ 2q þ 7fq
‘‘c’’ denotes the objects made primarily by casting with no noticeable thermo-mechanical treatments applied subsequently. ‘‘f’’ denotes the objects finished by forging. ‘‘q’’ denotes the objects given a quench treatment after casting. ‘‘fq’’ denotes the objects finished by forging and then quenching. Empty cells mean no object examined. ‘‘*’’ denotes the group from which one or more objects were chosen for the microstructure presentation in this article.
and q those that were quenched but not forged. The objects forged and then quenched are denoted by fq. The majority of the artifacts in Table 1 were made from the 4th century AD onward when the Three Kingdoms period was firmly established. In this era the Korean peninsula was divided by the four rival kingdoms, Koguryo in the north extending to Manchuria, Silla and Paekche in the southeast and southwest, respectively, and Kaya bordering the southern coast between Silla and Paekche. They had a formation time of approximately 300 years during which the Han colonies installed in northern Korea were removed by the indigenous polities. In the middle of the 7th century AD the Silla kingdom established the Unified Silla by conquering Paekche in AD 660 and Koguryo in AD 668 with the aid of the Chinese Tang forces. The Unified Silla kingdom was succeeded by the Koryo dynasty in the 10th century AD, which was then replaced by the Choseon dynasty in the late 14th century AD (Eckert et al., 1990). Prior to the Three
Kingdoms period, there was the Korean Bronze Age (ca. from BC 10th) followed by the two stages of Iron Age, Early (BC 3rdeBC 1st) and Late phase (AD 1steAD 3rd) (Kim, 1998). No artifacts from the Bronze Age and the Late Iron Age were available for examination. 3. Microstructure examination For microstructural examination specimens from the artifacts were mounted and polished following standard metallographic procedures and then etched with a solution of 100 ml distilled water, 30 ml hydrochloric acid and 10 g ferric chloride. They were examined using an optical microscope and a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). Because of the restriction in the amount of samples available their chemical compositions could only be estimated by analyzing the EDS spectra. And the
Table 2 Bronze artifacts presented for the illustration of microstructures No.
Artifact
Excavation site
Date (Century) historical period
Composition (mass %)
Treatments applied
1 2 3 4 5 6 7
Dagger Spearhead Mirror Dagger Bowl Bowl Spoon
Tanbangdong at Daejeon
BC3eBC2 Early Iron Age BC2eBC1 Early Iron Age AD5 Three Kingdoms
Sn: 8, Pb: 33 Sn: 22, Pb: 9 Sn: 32, Pb: Detected Sn: 17, Pb: Detected Sn: <15, Pb: Detected Almost pure Cu Sn: 22
Cast
8 9 10 11 12 13 14 15 16 17
Buckle Mirror Vessel Gilt Vessel Vessel Vessel Spoon Spoon Vessel Vessel
Wonboongni at Nonsan Great Hwangnam Tomb at Kyongju Wang-gyong site at Kyongju Boonhwang temple site at Kyongju
AD4eAD14 Three Kingdoms to Koryo AD7eAD10 Unified Silla
Boosoori at Boeun
AD11eAD14 Koryo
Gaodong at Daejeon
AD17eAD18 Choseon
Sn: Sn: Sn: Sn: Sn: Sn: Sn: Sn: Sn: Sn:
14, Pb: 12 27 24 22 22 22 22 22 21 23 Pb: 4, As: 1
Cast Cast Forged Forged Cast Cast Quenched Quenched Quenched and Forged
Quenched and Forged Cast
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results were given in weight fraction up to the nearest whole digit although the EDS provides the accuracy of a few tenth of a percent. This is sufficient for the present investigation. The microstructures were closely associated with the method of fabrication, and the objects made by the same technique had qualitatively similar microstructures. One or more representative articles were selected from each of the groups marked by an asterisk in Table 1, and a total of seventeen will be presented. Table 2 summarizes them in a chronological order. 3.1. Artifacts from the Tanbangdong site at Daejeon (BC 3rdeBC 2nd) Fig. 1a shows a Korean-style dagger excavated from the Tanbangdong district at Daejeon and dated to the 3rde2nd
century BC (Lee et al., 2001). Fig. 1b, a secondary electron image (SEI) micrograph showing the microstructure of the dagger at the area marked, consists of numerous bright areas of varying sizes in the dark background. The EDS analysis showed that the bright areas correspond to almost pure lead (Pb) and the background to the a phase of the CueSn alloys. The major elements included were copper (Cu), lead and tin (Sn) as shown in Fig. 1c, an EDS spectrum taken from the whole area of Fig. 1b. The average composition was estimated from the spectrum to be 59% Cue33% Pbe8% Sn. Fig. 1b shows that lead, having a low freezing temperature, existed as liquid droplets spread over the solid a phase before they solidified to form individual lead inclusions in the a matrix. It is evident that the dagger was shaped primarily by casting. No hint of mechanical working or thermal treatments after casting was observed. Fig. 2a shows a spearhead excavated together with the above dagger. Fig. 2b is a SEI micrograph of its microstructure. The bright regions, as in the dagger, correspond to almost pure lead, and the background consists of the a and d phases. The relative fractions of lead and tin were approximately 9% and 22%, respectively. The decreased lead content as compared with that of the dagger is apparent in the reduced lead areas, and the increased tin is verified in the presence of the high-tin d phase. No evidence of treatments other than casting is observed. 3.2. Artifacts from the Wonboongni site at Nonsan (BC 2ndeBC 1st) Fig. 3a shows a mirror, which is one of the bronze objects from the earth-pit tombs at the Wonboongni district in Nonsan (Cho
Fig. 1. Korean style dagger excavated from Tanbangdong at Daejeon, Korea. Estimated date: 3rde2nd century BC. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 1a. (c) EDS spectrum taken from Fig. 1b.
Fig. 2. Spearhead excavated from Tanbangdong at Daejeon, Korea. Estimated date: 3rde2nd century BC. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 2a.
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Fig. 4. Dagger excavated from Wonboongni at Nonsan, Korea. Estimated date: 2nde1st century BC. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 4a. Fig. 3. Mirror excavated from Wonboongni at Nonsan, Korea. Estimated date: 2nde1st century BC. (a) General appearance. (b) Sketch showing the cast-in geometric pattern on the back face of the mirror. (c) Optical micrograph showing microstructure at the area marked in Fig. 3a.
3.3. Artifacts from the Great Hwangnam Tomb at Kyongju (AD 5th)
et al., 2001), and Fig. 3b is a sketch of the peculiar cast-in geometric pattern on its back. This type of mirror is dated approximately to the 2nde1st century BC, which is in accord with the estimation on pottery typology. Fig. 3c, an optical micrograph, shows its microstructure consisting mostly of the d grains with a small amount of unknown phase between them, indicating that the general tin content is close to 32%. Of all the bronze artifacts so far reported in Korea this is one with the highest tin content. In view of the brittleness of the d or 3 phase the 32% tin level may be close to or beyond the practical upper limit for CueSn alloys. The experiment with a 65% Cue35% Sn alloy by Choi et al. (2003) showed that the de3 eutectoid, which can form above 32.5% Sn, was more prone to crack formation than the single d phase. The presence of lead was observed, but the amount was not enough to affect the overall composition. Fig. 4a shows a broken dagger from the same site as the mirror. Fig. 4b, an optical micrograph, shows the a dendrites in the background of the aed eutectoid, which is too fine to be resolved. The aed eutectoid forms in the CueSn alloys as a result of the decomposition of the g phase simultaneously into the a and d at 520 C. The circular crater marked by an arrow is filled with almost pure lead segregated during the solidification of the casting. The EDS data showed that the tin content was approximately 17% and that the lead content was negligible compared with copper and tin. Of the thirteen objects from the site the mirror and dagger represent the upper and lower limit in tin content. Lead inclusions such as that in Fig. 4b were found in most of them, but the level of lead was not significant.
Fig. 5a presents a bowl which is one of the cast or hammered bronze objects from the Great Hwangnam Tomb in Kyongju, the former capital of the Silla and the following Unified Silla kingdom. The tomb, the greatest of all in the neighborhood in size as well as in the amount of burial goods, was estimated to have been constructed in the 5th century AD (Kim et al., 1994; Portal, 2000). Fig. 5b, an optical micrograph, shows the dendrite pattern present in the structure, indicating that the artifact was manufactured primarily by casting. Fig. 5c, a SEI micrograph magnifying Fig. 5b, reveals micro voids and inclusions in the background. The EDS analysis showed that the inclusions were high in copper, lead and tin while the background was the a phase. These phases may have been affected by corrosion. The attempt to estimate the average composition from the EDS data was complicated by the corrosion of the sample as can be seen in Fig. 5b, and the results were inaccurate. The tin content inferred from the morphological characteristics in Fig. 5b should be below 15%. Fig. 6a is another bowl from the tomb. The bowls in Figs. 5a and 6a must have been made for a similar purpose, but differences were noted in the microstructure and chemical composition. No dendrite structure such as that in Fig. 5b is observed in Fig. 6b, an optical micrograph of the microstructure near the bottom of this bowl, and no other elements than copper were detected in the EDS analysis. The structure in Fig. 6b, therefore, corresponds to the a phase of almost pure copper. The matrix contains twin boundaries along with a number of dark spots well aligned in horizontal layers. The examination in the SEM showed that the spots correspond mostly to micro voids and occasionally to inclusions high in lead and tin.
