Indian Journal of Science and Technology, Vol 8(23), DOI: 10.17485/ijst/2015/v8i23/84418, September 2015
ISSN (Print) : 0974-6846
ISSN (Online) : 0974-5645
Scientific Analysis of the Gilt-bronze Incense Burner
of Baekje from the Neungsalli Temple
Site in Buyeo, South Korea
Hyung-Tae Kang* and Min-Jeong Koh
Department of Conservation Science, National Museum of Korea, Seoul, Korea;
[email protected], [email protected]
Abstract
The Gilt-bronze incense burner, excavated at the Buyeo Neungsalli temple site,was made in the 6th century. It consists
of a knob, lid, body and support, and features various patterns and ornaments representing Buddhism and Taoism.
Following the assessment of its overall design and aesthetic senses, the incense burner has been recognized as a
cultural masterpiece in East Asia. The present study analyzes the alloy composition, micro structure and plating layer
of the incense burner. Atomic absorption (AAS) and micro beam X-ray fluorescence (micro-XRF) methods applied to
the incense burner base. Analytical results revealed it to a binary alloy of copper and tin with compositions of 82 86% and 13 - 14%, respectively. The melting temperature and strength of the burner were estimated based on the tin
concentration. The micro structure and the nonmetallic inclusions were analyzed by optical microscopy and scanning
electron microscopy coupled with energy dispersive spectrometry (SEM-EDS). The analysis revealed an α+δ eutectoid
with the α phase featuring a high copper content, along with a small amount of 4 - 5um sized nonmetallic inclusions.
This composition revealed the use of sophisticated technologies in the fabrication and refinement of the burner. The
lead isotope ratios from the Thermal Ionization Mass Spectrometry (TIMS) suggested that the raw material of the
incense burner originated from Zone 3 (Yeongnam Massif, Okcheon Metamorphic Belt) of the southern part of Korea.
The chemical composition of the plating layer cross-section was revealed to comprise gold and copper at 60% and 37%,
respectively. The plating layer included both copper and gold owing to diffusion effect between the gold and the base
copper during the gold amalgam plating process. The surface color of the incense burner was, therefore most likely
reddish brown, not gold.
Keywords: Chemical Composition, Gilt-Bronze, Microstructure, Micro-XRF, SEM-EDS, TIMS
1. Introduction
The Neungsalli temple site is a major historical site
representing Baekje Buddhism in the Sabi Period (AD
479 - 672). Located between the Naseong site and the
Neungsalli ancient tombs, this site has produced many
Baekje treasures made 1,500 years ago1.2. The Neungsalli
temple site presents a typical architectural layout of Baekje
temples. It is characterized by a single pagoda and a single
central prayer hall in which the middle gate, pagoda,
* Author for correspondence
the central prayer hall and lecture hall are arranged in
a straight line and surrounded by galleries2. Thousands
of artifacts, including a gilt-bronze incense burner, giltbronze ornaments, jade ornaments, wood ornaments,
and earthen wares were excavated in an academic survey
performed by the Buyeo National Museum1,2.
Among these artifacts, the gilt-bronze incense burner,
was designated as National Treasure No. 287, as an example
of metal craft integrating artistic, unique and technical
capabilities of the Baekje people. This incense burner
Scientific Analysis of the Gilt-bronze Incense Burner of Baekje from the Neungsalli Temple Site in Buyeo, South Korea
consists of a knob, lid, body, and support. The dragon and
phoenix carved on the knob and the support symbolized
the yin and yang and the royal authorities. The mountains
and the group on the knob represented the Taoistic utopia
united with nature. The lotus on the body indicated the
born in a lotus blossom in Buddhism3. These elements of
the dragon, phoenix, figures, animals and plants carved
on the incense burner were organically connected and the
design charateristics assessed the burner to a cultural and
historical masterpiece in East Asia4.
How did the craftsmen manufacture this incense
burner? Where were the materials sourced, how were
they mixed and which methods were implemented to
manufacture the incense burner? Which plating methods
were introduced to manufacture the incense burner with
abundant gold color? A scientific investigation of the
incense burner was conducted to answer these questions.
