Spirit of a Bronze Caster: Techniques from the Shang and Zhou Dynasties
The Shang (ca. 1600–1046 BCE) and Zhou (ca. 1046–256 BCE) dynasties bronzes, dated more than 3,000 years ago, with their distinctive forms and intricate motifs, continue to fascinate us today. How these exquisite bronzes were cast has long been a subject of scholarly interest. Initially, scholars could only deduce the assemblage of the clay molds (taofan) used in bronze casting by analyzing traces left on the surfaces of the artifacts. Then in the 1930s, the Institute of History and Philosophy (IHP), Academia Sinica, had first excavated clay molds from Hsiao-t’un at the Yinxu, and later discoveries were made in areas centered in in Zhengzhou, Yanshi, and Luoyang, all of Henan province, as well as in Shanxi, Shaanxi, and Anhui provinces. However, many aspects of clay molds still elude current research.
In view of the above, the IHP, the Sen-Oku Hakukokan Museum (Kyoto, Japan), and Ashiyagamanosato (Fukuoka, Japan) in 2018 signed a collaborative agreement regarding the replication and joint research of bronze casting techniques. This exhibition is the result of multiple efforts. Organized by these three organizations, “Pouring One’s Soul” features both works cast by replication experiments and various artifacts of casting bronzes in IHP collections, with the hope of furthering our understanding of bronzes from the perspective of casting.
The exhibition is divided into two parts: the first introduces the casting of bronzes via graphic, textual, and video as demonstrated on the walls of the exhibition area, contents of which include the discovery of clay molds as well as findings from our replication efforts such as the raw materials of bronzes, melting copper, and the production of molds, among others. The second exhibits artifacts related to bronze casting held by the IHP, including materials used in melting and the clay molds as well as models of cast chüeh, ku, handled yu, square i, ting, chariot fittings, and weapons. Finally, the research and experiment results of early IHP researchers on clay molds are also introduced.
The Discovery of Clay Molds
During the 1930s, the IHP conducted several seasons of excavations at Hsiao-t’un village in the suburb of Anyang city, Henan province, discovering many foundations of large structures. Among them, a large number of artifacts related to casting, including clay molds, models, furnace wall fragments, and slag, were unearthed from the trash pits of Structure B5. It can thus be surmised that the site was a workshop for bronze casting prior to the construction of the temple-palace complex. In recent years, we have re-collated this group of clay molds, discovering that the majority belong to Yinxu Period II, with some being from Yinxu Period I, as well as clay molds similar in form to that of bronzes excavated from the tomb of Fuhao. Therefore, we have also hypothesized that the bronzes used by Shang elites during Yinxu Periods I and II were produced in Hsiao-t’un.
Early on IHP researcher Shih Chang-ju (1902–2004) observed that this group of bronzes had not been produced via “lost-wax casting,” as is the case with modern-day metalware, but rather by the technique of “section molds.”
How Are Bronzes Produced?
Raw Materials
The melting point of pure copper is approximately 1085°C. Tin (Sn) and lead (Pb) are thus added to copper (Cu) to lower the melting temperature to around 800°C, resulting in a bronze alloy. Pure copper has an orange-red color, but as tin is added to create an alloy the color becomes increasingly silver depending on the proportion. If exposed to air or buried in soil, the surface of the metal combines with oxygen (oxidation) to form copper oxide, which is red, green, or blue in color.
We have taken cross-cuts from damaged bronzes that had been excavated as samples and then analyzed them using SEM-EDS and EPMA electron microscopy, finding that the tin content ranged from 12–22% with a tin content of 16–18% being most common. Remarkably, this is consistent with Kao gong ji (The Record of Artificers) in the Rites of Zhou, which calls for “four parts gold and one-part tin” when discussing the alloy proportion.
Melting
Ashiyagamanosato in Fukuoka, Japan, has experimented with “zeng lu” (“Koshiki-ro” in Japanese; lit. “cauldron furnace”), a traditional Japanese melting furnace, to replicate bronze melting practices. The cylindrical furnace is first filled with charcoal and then alternating layers of charcoal and metal material. Employing airflow, the furnace is heated for several hours until the temperature reaches about 1000℃, at which point molten bronze flows out the lower egress.