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Fig. 6. Bowl excavated from the Great Hwangnam Tomb at Kyongju, Korea. Estimated date: 5th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 6a.
Fig. 5. Bowl excavated from the Great Hwangnam Tomb at Kyongju, Korea. Estimated date: 5th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 5a. (c) SEI micrograph magnifying Fig. 5b.
The voids aligned in layers and the twin boundaries are the direct evidence of heavy working and the subsequent annealing applied in fabrication. This bowl had thin walls of 1mm or less, and was the only object made of unalloyed copper among those in Table 1. 3.4. Artifact from the Wang-gyong site at Kyongju (AD 4the14th) Fig. 7a shows a broken spoon from the Wang-gyong site located in the city of Kyongju (Kim et al., 2002). The site covered most of the core districts of the former capital of the Silla and the Unified Silla kingdom, and was given the name Wanggyong, meaning the capital city. It continued to be a major city
in the following Koryo dynasty. Table 1 shows that spoons started to appear from the Koryo period. The only exception is that in Fig. 7a, which may also have been made during the later period of this site, i.e., the Koryo period. This spoon had a very thin wall of approximately 0.3 mm, and had a unique microstructure clearly distinguished from any of the other spoons. Fig. 7b, a SEI micrograph showing its microstructure, consists of the aed eutectoid in the background of the a grains containing straight twin boundaries. The spoon was evidently hot forged and then finished by slow cooling. The tin content was 22%, and no lead was detected anywhere in the specimen.
3.5. Artifacts from the Boonhwang temple site at Kyongju (AD 7the10th) Fig. 8a shows a buckle from the Boonhwang temple site at Kyongju (Jung and Park, 2005). Fig. 8b, an optical micrograph, retains in the dark regions the dendrite patterns formed in casting. The dark regions consist of the a phase, and the bright interdendritic regions are filled with the aed eutectoid. The white arrow in Fig. 8b locates one of the regions filled with almost pure lead. The average composition was 74% Cue14% Sne12% Pb. No evidence of treatments other than casting was observed.
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Fig. 7. Spoon excavated from the Wang-gyong site at Kyongju, Korea. Estimated date: 4the14th century AD. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 7a.
Fig. 9a is a mirror from the same site. Fig. 9b, a SEI micrograph, consists entirely of the fine aed eutectoid. This indicates that the tin content is close to 27%, which was also verified from the EDS data. No other elements than copper and tin were observed.
Fig. 8. Buckle excavated from the Boonhwang temple site at Kyongju, Korea. Estimated date: 7the10th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 8a.
Fig. 9. Mirror excavated from the Boonhwang temple site at Kyongju, Korea. Estimated date: 7the10th century AD. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 9a.
Fig. 10a shows a broken vessel from the site. In Fig. 10b, an optical micrograph, the large bright regions represent the a phase in dendrites, and the background consists of two constituents, the light acicular martensite and the dark matrix of the retained g phase. Laboratory trials showed that a structure resembling the background was obtained when the b phase was quenched from a relatively low temperature near 586 C. It seems likely that during the initial stage of quenching while the temperature drops from 586 C to 520 C, some of the b transforms to g to produce a mixture of b and g, which then transforms to the mixed structure as in the background. Even with this special thermal treatment the individual dendrite regions all remain as single crystals, implying that little or no mechanical working was applied. The overall tin content was 24%. No other elements were detected, and lead was not present even in the form of isolated inclusions. Fig. 11a is a fragment from the above site. The surface retained a thin gold layer at the bright areas, and it must have broken off a gilt bronze vessel. Fig. 11b, an optical micrograph, shows the bright a dendrites in the dark background of the g phase retained by rapid cooling. The dendrite regions were all single crystals. This artifact is similar to the above vessel in the treatments inferred from its microstructure. A difference is observed, however, in the exact tin content and the temperature from which it was quenched. The lower tin content, approximately 22%, is revealed by the increased
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Fig. 12. Vessels and spoon excavated from Boosoori at Boeun, Korea. Estimated date: 11the14th century AD. (a) General appearances. (b) Optical micrograph showing microstructure at the area marked in the left vessel in Fig. 12a. Fig. 10. Vessel excavated from the Boonhwang temple site at Kyongju, Korea. Estimated date: 7thV10th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 10a.
a regions. The background microstructure consisting exclusively of the g phase places the temperature from which the metal was quenched between 520 C and 586 C. No lead was detected anywhere in the specimen.
Fig. 11. Gilt bronze vessel excavated from the Boonhwang temple site at Kyongju, Korea. Estimated date: 7the10th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 11a.
3.6. Artifacts from other sites built after AD 11th century Fig. 12a shows three artifacts, two vessels and a spoon, excavated from a burial site at the Boosoori district in Boeun, which was constructed in the 11the14th century AD (Cho et al., 2004). Their microstructures were all similar to that in Fig. 12b, an optical micrograph taken from the vessel at the left. The average tin content for all is about 22%, and no lead was found. Fig. 12b shows the a regions and the martensite in the background as expected in 78% Cue22% Sn alloys when quenched. While no twins are present in the a regions of the artifacts described above, they are present in the structure shown in Fig. 12b. The twin formation by recrystallization occurs only in alloys which have been mechanically strained as by hammering. It is evident that the artifacts were mechanically worked before being quenched. Forging must have been essential for the thinning of the walls down to 0.3 mm or less. Fig. 13a shows three fragments from a spoon and two vessels from a burial site at the district of Gaodong in Daejeon, which was constructed between the late 17th and early 18th century AD (Cho et al., 2003; Park, 2003). The microstructure and chemical composition of the spoon and the vessel next to it were almost identical to those in Fig. 12a. The vessel on the right, however, varies significantly as can be seen in Fig. 13b, an optical micrograph, where the cast dendrite structure is retained. The average alloy composition was 72% Cue23% Sne4% Pbe1% As (arsenic). No evidence of forging or quenching was observed. The presence of lead and arsenic is apparently associated with the different fabrication technique.
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Fig. 13. Spoon and vessels excavated from Gaodong at Daejeon, Korea. Estimated date: 17the18th century AD. (a) General appearances. (b) Optical micrograph showing microstructure at the area marked in the vessel at the right hand side of Fig. 13a.