This study summarizes the manufacturing
technologies and their features based on scientific
analyzes of the incense burner. Samples were collected
from a protrusion inside the body and the support of
the incense burner. The major and minor elements were
analyzed to investigate the composition and mixing ratio
of the base alloy5,6, and partly conducted nondestructive
analysis with XRF7,8. The elemental composition was used
to estimate the melting point and strength of the alloy9.
The micro structure of the base was observed, and the
nonmetallic inclusions were analyzed to investigate the
refinement level from the size and the amount9. T
h e color
of the plating surface of it was reddish brown rather than
gold And so, studied the causes of this phenomenon. The
plating layer10,11 was observed and analyzed to determine
the elemental composition. The lead isotope ratios12–15
were analyzed to estimate the source of the materials used
to manufacture the alloy. Distribution map of galena also
used to inspect where these data belong to southern part
of Korea13.
2. Materials And Experimental
Figure 1 shows an overall picture of the gilt-bronze
incense burner of Baekje. Small metal parts protruding
from the body and the inside of the support used as
samples, and fragments of plating layer obtained from
naturally separated pieces.
2
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Figure 1. Gilt-bronze incense burner of Baekje (61.8 cm x19
cm) dated from the 6th century and designated as National
Treasure No. 287.
2.1 Elemental Analysis
Each sample melted in the aqua regia, and prepared the
sample solution with dilution. The standard solutions for
each element prepared with dilution depending on the
concentration range of the sample using the standard
solution for atomic absorption (Junsei Chemical Co.
Japan). Atomic absorption spectrometry (AAS; PerkinElmer 3030, USA) applied to the elemental analysis. T
he
absorbance of each element measured with an acetylene air flame at 249.2nm for Cu, 283.3nm for Pb and 213.9nm
for Zn [5,6]. A nitric oxide - acetylene flame used for the
Sn and analyzed immediately after manufacturing the
sample solution [5]. The concentrations of nine elements
(Cu, Sn, Pb, Zn, Ag, Ni, Co, Sb, and Fe) were determined
and the average value from three analyses for each element
was taken as the contents.
The surface of the incense burner was analyzed by using
a nondestructive X-ray fluorescence (XRF) spectrometer
(Portable XRF, ArtTAX Basic, Bruker AXS, Germany).
The analysis condition was a molybdenum (Mo) target,
50kV voltage, 600uA current and 150 seconds time with
no filter[7,8]. The analysis area was 0.65mm diameter,
and the analysis was performed three times for the lid, the
body, and the support of the incense burner.
Indian Journal of Science and Technology
Hyung-Tae Kang and Min-Jeong Koh
2.2 Micro Structure
The grain samples mounted with epoxy resin and finely
ground with a grinder (Struers Rotopol-11, Denmark).
The micro structures observed with an optical microscope
(Optical Microscope, Leica DMLP, Germany) without
etching to analyze the nonmetallic inclusions after the
fine grinding.
Then, the molded samples were observed by SEM
(Hitachi S-3500N, Japan) and several points were analyzed
by SEM-EDS (KevexSuperdry, USA). Here, the analysis
condition was 20kV voltage, 100mA current, 15.0mm
measurement distance and 150 seconds measurement
time. The size and the distribution of each phase were
analyzed by using image analysis software (Image
Analyzer, ImageJ).
composition showed that the incense burner was a bronze
alloy with a typical 2-element system consisting of only
copper and tin. As shown in Table 1, analytical results
of AAS for the body and the support showed the same
chemical compositions and agreed with the compositions
determined by XRF spectrometry in a certain range.
The copper and tin composition ranges in the incense
burner were 82 - 86% and 13 - 14%, respectively. Five
other components (Ag, Ni, Co, Sb, and Fe) existed as trace
amounts with less than 0.1%, except for Sb which ranged
from 0.22% to 0.27%.