Furnace buildings used in bronze casting comprised many overlapping smaller cylindrical furnace walls, if any of the cylindrical vessels damaged, they could be directly replaced. This is why the furnace wall fragments excavated from Hsiao-t’un show bands of clay that can be joined to a vessel cylindrical in shape.
Casting
Casting is the technique of liquefying metal at high temperatures and pouring it into a mold where it is then cooled and solidified. At Yinxu, clay molds were used in the casting process. During replication experiments, we first preheated the molds to 700℃ and then poured the molten bronze into the sprue of mold assemblages.
Failed Products of Replication Experiments
The casting of a successful bronze can only be realized if the temperature of the bronze and of the clay mold as well as the surrounding temperature and humidity are compatible. If any one of these conditions is not met, the casting will likely fail. In the case of the failed replicated bronze on display here, the molten bronze did not flow smoothly into the belly, resulting in holes and other imperfections in the finished work.
How Are Clay Molds Produced?
Soil Analysis and the Material Used in Mold Production
Regarding topsoil, the Anyang region is characterized by a 5 to 6-meter layer of loess. After comparing the data from a soil analysis with that of loess, we were able to determine that the raw material used in the production of clay molds in the region is indeed loess. An SEM-EDS electron microscopy analysis has further shown that the particle size of the loess is predominantly 5 to 10 μm, with some being as fine as 1 μm. Clay is not used because of its water content. When heated, the water evaporates and thus causes the mold to shrink and deform, creating gaps between the sections of the assemblage and preventing them from fitting together. Therefore, loess, which contains almost no water content, is most suitable.
Ashiya Fine-Grained Soil
While conducting our replication experiments, we searched for a suitable replacement soil and finally discovered a similar soil in Japan, which we call “Ashiya fine-grained soil.” It is the raw material used in our production of clay molds.
Adhesion Agent of Clay Molds
A pink or blackened coating-like material was found on the surface of where the clay mold assemblages were fit together. Following analysis, it is determined that the coating is a mixture of fine soil and gypsum, which acts as an adhesive agent.
Stand-Alone Motif Mold Technology Applications
Bronze inscriptions are often found on the surface of bronzes. During the period of the Shang and Zhou, the majority of inscriptions were incised. Other notable characteristics include the handwriting of each character varied, the cross section of the strokes in the shape of inverted Ω, as well as a difference in height between the inscription area and bronze surface.
In our replication experiments, we used a brush pen dipped in clay slurry to write the reversed characters layer by layer on a dry clay tablet, which is called “clay slurry technique.” Because each character is written individually, the handwriting naturally varied. The tablet with the inscription is then embedded into the core of the clay mold assemblage (creating the difference in height), which is finally placed into the model, literally termed “embedment technique.” This technique, which has been confirmed by our replication efforts, combines the above two processes and refers to as the “embedded tablet with clay slurry inscription technique.” This method is the application of “Stand-alone motif mold technique.”
Generally speaking, from the Shang and Zhou to the Warring States period (ca. 475–221 BCE), motifs and inscriptions were employed the “stand-alone motif mold technique” in remarkably diversified development.
Early Research on Clay Molds
In the 1930s, the IHP discovered a large number of clay molds used for casting bronzes at Hsiao-t’un, Yinxu. Early on IHP researcher Shih Chang-ju (1902–2004) observed that this group of bronzes had not been produced via “lost-wax casting,” as is the case with modern-day metalware, but rather by the technique of “section molds.”
Li Chi (1896–1979) investigated the clay molds and bronzes, specifically how their motifs were produced by examining the mold constructions for chüeh and ku vessels. Wan Chia-pao (1924–2009) then attempted to apply modern casting knowledge to his replication experiments on casting chüeh, ting, kuei, and yu vessels. The end of the feet of a tripodal vessel chüeh, for example, may be used as both the sprue (the inlet for pouring molten bronze) and an outlet for airflow. The mold is then placed upside down as the melted bronze is poured. But if the air inside the mold is not successfully expelled, holes or other imperfections appear in the bottom, as the bronze is unable to flow evenly throughout the mold.
The above research results and casting experiments conducted by the first-generation IHP researchers are invariably pioneering studies on clay molds, laying a robust foundation for subsequent research.