4. Discussion Table 1 summarizes the number of artifacts examined versus artifact types and dates along with the method of fabrication. As for the Bronze Age objects, the extensive data reported by Choi (1996) on chemical compositions are available for comparison. But no items from the 1st and 2nd century AD were included in his work. Collins (1931), by quoting the work by Umehara (1927), reported on some Korean bronzes belonging approximately to this period. They were excavated from the region near the modern city of Pyongyang, which was then occupied by the Han commandery, Lelang, and might have been made under strong Chinese influence. It is not clear why the items from this specific period are rarely available. Within these limitations, the data in Table 1 reveal some important trends, which may best be recognized in the last row where the total number of the objects belonging to each period is given. The data placed in three cells combined reflect the uncertainties in dating. First, the seventeen objects of the Early Iron Age were all made exclusively by casting; second, cast items continued to be made throughout the entire period; third, the mechanical working subsequent to casting started at the Three Kingdoms period, and the thermal treatment at the Unified Silla period; fourth, the treatment of forging followed by quenching became a standard recipe from the Koryo period. Forging and quenching were applied separately before they were combined. The benefit of the combined thermo-mechanical treatments seems to have been recognized gradually over several centuries until
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they became a new technical tradition at around the 10th century AD. The new tradition dominated the Korean bronze industry for the following one thousand years. Table 2 provides the supplementary information that is not evident in Table 1, especially on alloy compositions. The tin and lead contents in cast objects show a large variation. This has been also noticed in other research. The twenty four early Korean daggers in Choi’s work (1996) contained 0.25% to 25.0% Sn averaging 13.6%, and 0% to 14.22% Pb averaging 6.9%. As for Korean mirrors, Choi reported three containing 16.6 w 27.3% Sn and 4.0 w 11.4% Pb, and Chikashige’s analysis in Collins’ paper (1931) showed three containing 22.89 w 26.97% Sn and 1.54 w 2.06% Pb, and one with 49.05% Sn and 4.02% Pb. Chikashige (1920) reported two Korean mirrors of 21.9% Sne9.68% Pb and 11.76% Sne15.14% Pb. The 33% Pb at No. 1 of Table 2 is unusually high, far beyond the upper limit that has been reported. The amount of lead detected in the Wonboongni artifacts was too little to count as a deliberate addition (Craddock, 1979). No other elements than copper, tin and lead were detected in such an amount as to affect the overall compositions. The only exception was arsenic, which was observed only in cast objects as in the vessel No. 17. The presence of arsenic from 1.0% to 1.14% was also reported in three of Choi’s daggers. Other minor elements such as iron, zinc and antimony were observed by Choi and Chikashige. The compositions in the cast objects of the later period, No. 5, 8, 9 and 17, showed considerable variation. Table 2 showed that the introduction of thermo-mechanical treatments brought about a conspicuous change in the design of alloys. The tin content in those treated tended toward the peritectic point of the CueSn system, 22%, and lead was kept from alloys. The bowl No. 6 and the spoon No. 7 could be the earliest of the hammered objects. The former was dated to the mid 5th century AD, and the latter anywhere from the 4th to 14th century AD. The bowl has special importance since it was the only object made of copper without alloying, and was recovered together with cast bronze objects, one of which is the bowl No. 5. It seems obvious that unalloyed copper was seldom selected as a material for home use. This copper bowl may therefore be understood as a new attempt tried only for a limited time. In view of the difficulties that would have arisen in casting, the shaping of the unalloyed copper bowl must have required hammering, most probably at elevated temperatures. This was verified in Fig. 6b. Technically, hammering copper or any other metals to shape was certainly nothing new at the time, but it has a special meaning in this case as an early attempt at forging copper, which could be extended to other copper-based alloys or even to high tin bronzes if the working temperature was properly controlled. Fig. 7b showed that the high-tin spoon was actually hammer-shaped at high temperatures, followed by slow cooling. The d phase in the eutectoid, however, can cause serious brittleness in service, especially when the wall is thinned down to 0.3 mm or less as in this spoon and the other hammered objects. The absence of lead from this article may be understood as an effort to lessen this problem. But the potential advantage of hot forging could
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probably not be fully appreciated until the brittle d phase was appropriately taken care of. The two vessels from the Boonhwang temple site labeled No. 10 and 11 may be important examples of the early experimentation undertaken to solve the difficulties arising from the presence of the d phase. The microstructures in Figs. 10b and 11b show that they were quenched after casting, but not forged. The d phase was replaced by the tougher martensite or g phase. This allowed high tin alloys to be cast into thin-walled articles by taking advantage of their enhanced cast-ability and better strength and toughness. These vessels belong to the earliest of those in Table 1 given the quench treatment, and are distinguished from the later ones similarly treated. They were not hammered; the temperatures from which they were quenched were not consistent, and were lower than those of the later ones; one of them contained more tin than normal. These differences all indicate that the technique was under development with a standard recipe yet to be established. However, no lead was used. It is not even present as isolated inclusions. The subsequent development of technique came from combining the beneficial effects of both forging and quenching while the alloy composition was adjusted to 22% tin and negligible lead. The new technical tradition thus reached the climax characterized by the specific alloy composition, Cue22% Sn, and the unique thermo-mechanical treatments of forging and quenching. Useful bowls, vessels and spoons with thin walls could now be made at will using high tin alloys. The Great Hwangnam tomb, well dated to the mid 5th century AD, provides an important reference point in the tracing of the above transition. The excavation recovered more than 3000 iron objects including various weapons, tools, kitchenware and semi-finished products. The metallurgical examination revealed that the contemporary iron technology in smelting, steel making and various thermomechanical treatments was in a highly advanced state (Park and Jung, 2004). By contrast, the use of copper and copper-based alloys was restricted to home appliances such as bowls, smoothing irons and cauldrons. But no spoons of any kind, let alone those made of bronze, were recovered. A smoothing iron and a cauldron, examined but not presented here, were all cast from unleaded alloys of low tin content of approximately 3%. Experimentation with high tin alloys seemed not to have been seriously tried at this time, and thermo-mechanical treatments on copper-based alloys were in an early stage. The hammered bowl No. 6, however, implies that there was an attempt to find a new method that could give more freedom in fabrication than casting alone. This effort is seen in Table 1 to accelerate during the Unified Silla period. Now the major concern was not in hammering but in heat treatments, as demonstrated in Table 1 by the five high-tin artifacts quenched but not forged. The appearance of many hammered and quenched objects in the following Koryo period indicates that the discovery of this thermal technique was a technical breakthrough for the establishment of a new tradition, which was coincident with the first production of spoons. The spoons were all made by a combination of forging and quenching except that in Fig. 7a, which
could be dated to the earlier period, and therefore could be considered as an earlier trial product. High tin bronzes have a long history of production, especially in China (Barnard, 1961), and in Siberia, Middle East, India, Thailand, Roman worlds and Islamic Iran as well (Allen, 1979; Meeks, 1993; Rajpitak and Seeley, 1979; Srinivasan, 1998). Thermal and mechanical treatments on them are also known to have long been practiced in Central Asia, Thailand and India from as early as the 1st millennium BC. Ravich (1991) examined Scythian and Sarmatian bronze mirrors from Central Asia, and found that they were either cast or forged. The forged mirrors were free from lead and consisted of the twinned a and the quenched b phase. Shemakhanskaya (1991) also reported Central Asian bronze artifacts hot forged using alloys of 20e23% tin and no lead. Rajpitak and Seeley (1979) and Wheeler and Maddin (1976) reported ancient Thai bronze objects treated by forging and quenching. Srinivasan (1998) observed the similar treatments in Indian bronze vessels dating supposedly to 800 BC. Goodway and Conklin (1987) and Sun and Wang (1994) discussed the technical aspects of musical instruments made in Philippines and in China by hot forging and quenching of high tin bronzes. Allen (1979) mentioned the use of both casting and forging in high tin objects from the Islamic period of Iran. Melikian-Chirvani (1974), based on visual inspection, described some of the high-tin bronze vessels from early Islamic Iran that were supposedly made by the similar technique. Melikian-Chirvani proposed on typological grounds the spread of this Islamic high-tin technique to Korea and Japan via the Chinese Tang dynasty (AD 618e907). He suggested that the Japanese word sahari came from the Persian word safidruy meaning high tin bronze. According to the Japanese scholar Chikashige (1920), however, the word sahari refers to ‘Corean bronze’ which Koreans used for making spoons and others. (Korea was often spelled Corea at the time.) On the other hand, Collins (1931), in his quotation of Chikashige’s work, was reluctant to name the specific metal ‘Corean bronze’ because of his premise that the Koreans could not derive their metallurgy from others than the Chinese. It is interesting to see that Chikashige reported the compositions of two Korean spoons, one old and one new, to be Cue24.04% Sne 2.19% Ni and Cue21.62% Sne0.20% Zn, respectively. The absence of lead in both is consistent with the present results although the presence of nickel and zinc is at variance. Chikashige and Collins apparently did not recognize the technical aspects associated with the thermo-mechanical treatments. Considering the composition of the metal called ‘Corean bronze’ and its use in making spoons, it was apparently designed for forging and quenching. Barnard (1961) also quoted Chikashige’s old spoon along with two more Korean spoons. They were consistently made of unleaded high tin bronzes. Phillips (1922) also reported a similar Korean bronze spoon from the Koryo period. Many Korean bronze objects, especially spoons, reported previously even without recognizing the thermal and mechanical treatments showed distinctive characteristics in their well-defined tin content near 22% and their negligible lead.
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Some important factors need to be commented on for the future pursuit of the origin of the transition observed above. The data in Tables 1 and 2 indicate that the new technology was gradually established step by step over the centuries of the Unified Silla period. This period may be interpreted as the time required for the domestic bronze workers to become aware of the critical aspects relating to the new technology through repeated experimentation and technical innovation. The Unified Silla society at the time was under strong foreign influence as a result of the Chinese Tang involvement in the previous unification campaign. The new tradition incubated during this specific period could then be stimulated by the influx of foreign goods and ideas. The iron technology culminated in the preceding Three Kingdoms period, as evident in the number and variety of artifacts excavated (Jung et al., 2006; Park and Jung, 2004) must have had an impact in the initiation and development of this process. It took many centuries, however, for the concepts of thermal and mechanical treatments, so important and well-developed in iron works, to be practically introduced in bronze metallurgy. 5. Conclusion While the information currently available for the history of bronze production in Korea is insufficient to draw more general conclusions, a few key elements characterizing the Korean bronze technology were observed. The early Korean bronze tradition was based on casting and leaded high tin alloys. A later development introduced hot forging and quenching, probably during the Unified Silla period. The thermal and mechanical treatments were initially tried separately. But the discovery of the thermal technique to avoid the brittle d phase allowed them to be combined, and led to the establishment of a new tradition that has lasted up to the present. Lead was strictly kept from alloys and the tin content was settled near 22%. Leaded high tin alloys also continued in use, but only in cast objects, and with significant composition variation. The iron technology culminated during the Korean Three Kingdoms period and the involvement of Chinese Tang dynasty in the unification campaign are likely to have played a certain role in the initial phase of the technical transition. Acknowledgement The Jungang Research Center of the Cultural Heritage, the Kyongju National Research Institute of Cultural Properties and other contributors are acknowledged for offering the artifacts for metallurgical examination. The comments and suggestions from the reviewers were invaluable in the revision of the manuscript. This work was inspired by the various technical aspects observed in bronze artifacts from the Boonhwang temple site. They have been reported elsewhere (Jung and Park, 2005), and four of them are reproduced here. This work was finished while the author had his one year sabbatical leave at the Department of Geology and Geophysics, Yale University, as a visiting scholar from March 2006.