2.3 Lead Isotope Ratio
The fine sample powder placed in a Teflon vial, and added
2 - 3ml of the aqua regia. The lid closed and heated for
about 12 hours on a heating plate at a temperature of
150°C. The samples were heated and dried after opening
the lid of the vial, dried again with 1ml of 6N-HCl and
centrifuged by melting with 1ml of the 1N-HBr. The lead
was separated from the centrifuged sample solution using
negative ion exchange resin (AGI-X8, Chloride form,
#100 - 200) and 1N-HBr. The isotope ratio was measured
by thermal ionization mass spectrometry (TIMS; VG
Sector 54 - 30) after putting the separated lead on the Re
single filament13. The analysis data calibrated with the
standard materials (NBS, SRM 981)13.
3. Result and Discussion
3.1 Chemical Compositions of Alloy
The alloy composition of the base material burner was
quantified, Table 1 showed the results. The analytical
results of nondestructive XRF spectrometry, shown
in Figure 2, are also included in Table 1. The chemical
Figure 2. The surface of the lid being analyzed with microXRF spectrometry.
The chemical composition of the bronze closely related
to the cast technologies such as the melting temperature,
strength, and hardness. Figure 3 is a phase diagram of
the Cu-Sn alloy and was used to estimate the melting
temperature depending on the tin content. The diagram
[9] shows the status of the metal, including the liquid and
solid and their crystalline structures and dissolution point
with the X-axis of the tin concentration and the Y-axis of
the temperature. Tin content of 13 - 14% in the alloy was
in the range of 950 - 960°C melting temperature.
Figure 4 shows the mechanical properties of copper
alloy depending on the tin content [9]. As shown in the
figure, the hardness increases with increasing tin content
but alloy becomes increasingly fragile as the tin content
Table 1. Comparison of analytical data with aas and xrf for the incense burner
Method
AAS
XRFa
Position
Body
Support
Surface
Cu
81.5
83.6
85.9
Sn
14.3
12.9
13.5
Chemical Composition(weight %)
Pb
Zn
Ag
Ni
Co
0.07
0.04
0.04
0.02
0.05
0.02
0.07
0.03
0.55
Sb
0.22
0.27
Fe
0.07
0.09
a: Nondestructive XRF analyzes were performed for the spots of non-corroded surfaces of the incense burner.
Vol 8 (23) | September 2015 | www.indjst.org
Indian Journal of Science and Technology
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Scientific Analysis of the Gilt-bronze Incense Burner of Baekje from the Neungsalli Temple Site in Buyeo, South Korea
exceeds 14%. As shown in Figure 3, higher tin content of
alloy forms a mass by agglomeration of various crystals
(α, β, γ). Therefore overall strength is decreased due to
different contraction rates or directions for each crystal.
In a real cast, it would be difficult to make a product
because the alloy is easily broken at high tin content.
T
h e incense burner according to the tin content shown
in Figure 4 was 80 - 83HB of Brinell hardness and 48 50kgf/mm2 of tensile strength, respectively [9]. Therefore,
it was suggested that tin content in alloy was introduced
to establish the maximum hardness and tensile strength.
3.2 Investigation of Micro Structures
As shown in Figure 5(a), the micro structure of the body
was determined to be α phase with high copper content
and a α+δ eutectoid was located in between. The light gray
indicates the δ phase at higher tin content than the α phase.
The size of the dark gray nonmetallic inclusions was about
4 - 5um and a smaller black phase existed. The contents
and the size were small except for the corroded part. Figure
5(b) shows the micro structure of the support. The α phase
formed the basis, as for the body, and a α+δ eutectoid
located in between. The contents and the size were larger
than the body. The large, dark gray nonmetallic inclusions
were about 10um long. Micro phase of the support shows
the same tendency as that of the body.
Figure 6(a) and (b) showed the back-scattered SEM
images from the same samples. The elements such as Cu,
Sn, Pb, S, Bi and O were observed from the nano-phase of
No. 1 and Cu, Pb, Sn, Bi, As and O were detected from No.
2. It showed that the Pb and Bi contents were higher than
those of the other components. T
h e nano-phase of No. 3
detected elements of Cu, Sn, Pb, S, Bi, O, and Fe, and the
S content was the highest. No. 4 shows the contents of Pb,
Bi, Cu, As, Sn and O and particularly Pb and Bi had the
highest contents.
