Study on the conditions of bronzes excavated from Jin State
cemetery at Tianma-Qucun Site
Liu yu
(Beijing, 100710, Institute of Archaeology, CASS)
Zhang xiaomei Yang xianwei Yuan sixun
(Beijing, 100871, Department of Archaeology, Peking University)
Key words: bronze
corrosion
Jin state cemetery at Tianma-Qucun Site
Bronzes undergo a complex corrosion process and gradually reach a dynamic
equilibrium in the burial context. However, once excavated the equilibrium is
interrupted and potential harmful factors will influence the bronzes again,
accelerating the corrosion process. So it’s important to study the corrosion and
preserving conditions of ancient bronzes.
Tianma-Qucun site, excavated by the Dept. of Archaeology of Peking University in
cooperation with the Institute of Archaeology of Shanxi province, is one of the most
important bronze age sites in China in recent years. It is located in Quwuo County,
Shanxi Province, the most important part of which is the cemetery of Jin state of
Zhou dynasty. The cemetery is composed of two sections：The “Gong Tombs” section
(G for short) is in the north of central site, which includes the tombs of 8 Jin
marquis and their wives; The “Bang Tombs” section (B for short) is in the west
of the site, which includes the tombs of lower class nobles and commoners.
This research is mainly about the corrosion and preserving conditions of those
bronzes excavated from Jin state cemetery. Samples are taken from the 298 bronzes
of G and the 138 bronzes of B. Through investigating and analyzing the samples, we
can discuss the type, degree and cause of the corrosion, which will be useful for
conservation.
1 The conditions of bronzes
Most Qucun bronzes are corroded heavily and covered with coarse, thick and colorful
corrosion layer. Some have light green, powdery products, commonly referred to as
“bronze disease”. The bronzes having thin hard reflective surface with black, pea
green, leaden gray surface color and the bronzes with soot on the bottom are often
found in good condition.
Observation with naked eyes shows the conditions of the bronzes are different.
For example, the bronzes unearthed from B are better than those from G. In the latter,
those from late tombs are better than from early ones, from marquise tombs are better
than from marquis ones. The conditions of bronzes excavated from the same tomb are
more similar than those from different tombs，although in some cases the conditions
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of bronzes from the same tomb such as M64 can be different.
We used X-ray method to test the inner conditions of the bronzes. The result shows
that most of bronzes are corroded to different degrees. The radiograph of one bronze
shows significant differences in darkness among the different parts, which are
believed caused by uneven corrosion. Many bronzes are found to have many internal
defects or corrosion parts even with nice surface.
2 Analysis of corrosion products and soils:
2.1 XRD Analysis
62 corrosion product samples of bronzes, from 12 tombs of G and 4 of B, are analyzed
by XRD. The result indicates that the major corrosion products include malachite,
azurite, cuprite, silicon oxide, atacamite, antlerite, brochantite. Several samples
have chalcopyrite, stannite，cerussite, phosgenite, etc.
2.2 Analysis of Chloride ion and Sulphate radical ion
336 powdery corrosion products samples are analyzed by chemical method, the
result shows that about 40% samples have high content of chloride ion and quite some
have high content of sulphate radical ion.
2.3 Corrosiveness of soils
The soil resistivity of 7 excavating locales are measured, including M31, M93,
M91, M9 and some Bang tombs etc. The result shows that all measured soils are
moderately corrosive, and the soils of M91 and some Bang tombs are more corrosive
than others. The PH of the soil samples from M8 chariot pit, M93 and M91 are 7.9-8.2,
which are slightly alkaline. Soluble salt analysis shows that the content of Sulphate
radical ion is high in all 3 samples, especially M91. The content of Chloride ion
is also much higher in soil sample from M91 than other two samples.
3 Discussion
The alloy of these bronzes are mainly composed of copper and tin, some also have
lead. Most bronzes consist of two phases known as α(copper rich)and δ(tin rich),
some just consist of α . The unequal composition distribution will cause
electrochemical corrosion. The grain boundaries are more reactive and anodic than
the grains, so corrosion is concentrated along grain boundaries. There is the
tendency for the tin rich areas to be attacked preferentially during corrosion, the
segregation of tin to the grain boundaries enhances this localized attack. In dry
condition all the chemical on the surface is stable, if moisture can penetrate
through the outer loose layer containing atacamite, sulphate etc., the cuprous
chloride layer below them becomes unstable, absorbs water, and changes to be basic
cupric chloride which can produce powdery light green spots. This condition is known
as bronze disease. The bronzes with many casting defects are always heavily corroded
because defects can serve as the corrosion passage.
The bronzes with reflective surface or with soot are usually in good conditions.
The study indicates that the special patinas are composed of many kinds of oxides
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with low value of Gibbs Energy (standard free energy), especially stannic oxide,
and carbon. The patinas can protect the bronzes from corrosion. However, if the
patinas are cracked or broken, the function for preserving them will be decreased.
The conditions of unearthed bronzes are influenced by the burial context,
especially the characteristics of soils such as the type of soil, the salts present,
the PH, conductivity,etc. For example, the soil of M91 is more corrosive than others,
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the content of Cl , SO4 much higher, which in turn cause higher Cl , SO4 content in the
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corrosion products of bronzes. Furthermore, the tomb chamber of M91 was subsided,
some harmful matters such as fertilizer can flow into the tomb with rainfall, making
the burial context of M91unsuitable for preserving the bronzes.
Because of the high content of chloride ion in the corrosion products of about
40% bronzes, the “bronzes disease” will develop again while suitable temperature
and humidity in the environment is present especially if the corrosion products layer
are cracked or removed by various reasons. Environment control is therefore as
important as chemical treatment in the conservation work of bronzes.
References:
(1) Department of Archaeology of Peking University and Institute of archaeology of
Shanxi Province,“Excavated the tombs at the Tianma-Qucun Site in 1992”, cultural
relics, Mar. (1993):11-30; “Second excavation of the cemetery of the cemetery
of Jin state at Beizhao in the Tianma-Qucun Site ” , cultural relics ，
Jan.(1994):4-28; “Third excavation of the cemetery of the Jin state at Beizhao
in the Tianma-Qucun Site”，cultural relics ，Aug(1994):22-33;“Fourth excavation
of the cemetery of the Jin State at Beizhao in the Tianma-Qucun Site”
，cultural
relics，Aug(1994):4-21;“Fifth excavation of the cemetery of the Jin State at
Beizhao in the Tianma-Qucun Site”, cultural relics，Jul(1995):4-39.
(2) Fan chongzheng et al., “Study on the bronze disease and the conservation of
bronzes ” , In Selected papers of Archaeometry, University of science and
technology Press, 1991.
(3) Zhu rizhang et al,“ Corrosion of Metals”
，Metallurgical Industry Press, 1989.
(4) Corrosion testing methods of materials in soil，(Editors: national testing
stations of soil corrosion), Science Press, 1990.
(5) Liuyu et al., “Research on the bronze wares excavated from Tianma-Qucun Site
of Jin State in Zhou Dynasty”, Sciences of conservation &archaeology, No.2,
2000.
(6) Liu cheng et al, “Study on the corrosion mechanism of ancient bronzes”, In
Selected papers of Archaeometry, University of science and technology Press,
1991.
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