2001
References Allen, J.W., 1979. Persian Metal Technology 700e1300. Ithaca Press, London. Barnard, N., 1961. Bronze Casting and Bronze Alloys in Ancient China. Monumenta Serica Monograph 14. The Australian National University and Monumenta Serica, Canberra. Barnes, G.L., 1999. The Rise of Civilization in East Asia e The Archaeology of China, Korea and Japan. Thames and Hudson, London. Chikashige, M., 1920. The composition of ancient eastern bronzes. Journal of the Chemical Society (London) 117, 917e922. Cho, S.G., Hong, J.Y., Cho, J.K., Lee, M.H., Kim, H.I., Oh, J.H., Byun, Y.H., Lee, H.J., Lee, S.B., Shin, C.H., Gal, J.E., Choi, S.M., 2001. Excavation Report of the Nonsan Wonboongni Site. Jungang Research Center of the Cultural Heritage, Daejeon Korea (in Korean). Cho, S.G., Cho, J.K., Shin, N.S., Do, M.S., 2003. Excavation Report of the Daejeon Gaodong Site. Jungang Research Center of the Cultural Heritage, Seoul Korea (in Korean). Cho, S.G., Shin, N.S., Do, M.S., Lee, H.J., 2004. Excavation Report of the Tomb Complex at Boeun Boosoori. Jungang Research Center of the Cultural Heritage, Seoul Korea (in Korean). Choi, J., 1996. The Feature of the Korean Bronze Age-From the Metallurgical Viewpoint. Proceedings of the Forum for the Fourth International Conference on the Beginning of the Use of Metals and Alloys (BUMA-IV), Shimane, Japan, pp. 25e46. Choi, K.J., Park, J.S., 2004. On the manufacturing technology of some Koryo bronze artifacts. Journal of the Korean Society for Heat Treatment 17.1, 2e9 (in Korean, with English abstract). Choi, K.J., Jee, T.G., Park, J.S., 2003. Microstructural evolution in the unidirectional heat treatment of Cue35% Sn alloys. Journal of the Korean Society for Heat Treatment 16.6, 320e328 (in Korean, with English abstract). Collins, W.F., 1931. The corrosion of early Chinese bronzes. The Journal of Institute of Metals XLV, 23e55. Craddock, P.T., 1979. The copper alloys of the medieval Islamic world e inheritors of the classical tradition. World Archaeology 11.1, 68e79. Eckert, C.J., Lee, K.B., Lew, Y.I., Robinson, M., Wagner, E.W., 1990. Korea Old and New e A History. The Korea Institute at Harvard University, Seoul Korea. Goodway, M., Conklin, H.C., 1987. Quenched high-tin bronzes from the Philippines. Archaeomaterials 2.1, 1e27. Jung, Y.D., Park, J.S., 2005. Technical transitions observed in the bronze artifacts excavated from the Boonhwang Temple Site in Kyongju. Journal of the Korean Institute of Metals and Materials 43.1, 80e86 (in Korean with English abstract). Jung, Y.D., Jung, W.S., Choi, K.J., Park, J.S., 2006. Iron technology as observed in forged iron artifacts from the Silla Tomb Complex at Wolsanni Kyongju. Journal of the Korean Institute of Metals and Materials 44.5, 359e366 (in Korean with English abstract). Kang, H.T., Yu, H.S., Moon, S.Y., Kwon, H.N., 2000. Scientific analysis of bronze materials of Sanoisa Temple in Cheongju (I). Conservation Science in Museum 2, 57e68 (in Korean with English abstract). Kim, J.G., Kim, D.H., Choi, B.H., Park, Y.B., Lee, J.S., Yoon, K.I., Cho, Y.J., Shin, C.S., 1994. The Great HwangNam Tomb: Excavation Report on the South Tomb (Text and Photos). Kyongju National Research Institute of Cultural Properties, Kyongju Korea (in Korean). Kim, K.N., Hwang, I.H., Lee, S.K., Moon, S.T., 2002. Excavation Report of the Silla Wang-gyong Site (Text and Photos). Kyongju National Research Institute of Cultural Properties, Kyongju Korea (in Korean). Kim, W.Y., 1986. Art and Archaeology of Ancient Korea. Taekwang Publ. Co, Seoul Korea. Kim, W.Y., 1998. Introduction to Korean Archaeology. Iljisa, Seoul Korea (in Korean). Lee, D.N., Lee, H.J., 2002. Metallurgical Characteristics of Jing and Kkwaenggwari (Percussion Instruments). Proceedings of the Fifth International Conference on the Beginning of the Use of Metals and Alloys (BUMA-V), Kyongju Korea, pp. 249e259.
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Lee, K.S., Kang, H.T., Chung, K.Y., 2001. Chemical compositions and lead isotope ratios of the bronze artifacts excavated from the historical sites located at Moonhwadong, Tanbangdong and Biraedong in Daejeon. Gogohakji 12, 119e131 (in Korean). Meeks, N., 1993. Surface characterization of tinned bronze, high-tin bronze, tinned iron and arsenical bronze. In: Niece, S.L., Craddock, P. (Eds.), Metal Plating & Patination. Butterworth-Heinemann Ltd, Great Britain, pp. 247e275. Melikian-Chirvani, A.S., 1974. The white bronzes of early Islamic Iran. Metropolitan Museum Journal 9, 123e151. Park, J.S., 2003. On the manufacturing technology of the bronze artifacts excavated from the historic site at Gaodong, Daejeon. In: Cho, S.G., Cho, J.K., Shin, N.S., Do, M.S. (Eds.), Excavation Report of the Daejeon Gaodong Site. Jungang Research Center of the Cultural Heritage, Daejeon Korea, pp. 264e291 (in Korean). Park, J.S., Jung, Y.D., 2004. Iron technology of Silla as observed in the microstructure of a sword and a spear excavated from the Great Hwangnam Tomb. Journal of the Korean Institute of Metals and Materials 42.3, 307e313 (in Korean with English abstract). Phillips, G.B., 1922. The composition of some ancient bronze in the dawn of the art of metallurgy. American Anthropologist New Series 24.2, 129e143.
Portal, J., 2000. Korea Art and Archaeology. British Museum Press, London. Rajpitak, W., Seeley, N.J., 1979. The bronze bowls from Ban Don Ta Phet, Thailand: an enigma of prehistoric metallurgy. World Archaeology 11.1, 26e31. Ravich, I.G., 1991. Study of the composition of Scythian and Sarmatian bronze mirrors and technologies of their manufacture. Bulletin of the Metals Museum 16, 20e31. Shemakhanskaya, M.S., 1991. New aspect of the origin of ancient bronze. Bulletin of the Metals Museum 16, 3e9. Srinivasan, S., 1998. The use of tin and bronze in prehistoric southern Indian metallurgy. Journal of Metals 50.7, 44e50. Sun, S., Wang, K., 1994. An experimental study of Chinese gongs and cymbals. Bulletin of the Metals Museum 21, 19e34. Umehara, S., 1927. Chemical analysis of the ancient bronzes of China. Artibus Asiae 2.4, 247e264 (in French). Voce, E., 1951. Examination of specimens from the Pitt Rivers Museum. In: Coghlan, H.H. (Ed.), Notes on the Prehistoric Metallurgy of Copper and Bronze in the Old World (Pitt Rivers Museum Occasional Papers on Technology, 4). The University Press, Oxford, pp. 105e111. Wheeler, T.S., Maddin, R., 1976. The techniques of the early Thai metalsmith. Expedition 18.4, 38e47.