The micro structure observations revealed a
sophisticated technology level related to the refinement
owing to the low contents of the nonmetallic inclusions.
T
h e refinement of the copper from the raw ore produced
the dark gray phase in the nonmetallic inclusions, and
commonly detected small amounts of Cu, S, O, and
Fe. Further investigation will be required to determine
whether this is the result of refinement of the sulfide ore
(for example: chalcocite, Cu2S) or the bornite (Cu5FeS4).
4
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3.3 Provenance of Raw Material
The analysis of the lead isotope ratio is one of the natural
scientific methods that have been developed to estimate the
origin of raw materials of bronze14. The lead contains four
isotopes with different mass numbers. Among them, 204 is
the stable isotope existing in the nature and 206, 207 and
208 are the isotopes produced by the radiation decay and
their mixture forms galena14. Galena is used to estimate
the place of origin because each mining place contained
its peculiar lead isotope ratio. Bronze artifacts contain lead
as a minor or trace element. Therefore, the place of origin
may be estimated because the lead isotope ratio remains
constant in the bronze if the raw material in a certain place
was used to manufacture bronze objects in the ancient era.
A recent study determined the lead isotope ratio
data for galena in the southern part of Korea and drew
a distribution map13. Geologically, the southern part of
Korea consists of various geological structures, including
the Gyeongsang Basin, Yeongnam Massif, Okcheon
Metamorphic Belt, Mt. Taebaeksan Basin and Gyeonggi
Massif. The study collected 215 galena samples from 69
local mining fields, analyzed the lead isotope ratio and
drew the local distribution map13. Then, the southern part
of Korea was categorized into four zones from Zone 1 to
Zone 4 considering the analysis results of the lead isotope
ratio, the earth’s crust structure, and the regional locations.
Figure 7 shows the distribution area selected from the
analysis results of the lead isotope ratio and the location of
the galena mining fields in the area13.
Table 2. Lead isotope ratios data for two positions of
the incense burner
Position
Body
Support
206/204
18.026
18.078
Lead Isotope Ratios
207/204 208/204 207/206 208/206
15.615
38.395 0.8663
2.130
15.654
38.543 0.8659
2.132
In the present study, TIMS used to measure the lead
isotope ratio of two samples from the base of the incense
burner, and the Table 2 showed the results. As shown in Figure
8 (a) and (b), the lead isotope ratios of samples included
into the recently published distribution map of galena in the
southern part of Korea. It showed that these samples included
in the minerals from Zone 3, mainly in the Yeongnam Massif
and the Okcheon Metamorphic Belt. The area was close to the
Neungsalli site where the incense burner was discovered.
Indian Journal of Science and Technology
Hyung-Tae Kang and Min-Jeong Koh
3.4 Elemental Mapping of Plating layer
Most of the gilt-bronze crafts boast a splendid gold color.
However, as shown in Figure 1, the color of the surface of
the incense burner, including the plating layer, is reddish
brown and dark brown rather than gold. This study
analyzed the structure and composition of the plating
layer to investigate the cause.
Table 3. Nondestructive analytical results with colors
of the incense burner
Position
Color
Au
Lid
Golden
****
Reddish brown **
Black
**
Body
Golden
***
Reddish brown **
Support Golden
****
Reddish brown
*
Brown
**
Hg
**
*
*
*
*
**
*
*
Cu
**
**
***
**
**
**
**
**
Pb
*
**
*
*
*
*
**
**
*: indicates qualitative data of the XRF spectrogram of each position.
* Below 3%, ** 3~15%, ***15~50%, **** Above 50%
Fe
*
**
**
*
**
*
***
**
As shown in Figure 2, the overall surface and various
points on the lid, the body, and the support of the incense
burner were analyzed with the nondestructive X-ray
fluorescence analyzer and it was shown in Table 3. All
points of the burner detected the gold and the mercury,
which confirmed that the gold amalgam plating method
was used in the fabrication of the incense burner. The
table also shows that copper was the base element of the
incense burner.
