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Mystery of Ancient Bronze Casting Unveiled in Sword

An ancient bronze casting technology -- technology that has amazed archaeologists and others who have wondered how people 2,500 years ago in China could cast such exquisite work in bronze -- has been deciphered though analysis of a bronze sword by a group of scholars from Shanghai Museum, Nanjing Museum and Shanghai Institute of Materials.

The discovery involving bronze casting in Wu and Yue (names of two ancient states in China, covering Zhejiang, Jiangsu and southern Shandong provinces) was announced at a meeting of “Studies on Bronze Casting Technology in Wu and Yue Regions” held by the Administrative Committee of Cultural and Historical Relics in Shanghai and organized by the State Administration of Cultural Heritage.

In 1965 a bronze sword with the inscription of "made and used exclusively by Goujian, king of the State of Yue" was unearthed from the Chu tomb in Jiangling, Hubei Province. The diamond patterns on the surface and the concentric circles as thin as paper at the tip of the sword aroused great interests of archaeologists and scientific and technological historians. Starting at the end of the 1970s, researchers from China, the United States and Canada began to analyze the composition of the diamond patterns. However, there was a deep chasm in their speculations on the casting technology, with no overall conclusions.

For the past two decades, south China has seen a large number of unearthed bronzes. The Wu and Yue bronze wares amazed academic circles with their exquisite and thin-walled casting, unique surface decoration as well as composite bronze technology. The diamond patterns, the concentric circles at the tip of the sword and the composite bronze sword -- also called the twin-colored sword -- can all be rated as consummate works of the bronze weapon casting technology in Wu and Yue, representing the enigma of ancient science and technology. The research group used modern technology and techniques to examine the archaeological specimens. After a comprehensive analysis of the acquired information, the mystery was unveiled: The diamond patterns are the result of the thermal diffusion of alloys. First powder of the stannum-group alloy was applied to the surface of the bronze sword. The elements of the alloy permeated the body of the sword through heating, causing the powder-covered area to become white while the rest of the sword remained copper-yellow. Simulated experiments demonstrate that more than 2,500 years ago the alloying technology on metal surface had been invented and applied in ancient China to make twin-colored diamond patterns in a non-mechanical way. The research also revealed that the white-colored area on the surface of the sword is not only full of tin but also abundant with grained crystals to strengthen the corrosion resistance tremendously. After thousands of years’ corroding, due to the varying corrosion resistance, the unique diamond patterns checkered with black and grey present themselves before our eyes.

A precise pottery-matrix casting technology appeared in ancient China more than 2,500 years ago. The thin-walled concentric circles at the tip of the sword with extremely short distance between and fine cord lines in relief at the base could not be made by machining but casting. In the light of the wheel method to make the eggshell-like pottery during the Neolithic period, following the ancient formula to compound special mould materials, after repeated trials the research group manufactured the pottery matrix of concentric circles and founded similar decorations to those on the archaeological specimens.

Six proportions of the alloy of copper with tin were recorded in the Book of Diverse Crafts compiled during the Spring and Autumn Period to cast bronze wares with different applications. Hard bronze with a high proportion of tin can be used to make a sharp sword; however, the sword could be snapped very easily in a fight. To avoid this, workers in Wu and Yue adopted the double casting technology using bronze of different proportions. They chose bronze with low proportions of tin for the edge of the sword, and bronze with high proportion of tin to cast the blade that in turn covers the edge. The bronze sword founded in this way tempers toughness with flexibility, becoming a deadly weapon on the ancient battleground. The part with the high proportion of tin is tinged with yellow and white while the area with low proportion of tin has a tincture of dark red, which can explain its nickname of "twin-colored sword." The findings demonstrate that Chinese workers were able to make use of composite bronze materials to improve a weapon’s performance 2,500 years ago. The research also brings to light the superb lost wax casting technology to found three-dimensioned patterns on a plane surface and the pottery-matrix casting technology to make fine and embossed patterns, both of which were applied by workers in ancient Wu and Yue.

(中国文物报 [China Cultural Relics Biweekly], translated by Shao Da for china.org.cn, May 14, 2002)

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