Traditions and transitions in Korean bronze technology Jang-Sik Park a,*, Robert B. Gordon b,1 a
Department of Metallurgical Engineering, Hongik University, Chochiwon, Choongnam 339-701, Republic of Korea b Department of Geology and Geophysics, 207 KGL, Yale University, New Haven, CT 06520-8109, USA Received 5 January 2006; received in revised form 17 January 2007; accepted 23 January 2007
Abstract Metallurgical examination of Korean bronze artifacts shows that a technical tradition based on casting and use of leaded high-tin alloys was established in Korea at the early stages of bronze use. After the subsequent discovery of quenching methods that suppress formation of the brittle d phase, new thermo-mechanical techniques were introduced between the 7th and 10th centuries AD. Lead-free alloys were used, and tin contents near that of the peritectic point in the CueSn phase diagram were chosen. Leaded high tin alloys continued in use, but only in cast objects, and with significant composition variation. The unique conditions during the time of innovation suggest that the transition to new metallurgical techniques was gradually achieved through domestic technical innovation inspired by external influences. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Korea; Bronze technology; Alloy composition; Casting; Hot forging; Quenching
1. Introduction It is believed that bronze objects were first used in Korea around the turn of the 1st millennium BC. It is yet to be determined if they were imported or locally produced. Local bronze production is estimated from archaeological evidence to have started no later than the 6th century BC (Kim, 1998). The Korean bronze culture at the early stage is characterized by the two prominent elements, daggers and mirrors. The early Liaoning style daggers were soon replaced by the so-called Korean-style daggers with a substantial modification in shape (Kim, 1986). The mirrors are famous for the fine, cast-in decoration on their back faces based on a linear geometric pattern depicting arrays of hatched triangles. Attempts have been made to characterize the early Korean bronze culture on these typological grounds (Barnes, 1999; Kim, 1986). The results show that this culture received a significant influence from the arts of the Ordos and Siberian steppe, which are distinguished from the * Corresponding author. Tel.: þ82 41 860 2562; fax: þ82 41 866 8493. E-mail addresses: [email protected] (J.-S. Park), robert. [email protected] (R.B. Gordon). 1 Tel.: þ1 203 432 3125; fax: þ1 203 432 3134. 0305-4403/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2007.01.010
style of Mainland China. The unique transitions found thereafter in both the shape of daggers and the decoration patterns of mirrors, which may be traced in a chronological order, are interpreted as a sign of localized development. Later, iron was introduced and spread throughout the Korean peninsula, and new articles for varying purposes such as vessels, bowls, spoons, ornaments and horse trappings (Kim, 1986, 1998) were made while the production of bronze daggers and tools was abandoned. The changes apparent in artifact typologies raise a question concerning the transitions in technology. Recent research shows that the important feature of the early Korean bronze is the use of casting as the major fabrication technique and the use of relatively high tin alloys with frequent lead additions (Choi, 1996; Lee et al., 2001). For the later development, Voce (1951) examined the microstructures of two Korean bronze bowls with an unknown provenance dated to the 12th to 14th century AD, and found the application of hot forging and quenching on alloys of about 20% tin. His work may be the earliest example showing Korean high tin bronzes consisting of the twinned a phase in the matrix of the quenched b phase. Recent metallurgical work by Choi and Park (2004) on objects from this period showed that similar
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technique and alloys were employed for kitchenware including spoons, chopsticks and large vessels. Lee and Lee (2002) reported that the traditional musical instruments are made in Korea today by the same technique. In their work on the artifacts from a historic site of the 7th to 10th century AD, Jung and Park (2005) found evidence of a gradual technical transition in high tin bronzes involving forging or quenching. Kang et al. (2000) found that bronze objects from a Korean temple site in use from the 10th to 14th century AD were either cast or forged. The forged objects contained 20% tin but no lead, and were finished by quenching. The cast objects contained approximately 10% tin and up to 20% lead. It is apparent that the early Korean bronze technology underwent a significant transition with the introduction of thermo-mechanical treatments. These treatments, applied subsequently after the initial casting operation, must have required a closer adjustment of alloy composition than in earlier practice. This study interprets the microstructures of Korean bronze objects spanning approximately 2000 years, with a special emphasis on tracing the particular sequence of thermal and mechanical treatments chosen for a given alloy composition. Trends were found in the temporal variation in alloy compositions and the associated fabrication techniques. These give us a better understanding of the process of technical development between the 7th and 10th century AD in Korea.
2. Comments on artifacts and history A hundred and five artifacts from the southern part of the Korean peninsula were examined. Map 1 locates some of the major sites from which they were excavated. The dates estimated on typological grounds extend from the 3rd century BC to 18th century AD. Table 1 summarizes the number of objects examined, the artifact types and dates. Those from the same site are expected to have a similar age, but quite a few artifacts were excavated from temple sites, residential areas and military fortresses used by more than one kingdom. They were placed in Table 1 in such a way as to span the entire probable period. Kitchenware was the item excavated most frequently, and consisted primarily of bowls, vessels and spoons. Spoons had special significance in their quantity and microstructures, and were placed separately. The weapon included five daggers and two spearheads and an arrowhead, and the tool included four axes and two chisels. The weapons and tools were distinguished from the others in that none of them were recovered from the sites of the Korean Three Kingdoms period (ca. 300 ADe668 AD) and afterwards. The character next to each number in Table 1 indicates the method of fabrication as observed in the microstructure examination. Objects made primarily by casting with no other noticeable treatments applied subsequently are designated c. The letter f indicates items finished by hot forging with no quenching,
Map. 1. Map of the Korean peninsula locating some of the major historical sites from which the present bronze artifacts were excavated.
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Table 1 Number of Korean bronze objects examined versus artifact type and date Date
BC 3eBC 1
AD 4e7
AD 7e10
AD 10e14
AD 14e18
Type of artifact
Early Iron Age
Three Kingdoms
Unified Silla
Koryo
Choseon
Weapon Tool Mirror
8c* 6c 3c*
1c
2c*
Total
1c
1c
8c 6c 9c
10fq*
6c* þ 6fq*
6c þ 1q 19c þ 1f þ 6q þ 19fq
7fq*
10fq
1f þ 21fq
5c þ 1f 6c þ 1f þ 17fq
1c 8c þ 16fq
7c þ 1f 105 (55c þ 3f þ 7q þ 40fq)
1c Ornament Kitchenware
2c 4c* þ 1f*
4c* þ 1q 3c þ 4q* 6c þ 2q þ 3fq
Spoon 1f* þ 4fq Others Total
1c 8c þ 1f
17c
9c þ 5q 7c þ 1f þ 2q þ 7fq
‘‘c’’ denotes the objects made primarily by casting with no noticeable thermo-mechanical treatments applied subsequently. ‘‘f’’ denotes the objects finished by forging. ‘‘q’’ denotes the objects given a quench treatment after casting. ‘‘fq’’ denotes the objects finished by forging and then quenching. Empty cells mean no object examined. ‘‘*’’ denotes the group from which one or more objects were chosen for the microstructure presentation in this article.
and q those that were quenched but not forged. The objects forged and then quenched are denoted by fq. The majority of the artifacts in Table 1 were made from the 4th century AD onward when the Three Kingdoms period was firmly established. In this era the Korean peninsula was divided by the four rival kingdoms, Koguryo in the north extending to Manchuria, Silla and Paekche in the southeast and southwest, respectively, and Kaya bordering the southern coast between Silla and Paekche. They had a formation time of approximately 300 years during which the Han colonies installed in northern Korea were removed by the indigenous polities. In the middle of the 7th century AD the Silla kingdom established the Unified Silla by conquering Paekche in AD 660 and Koguryo in AD 668 with the aid of the Chinese Tang forces. The Unified Silla kingdom was succeeded by the Koryo dynasty in the 10th century AD, which was then replaced by the Choseon dynasty in the late 14th century AD (Eckert et al., 1990). Prior to the Three
Kingdoms period, there was the Korean Bronze Age (ca. from BC 10th) followed by the two stages of Iron Age, Early (BC 3rdeBC 1st) and Late phase (AD 1steAD 3rd) (Kim, 1998). No artifacts from the Bronze Age and the Late Iron Age were available for examination. 3. Microstructure examination For microstructural examination specimens from the artifacts were mounted and polished following standard metallographic procedures and then etched with a solution of 100 ml distilled water, 30 ml hydrochloric acid and 10 g ferric chloride. They were examined using an optical microscope and a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). Because of the restriction in the amount of samples available their chemical compositions could only be estimated by analyzing the EDS spectra. And the
Table 2 Bronze artifacts presented for the illustration of microstructures No.