Table 4. Analytical result of sem-eds for the cross
section of the plating layer
Positiona
1
2
3
Mean
Chemical Composition(weight %)
Au
Cu
O
59.4
37.4
3.2
60.0
37.8
2.2
62.1
35.5
2.4
60.5
36.9
2.6
a: Each analytical position is the same as the number of Figure 6.
The cross-section of the plating layer observed with
SEM, and Table 4 and Figure 9 showed the EDS analysis
results. T
he mapping result of Figure 9 indicated the
elemental distribution of the plating layer with each color. It
also showed that the upper portions ( ) were predominant
with copper and the below portions ( ) were mixed with
copper and gold. Only some golds ( ) appeared in the pores
Vol 8 (23) | September 2015 | www.indjst.org
as the grain type. The gold mixed with the copper to a large
amount in some portions. T
h e result of the chemical analysis
of 2 points ( ) for Nos. 1 and 2 in the Table 4 showed the
composition of 60% for gold and 37% for copper.
This result explained that the diffusion phenomenon
of the components occurred in the process of the gold
amalgam plating10. In the gold amalgam plating process,
the amalgam mixture is heated to volatilize the mercury17.
Here, the diffusion progress between the gold of the
plating layer and the copper in the base metal occurred,
and dissipating the boundary layer10,18,19. In addition,
amalgam plating process may produced a porous plating
layer, leading the copper element to penetrate into
the pores10. The size and the number depended on the
heating temperature and time. The above result shows
that the incense burner was heated to a high temperature
during the amalgam plating process, causing the diffusion
between the gold and the copper. Consequently, the
copper was diffused into the plating layer and the layer
showed a reddish brown.
4. Conclusions
The gilt-bronze incense burner of Baekje excavated
from the Neungsalli site was assessed as being a cultural
and historical masterpiece in East Asia. This study is to
investigate the manufacturing method by analyzing the
chemical composition, micro structure, and the plating
layer of the incense burner in these respects.
The bronze alloy product was manufactured by adding
13 - 14% tin to copper. The physical characteristics such
as melting temperature, hardness and tensile strength
were to be 950 - 960°C, 80 - 83HB and 48 - 50kgf/mm2,
respectively. It showed that incense burner manufactured
with a tin content to give close to the maximum strength.
The craftsmen of the burner clearly understood the
changes in the physical features of the metal depending
on the compositional changes. It suggested that they
have designed and manufactured the incense burner by
applying these knowledges
The micro structure observation revealed a
sophisticated technology level related to the refinement
owing to the low content of nonmetallic inclusions.
The dark gray phase in the nonmetallic inclusions was
produced by the refinement of the copper from the raw
ore, and the commonly detected the elements of Cu, S,
and O and Fe at low contents. Further analysis of the
Indian Journal of Science and Technology
5
Scientific Analysis of the Gilt-bronze Incense Burner of Baekje from the Neungsalli Temple Site in Buyeo, South Korea
mixed micro phases required to determine whether a
sulfide ore such as chalcocite Cu2S) or bornite (Cu5FeS4)
containing the iron was used as the copper material or
their sophisticated refinement.
The lead isotope ratios were used to investigate the
source of the origin materials of the incense burner. By
inspection of the distribution map of galena, it suggested
that the copper source of the incense burner collected
from Zone 3, mainly comprised of the Yeongnam Massif
and the Okcheon Metamorphic Belt. This area was not far
from the place of the Neungsalli site, Buyeo, where the
incense burner was excavated.
The nondestructive analysis of the plating layer showed
that the gold amalgam plating method was used due to
the detection of gold and mercury as a whole. The overall
color of the plating layer of the incense burner is reddish
brown. The SEM-EDS mapping result of the plating layer
from the incense burner revealed a gold/copper mixing
ratio of about 6:4. This meant that diffusion bonding was
formed as a diffusion phenomenon between the gold in
the plating layer and the copper in the base. Further, the
copper component penetrated into the pores underwent
an oxidation process. These factors contributed to the
reddish brown color of the plating layer.
This study have increased our understanding of the
manufacturing technologies of metal crafts in the Baekje
Period and will support future research.
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Indian Journal of Science and Technology