Artifact
Excavation site
Date (Century) historical period
Composition (mass %)
Treatments applied
1 2 3 4 5 6 7
Dagger Spearhead Mirror Dagger Bowl Bowl Spoon
Tanbangdong at Daejeon
BC3eBC2 Early Iron Age BC2eBC1 Early Iron Age AD5 Three Kingdoms
Sn: 8, Pb: 33 Sn: 22, Pb: 9 Sn: 32, Pb: Detected Sn: 17, Pb: Detected Sn: <15, Pb: Detected Almost pure Cu Sn: 22
Cast
8 9 10 11 12 13 14 15 16 17
Buckle Mirror Vessel Gilt Vessel Vessel Vessel Spoon Spoon Vessel Vessel
Wonboongni at Nonsan Great Hwangnam Tomb at Kyongju Wang-gyong site at Kyongju Boonhwang temple site at Kyongju
AD4eAD14 Three Kingdoms to Koryo AD7eAD10 Unified Silla
Boosoori at Boeun
AD11eAD14 Koryo
Gaodong at Daejeon
AD17eAD18 Choseon
Sn: Sn: Sn: Sn: Sn: Sn: Sn: Sn: Sn: Sn:
14, Pb: 12 27 24 22 22 22 22 22 21 23 Pb: 4, As: 1
Cast Cast Forged Forged Cast Cast Quenched Quenched Quenched and Forged
Quenched and Forged Cast
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results were given in weight fraction up to the nearest whole digit although the EDS provides the accuracy of a few tenth of a percent. This is sufficient for the present investigation. The microstructures were closely associated with the method of fabrication, and the objects made by the same technique had qualitatively similar microstructures. One or more representative articles were selected from each of the groups marked by an asterisk in Table 1, and a total of seventeen will be presented. Table 2 summarizes them in a chronological order. 3.1. Artifacts from the Tanbangdong site at Daejeon (BC 3rdeBC 2nd) Fig. 1a shows a Korean-style dagger excavated from the Tanbangdong district at Daejeon and dated to the 3rde2nd
century BC (Lee et al., 2001). Fig. 1b, a secondary electron image (SEI) micrograph showing the microstructure of the dagger at the area marked, consists of numerous bright areas of varying sizes in the dark background. The EDS analysis showed that the bright areas correspond to almost pure lead (Pb) and the background to the a phase of the CueSn alloys. The major elements included were copper (Cu), lead and tin (Sn) as shown in Fig. 1c, an EDS spectrum taken from the whole area of Fig. 1b. The average composition was estimated from the spectrum to be 59% Cue33% Pbe8% Sn. Fig. 1b shows that lead, having a low freezing temperature, existed as liquid droplets spread over the solid a phase before they solidified to form individual lead inclusions in the a matrix. It is evident that the dagger was shaped primarily by casting. No hint of mechanical working or thermal treatments after casting was observed. Fig. 2a shows a spearhead excavated together with the above dagger. Fig. 2b is a SEI micrograph of its microstructure. The bright regions, as in the dagger, correspond to almost pure lead, and the background consists of the a and d phases. The relative fractions of lead and tin were approximately 9% and 22%, respectively. The decreased lead content as compared with that of the dagger is apparent in the reduced lead areas, and the increased tin is verified in the presence of the high-tin d phase. No evidence of treatments other than casting is observed. 3.2. Artifacts from the Wonboongni site at Nonsan (BC 2ndeBC 1st) Fig. 3a shows a mirror, which is one of the bronze objects from the earth-pit tombs at the Wonboongni district in Nonsan (Cho
Fig. 1. Korean style dagger excavated from Tanbangdong at Daejeon, Korea. Estimated date: 3rde2nd century BC. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 1a. (c) EDS spectrum taken from Fig. 1b.
Fig. 2. Spearhead excavated from Tanbangdong at Daejeon, Korea. Estimated date: 3rde2nd century BC. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 2a.
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1995
Fig. 4. Dagger excavated from Wonboongni at Nonsan, Korea. Estimated date: 2nde1st century BC. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 4a. Fig. 3. Mirror excavated from Wonboongni at Nonsan, Korea. Estimated date: 2nde1st century BC. (a) General appearance. (b) Sketch showing the cast-in geometric pattern on the back face of the mirror. (c) Optical micrograph showing microstructure at the area marked in Fig. 3a.
3.3. Artifacts from the Great Hwangnam Tomb at Kyongju (AD 5th)
et al., 2001), and Fig. 3b is a sketch of the peculiar cast-in geometric pattern on its back. This type of mirror is dated approximately to the 2nde1st century BC, which is in accord with the estimation on pottery typology. Fig. 3c, an optical micrograph, shows its microstructure consisting mostly of the d grains with a small amount of unknown phase between them, indicating that the general tin content is close to 32%. Of all the bronze artifacts so far reported in Korea this is one with the highest tin content. In view of the brittleness of the d or 3 phase the 32% tin level may be close to or beyond the practical upper limit for CueSn alloys. The experiment with a 65% Cue35% Sn alloy by Choi et al. (2003) showed that the de3 eutectoid, which can form above 32.5% Sn, was more prone to crack formation than the single d phase. The presence of lead was observed, but the amount was not enough to affect the overall composition. Fig. 4a shows a broken dagger from the same site as the mirror. Fig. 4b, an optical micrograph, shows the a dendrites in the background of the aed eutectoid, which is too fine to be resolved. The aed eutectoid forms in the CueSn alloys as a result of the decomposition of the g phase simultaneously into the a and d at 520 C. The circular crater marked by an arrow is filled with almost pure lead segregated during the solidification of the casting. The EDS data showed that the tin content was approximately 17% and that the lead content was negligible compared with copper and tin. Of the thirteen objects from the site the mirror and dagger represent the upper and lower limit in tin content. Lead inclusions such as that in Fig. 4b were found in most of them, but the level of lead was not significant.
Fig. 5a presents a bowl which is one of the cast or hammered bronze objects from the Great Hwangnam Tomb in Kyongju, the former capital of the Silla and the following Unified Silla kingdom. The tomb, the greatest of all in the neighborhood in size as well as in the amount of burial goods, was estimated to have been constructed in the 5th century AD (Kim et al., 1994; Portal, 2000). Fig. 5b, an optical micrograph, shows the dendrite pattern present in the structure, indicating that the artifact was manufactured primarily by casting. Fig. 5c, a SEI micrograph magnifying Fig. 5b, reveals micro voids and inclusions in the background. The EDS analysis showed that the inclusions were high in copper, lead and tin while the background was the a phase. These phases may have been affected by corrosion. The attempt to estimate the average composition from the EDS data was complicated by the corrosion of the sample as can be seen in Fig. 5b, and the results were inaccurate. The tin content inferred from the morphological characteristics in Fig. 5b should be below 15%. Fig. 6a is another bowl from the tomb. The bowls in Figs. 5a and 6a must have been made for a similar purpose, but differences were noted in the microstructure and chemical composition. No dendrite structure such as that in Fig. 5b is observed in Fig. 6b, an optical micrograph of the microstructure near the bottom of this bowl, and no other elements than copper were detected in the EDS analysis. The structure in Fig. 6b, therefore, corresponds to the a phase of almost pure copper. The matrix contains twin boundaries along with a number of dark spots well aligned in horizontal layers. The examination in the SEM showed that the spots correspond mostly to micro voids and occasionally to inclusions high in lead and tin.
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Fig. 6. Bowl excavated from the Great Hwangnam Tomb at Kyongju, Korea. Estimated date: 5th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 6a.
Fig. 5. Bowl excavated from the Great Hwangnam Tomb at Kyongju, Korea. Estimated date: 5th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 5a. (c) SEI micrograph magnifying Fig. 5b.
The voids aligned in layers and the twin boundaries are the direct evidence of heavy working and the subsequent annealing applied in fabrication. This bowl had thin walls of 1mm or less, and was the only object made of unalloyed copper among those in Table 1. 3.4. Artifact from the Wang-gyong site at Kyongju (AD 4the14th) Fig. 7a shows a broken spoon from the Wang-gyong site located in the city of Kyongju (Kim et al., 2002). The site covered most of the core districts of the former capital of the Silla and the Unified Silla kingdom, and was given the name Wanggyong, meaning the capital city. It continued to be a major city
in the following Koryo dynasty. Table 1 shows that spoons started to appear from the Koryo period. The only exception is that in Fig. 7a, which may also have been made during the later period of this site, i.e., the Koryo period. This spoon had a very thin wall of approximately 0.3 mm, and had a unique microstructure clearly distinguished from any of the other spoons. Fig. 7b, a SEI micrograph showing its microstructure, consists of the aed eutectoid in the background of the a grains containing straight twin boundaries. The spoon was evidently hot forged and then finished by slow cooling. The tin content was 22%, and no lead was detected anywhere in the specimen.
3.5. Artifacts from the Boonhwang temple site at Kyongju (AD 7the10th) Fig. 8a shows a buckle from the Boonhwang temple site at Kyongju (Jung and Park, 2005). Fig. 8b, an optical micrograph, retains in the dark regions the dendrite patterns formed in casting. The dark regions consist of the a phase, and the bright interdendritic regions are filled with the aed eutectoid. The white arrow in Fig. 8b locates one of the regions filled with almost pure lead. The average composition was 74% Cue14% Sne12% Pb. No evidence of treatments other than casting was observed.
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Fig. 7. Spoon excavated from the Wang-gyong site at Kyongju, Korea. Estimated date: 4the14th century AD. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 7a.
Fig. 9a is a mirror from the same site. Fig. 9b, a SEI micrograph, consists entirely of the fine aed eutectoid. This indicates that the tin content is close to 27%, which was also verified from the EDS data. No other elements than copper and tin were observed.
Fig. 8. Buckle excavated from the Boonhwang temple site at Kyongju, Korea. Estimated date: 7the10th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 8a.
Fig. 9. Mirror excavated from the Boonhwang temple site at Kyongju, Korea. Estimated date: 7the10th century AD. (a) General appearance. (b) SEI micrograph showing microstructure at the area marked in Fig. 9a.
Fig. 10a shows a broken vessel from the site. In Fig. 10b, an optical micrograph, the large bright regions represent the a phase in dendrites, and the background consists of two constituents, the light acicular martensite and the dark matrix of the retained g phase. Laboratory trials showed that a structure resembling the background was obtained when the b phase was quenched from a relatively low temperature near 586 C. It seems likely that during the initial stage of quenching while the temperature drops from 586 C to 520 C, some of the b transforms to g to produce a mixture of b and g, which then transforms to the mixed structure as in the background. Even with this special thermal treatment the individual dendrite regions all remain as single crystals, implying that little or no mechanical working was applied. The overall tin content was 24%. No other elements were detected, and lead was not present even in the form of isolated inclusions. Fig. 11a is a fragment from the above site. The surface retained a thin gold layer at the bright areas, and it must have broken off a gilt bronze vessel. Fig. 11b, an optical micrograph, shows the bright a dendrites in the dark background of the g phase retained by rapid cooling. The dendrite regions were all single crystals. This artifact is similar to the above vessel in the treatments inferred from its microstructure. A difference is observed, however, in the exact tin content and the temperature from which it was quenched. The lower tin content, approximately 22%, is revealed by the increased
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Fig. 12. Vessels and spoon excavated from Boosoori at Boeun, Korea. Estimated date: 11the14th century AD. (a) General appearances. (b) Optical micrograph showing microstructure at the area marked in the left vessel in Fig. 12a. Fig. 10. Vessel excavated from the Boonhwang temple site at Kyongju, Korea. Estimated date: 7thV10th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 10a.
a regions. The background microstructure consisting exclusively of the g phase places the temperature from which the metal was quenched between 520 C and 586 C. No lead was detected anywhere in the specimen.
Fig. 11. Gilt bronze vessel excavated from the Boonhwang temple site at Kyongju, Korea. Estimated date: 7the10th century AD. (a) General appearance. (b) Optical micrograph showing microstructure at the area marked in Fig. 11a.
3.6. Artifacts from other sites built after AD 11th century Fig. 12a shows three artifacts, two vessels and a spoon, excavated from a burial site at the Boosoori district in Boeun, which was constructed in the 11the14th century AD (Cho et al., 2004). Their microstructures were all similar to that in Fig. 12b, an optical micrograph taken from the vessel at the left. The average tin content for all is about 22%, and no lead was found. Fig. 12b shows the a regions and the martensite in the background as expected in 78% Cue22% Sn alloys when quenched. While no twins are present in the a regions of the artifacts described above, they are present in the structure shown in Fig. 12b. The twin formation by recrystallization occurs only in alloys which have been mechanically strained as by hammering. It is evident that the artifacts were mechanically worked before being quenched. Forging must have been essential for the thinning of the walls down to 0.3 mm or less. Fig. 13a shows three fragments from a spoon and two vessels from a burial site at the district of Gaodong in Daejeon, which was constructed between the late 17th and early 18th century AD (Cho et al., 2003; Park, 2003). The microstructure and chemical composition of the spoon and the vessel next to it were almost identical to those in Fig. 12a. The vessel on the right, however, varies significantly as can be seen in Fig. 13b, an optical micrograph, where the cast dendrite structure is retained. The average alloy composition was 72% Cue23% Sne4% Pbe1% As (arsenic). No evidence of forging or quenching was observed. The presence of lead and arsenic is apparently associated with the different fabrication technique.
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Fig. 13. Spoon and vessels excavated from Gaodong at Daejeon, Korea. Estimated date: 17the18th century AD. (a) General appearances. (b) Optical micrograph showing microstructure at the area marked in the vessel at the right hand side of Fig. 13a.
4. Discussion Table 1 summarizes the number of artifacts examined versus artifact types and dates along with the method of fabrication. As for the Bronze Age objects, the extensive data reported by Choi (1996) on chemical compositions are available for comparison. But no items from the 1st and 2nd century AD were included in his work. Collins (1931), by quoting the work by Umehara (1927), reported on some Korean bronzes belonging approximately to this period. They were excavated from the region near the modern city of Pyongyang, which was then occupied by the Han commandery, Lelang, and might have been made under strong Chinese influence. It is not clear why the items from this specific period are rarely available. Within these limitations, the data in Table 1 reveal some important trends, which may best be recognized in the last row where the total number of the objects belonging to each period is given. The data placed in three cells combined reflect the uncertainties in dating. First, the seventeen objects of the Early Iron Age were all made exclusively by casting; second, cast items continued to be made throughout the entire period; third, the mechanical working subsequent to casting started at the Three Kingdoms period, and the thermal treatment at the Unified Silla period; fourth, the treatment of forging followed by quenching became a standard recipe from the Koryo period. Forging and quenching were applied separately before they were combined. The benefit of the combined thermo-mechanical treatments seems to have been recognized gradually over several centuries until
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they became a new technical tradition at around the 10th century AD. The new tradition dominated the Korean bronze industry for the following one thousand years. Table 2 provides the supplementary information that is not evident in Table 1, especially on alloy compositions. The tin and lead contents in cast objects show a large variation. This has been also noticed in other research. The twenty four early Korean daggers in Choi’s work (1996) contained 0.25% to 25.0% Sn averaging 13.6%, and 0% to 14.22% Pb averaging 6.9%. As for Korean mirrors, Choi reported three containing 16.6 w 27.3% Sn and 4.0 w 11.4% Pb, and Chikashige’s analysis in Collins’ paper (1931) showed three containing 22.89 w 26.97% Sn and 1.54 w 2.06% Pb, and one with 49.05% Sn and 4.02% Pb. Chikashige (1920) reported two Korean mirrors of 21.9% Sne9.68% Pb and 11.76% Sne15.14% Pb. The 33% Pb at No. 1 of Table 2 is unusually high, far beyond the upper limit that has been reported. The amount of lead detected in the Wonboongni artifacts was too little to count as a deliberate addition (Craddock, 1979). No other elements than copper, tin and lead were detected in such an amount as to affect the overall compositions. The only exception was arsenic, which was observed only in cast objects as in the vessel No. 17. The presence of arsenic from 1.0% to 1.14% was also reported in three of Choi’s daggers. Other minor elements such as iron, zinc and antimony were observed by Choi and Chikashige. The compositions in the cast objects of the later period, No. 5, 8, 9 and 17, showed considerable variation. Table 2 showed that the introduction of thermo-mechanical treatments brought about a conspicuous change in the design of alloys. The tin content in those treated tended toward the peritectic point of the CueSn system, 22%, and lead was kept from alloys. The bowl No. 6 and the spoon No. 7 could be the earliest of the hammered objects. The former was dated to the mid 5th century AD, and the latter anywhere from the 4th to 14th century AD. The bowl has special importance since it was the only object made of copper without alloying, and was recovered together with cast bronze objects, one of which is the bowl No. 5. It seems obvious that unalloyed copper was seldom selected as a material for home use. This copper bowl may therefore be understood as a new attempt tried only for a limited time. In view of the difficulties that would have arisen in casting, the shaping of the unalloyed copper bowl must have required hammering, most probably at elevated temperatures. This was verified in Fig. 6b. Technically, hammering copper or any other metals to shape was certainly nothing new at the time, but it has a special meaning in this case as an early attempt at forging copper, which could be extended to other copper-based alloys or even to high tin bronzes if the working temperature was properly controlled. Fig. 7b showed that the high-tin spoon was actually hammer-shaped at high temperatures, followed by slow cooling. The d phase in the eutectoid, however, can cause serious brittleness in service, especially when the wall is thinned down to 0.3 mm or less as in this spoon and the other hammered objects. The absence of lead from this article may be understood as an effort to lessen this problem. But the potential advantage of hot forging could
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probably not be fully appreciated until the brittle d phase was appropriately taken care of. The two vessels from the Boonhwang temple site labeled No. 10 and 11 may be important examples of the early experimentation undertaken to solve the difficulties arising from the presence of the d phase. The microstructures in Figs. 10b and 11b show that they were quenched after casting, but not forged. The d phase was replaced by the tougher martensite or g phase. This allowed high tin alloys to be cast into thin-walled articles by taking advantage of their enhanced cast-ability and better strength and toughness. These vessels belong to the earliest of those in Table 1 given the quench treatment, and are distinguished from the later ones similarly treated. They were not hammered; the temperatures from which they were quenched were not consistent, and were lower than those of the later ones; one of them contained more tin than normal. These differences all indicate that the technique was under development with a standard recipe yet to be established. However, no lead was used. It is not even present as isolated inclusions. The subsequent development of technique came from combining the beneficial effects of both forging and quenching while the alloy composition was adjusted to 22% tin and negligible lead. The new technical tradition thus reached the climax characterized by the specific alloy composition, Cue22% Sn, and the unique thermo-mechanical treatments of forging and quenching. Useful bowls, vessels and spoons with thin walls could now be made at will using high tin alloys. The Great Hwangnam tomb, well dated to the mid 5th century AD, provides an important reference point in the tracing of the above transition. The excavation recovered more than 3000 iron objects including various weapons, tools, kitchenware and semi-finished products. The metallurgical examination revealed that the contemporary iron technology in smelting, steel making and various thermomechanical treatments was in a highly advanced state (Park and Jung, 2004). By contrast, the use of copper and copper-based alloys was restricted to home appliances such as bowls, smoothing irons and cauldrons. But no spoons of any kind, let alone those made of bronze, were recovered. A smoothing iron and a cauldron, examined but not presented here, were all cast from unleaded alloys of low tin content of approximately 3%. Experimentation with high tin alloys seemed not to have been seriously tried at this time, and thermo-mechanical treatments on copper-based alloys were in an early stage. The hammered bowl No. 6, however, implies that there was an attempt to find a new method that could give more freedom in fabrication than casting alone. This effort is seen in Table 1 to accelerate during the Unified Silla period. Now the major concern was not in hammering but in heat treatments, as demonstrated in Table 1 by the five high-tin artifacts quenched but not forged. The appearance of many hammered and quenched objects in the following Koryo period indicates that the discovery of this thermal technique was a technical breakthrough for the establishment of a new tradition, which was coincident with the first production of spoons. The spoons were all made by a combination of forging and quenching except that in Fig. 7a, which
could be dated to the earlier period, and therefore could be considered as an earlier trial product. High tin bronzes have a long history of production, especially in China (Barnard, 1961), and in Siberia, Middle East, India, Thailand, Roman worlds and Islamic Iran as well (Allen, 1979; Meeks, 1993; Rajpitak and Seeley, 1979; Srinivasan, 1998). Thermal and mechanical treatments on them are also known to have long been practiced in Central Asia, Thailand and India from as early as the 1st millennium BC. Ravich (1991) examined Scythian and Sarmatian bronze mirrors from Central Asia, and found that they were either cast or forged. The forged mirrors were free from lead and consisted of the twinned a and the quenched b phase. Shemakhanskaya (1991) also reported Central Asian bronze artifacts hot forged using alloys of 20e23% tin and no lead. Rajpitak and Seeley (1979) and Wheeler and Maddin (1976) reported ancient Thai bronze objects treated by forging and quenching. Srinivasan (1998) observed the similar treatments in Indian bronze vessels dating supposedly to 800 BC. Goodway and Conklin (1987) and Sun and Wang (1994) discussed the technical aspects of musical instruments made in Philippines and in China by hot forging and quenching of high tin bronzes. Allen (1979) mentioned the use of both casting and forging in high tin objects from the Islamic period of Iran. Melikian-Chirvani (1974), based on visual inspection, described some of the high-tin bronze vessels from early Islamic Iran that were supposedly made by the similar technique. Melikian-Chirvani proposed on typological grounds the spread of this Islamic high-tin technique to Korea and Japan via the Chinese Tang dynasty (AD 618e907). He suggested that the Japanese word sahari came from the Persian word safidruy meaning high tin bronze. According to the Japanese scholar Chikashige (1920), however, the word sahari refers to ‘Corean bronze’ which Koreans used for making spoons and others. (Korea was often spelled Corea at the time.) On the other hand, Collins (1931), in his quotation of Chikashige’s work, was reluctant to name the specific metal ‘Corean bronze’ because of his premise that the Koreans could not derive their metallurgy from others than the Chinese. It is interesting to see that Chikashige reported the compositions of two Korean spoons, one old and one new, to be Cue24.04% Sne 2.19% Ni and Cue21.62% Sne0.20% Zn, respectively. The absence of lead in both is consistent with the present results although the presence of nickel and zinc is at variance. Chikashige and Collins apparently did not recognize the technical aspects associated with the thermo-mechanical treatments. Considering the composition of the metal called ‘Corean bronze’ and its use in making spoons, it was apparently designed for forging and quenching. Barnard (1961) also quoted Chikashige’s old spoon along with two more Korean spoons. They were consistently made of unleaded high tin bronzes. Phillips (1922) also reported a similar Korean bronze spoon from the Koryo period. Many Korean bronze objects, especially spoons, reported previously even without recognizing the thermal and mechanical treatments showed distinctive characteristics in their well-defined tin content near 22% and their negligible lead.
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Some important factors need to be commented on for the future pursuit of the origin of the transition observed above. The data in Tables 1 and 2 indicate that the new technology was gradually established step by step over the centuries of the Unified Silla period. This period may be interpreted as the time required for the domestic bronze workers to become aware of the critical aspects relating to the new technology through repeated experimentation and technical innovation. The Unified Silla society at the time was under strong foreign influence as a result of the Chinese Tang involvement in the previous unification campaign. The new tradition incubated during this specific period could then be stimulated by the influx of foreign goods and ideas. The iron technology culminated in the preceding Three Kingdoms period, as evident in the number and variety of artifacts excavated (Jung et al., 2006; Park and Jung, 2004) must have had an impact in the initiation and development of this process. It took many centuries, however, for the concepts of thermal and mechanical treatments, so important and well-developed in iron works, to be practically introduced in bronze metallurgy. 5. Conclusion While the information currently available for the history of bronze production in Korea is insufficient to draw more general conclusions, a few key elements characterizing the Korean bronze technology were observed. The early Korean bronze tradition was based on casting and leaded high tin alloys. A later development introduced hot forging and quenching, probably during the Unified Silla period. The thermal and mechanical treatments were initially tried separately. But the discovery of the thermal technique to avoid the brittle d phase allowed them to be combined, and led to the establishment of a new tradition that has lasted up to the present. Lead was strictly kept from alloys and the tin content was settled near 22%. Leaded high tin alloys also continued in use, but only in cast objects, and with significant composition variation. The iron technology culminated during the Korean Three Kingdoms period and the involvement of Chinese Tang dynasty in the unification campaign are likely to have played a certain role in the initial phase of the technical transition. Acknowledgement The Jungang Research Center of the Cultural Heritage, the Kyongju National Research Institute of Cultural Properties and other contributors are acknowledged for offering the artifacts for metallurgical examination. The comments and suggestions from the reviewers were invaluable in the revision of the manuscript. This work was inspired by the various technical aspects observed in bronze artifacts from the Boonhwang temple site. They have been reported elsewhere (Jung and Park, 2005), and four of them are reproduced here. This work was finished while the author had his one year sabbatical leave at the Department of Geology and Geophysics, Yale University, as a visiting scholar from March 2006.
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Lee, K.S., Kang, H.T., Chung, K.Y., 2001. Chemical compositions and lead isotope ratios of the bronze artifacts excavated from the historical sites located at Moonhwadong, Tanbangdong and Biraedong in Daejeon. Gogohakji 12, 119e131 (in Korean). Meeks, N., 1993. Surface characterization of tinned bronze, high-tin bronze, tinned iron and arsenical bronze. In: Niece, S.L., Craddock, P. (Eds.), Metal Plating & Patination. Butterworth-Heinemann Ltd, Great Britain, pp. 247e275. Melikian-Chirvani, A.S., 1974. The white bronzes of early Islamic Iran. Metropolitan Museum Journal 9, 123e151. Park, J.S., 2003. On the manufacturing technology of the bronze artifacts excavated from the historic site at Gaodong, Daejeon. In: Cho, S.G., Cho, J.K., Shin, N.S., Do, M.S. (Eds.), Excavation Report of the Daejeon Gaodong Site. Jungang Research Center of the Cultural Heritage, Daejeon Korea, pp. 264e291 (in Korean). Park, J.S., Jung, Y.D., 2004. Iron technology of Silla as observed in the microstructure of a sword and a spear excavated from the Great Hwangnam Tomb. Journal of the Korean Institute of Metals and Materials 42.3, 307e313 (in Korean with English abstract). Phillips, G.B., 1922. The composition of some ancient bronze in the dawn of the art of metallurgy. American Anthropologist New Series 24.2, 129e143.
Portal, J., 2000. Korea Art and Archaeology. British Museum Press, London. Rajpitak, W., Seeley, N.J., 1979. The bronze bowls from Ban Don Ta Phet, Thailand: an enigma of prehistoric metallurgy. World Archaeology 11.1, 26e31. Ravich, I.G., 1991. Study of the composition of Scythian and Sarmatian bronze mirrors and technologies of their manufacture. Bulletin of the Metals Museum 16, 20e31. Shemakhanskaya, M.S., 1991. New aspect of the origin of ancient bronze. Bulletin of the Metals Museum 16, 3e9. Srinivasan, S., 1998. The use of tin and bronze in prehistoric southern Indian metallurgy. Journal of Metals 50.7, 44e50. Sun, S., Wang, K., 1994. An experimental study of Chinese gongs and cymbals. Bulletin of the Metals Museum 21, 19e34. Umehara, S., 1927. Chemical analysis of the ancient bronzes of China. Artibus Asiae 2.4, 247e264 (in French). Voce, E., 1951. Examination of specimens from the Pitt Rivers Museum. In: Coghlan, H.H. (Ed.), Notes on the Prehistoric Metallurgy of Copper and Bronze in the Old World (Pitt Rivers Museum Occasional Papers on Technology, 4). The University Press, Oxford, pp. 105e111. Wheeler, T.S., Maddin, R., 1976. The techniques of the early Thai metalsmith. Expedition 18.4, 38e47.