Anatomy of Avalokitesvara

An Ancient Chinese Cast-Iron Statue

     Recently we had an intriguing and challenging metallurgical assignment, namely, to determine whether a cast iron statue in the possession of a Portland family for nearly a century is of ancient origin or merely a century old souvenir.

     Unlike biological artifacts, whose age can be ascertained by techniques such as carbon dating, determining the age of metallurgical artifacts is more complex and requires an understanding of the evolution of metallurgical processes in the region of interest from ancient times. The study of metallurgical artifacts is known as Archeometallurgy and is an evolving field gaining more and more interest in the scientific and academic community.

Background

      The tale of the statue is interesting in itself, its possible link to antiquity notwithstanding. We became involved when a long time Portland resident, Scott S. Corbett, Jr. contacted us to ask if we could evaluate the origins of a statue he had inherited from his father. Mr. Corbett's father had been born in Chefoo, China in 1892, the son of a Presbyterian missionary who had arrived in China in 1863. Mr. Corbett himself was born in Tsinan, China in 1917, and in 1928, when he was 11 years old, the whole family moved to Portland.

     It seems Mr. Corbett's father had developed quite an extraordinary collection of Chinese antiques, and at his death in 1983, Mr. Corbett inherited the statue. Unfortunately, the family had not determined from Mr. Corbett's father the details of his acquisition of the statue; however, they did know that he had acquired it sometime between 1915 and 1920 in northern China, and that he had always stated it was very old.

The Statue

     The statue is made of cast iron and stands 18½ inches tall and weighs 26 pounds (Figures I & II). It depicts "Avalokitesvara," the Bodhisattva (one whose essence is enlightenment) of compassion and relates to the Hinayana period of Buddhism, which spanned the later part of the first millennium in the northwestern part of China.

     The use of cast irons in China dates back nearly 25 centuries, to the 5^th century BC; by the 3^rd century BC a form of malleable cast iron was being widely used in China for tools and agricultural implements. It is interesting to note the development of similar cast irons in the West didn't occur until the 18^th century! Cognizant of the differences between modern and ancient metallurgical processes, our focus was to evaluate the metallurgical properties of the statue, from which we would be able to ascertain its origin.

Autopsy

     Now, if the statue were of ancient origin, it would be an invaluable museum piece; thus, we devised a plan to gather as much information as possible with a minimum of intrusion. Two dime-sized spots on the backside of the statue (Arrows A and C, Figure II) were sanded, polished, and etched for metallographic examination of the surface microstructure (No lost material except for the surface patina at these spots!). Additionally, a small (0.15-inch diameter by 0.35-inch long) core sample was carefully removed by diamond drill from the back (Arrow B, Figure II) for cross-sectional metallography (Figure III) and chemical analysis. From these three samples, we were able to learn a great deal about the statue without affecting its integrity or appearance.

 

Analysis      The cast-iron statue was a single-piece casting poured from a single melt. It exhibited interdendritic shrinkage (Figure III) caused by inadequate feeding during solidification. The near-surface microstructure (Figure IV) was slightly finer than the interior microstructure; however, the microstructural constituents were the same, and there was no chilled layer (such as occurs from rapid cooling), indicating a relatively slow cooling rate. The implication: the statue was probably cast in a heated loam mold.     The microstructure consisted of coarse pearlite grains surrounded by a light-gray, grain-boundary eutectic phase containing ledeburite-steadite (iron-iron carbide-iron phosphide). Microhardness measurements showed a wide range of hardnesses, from 24 to 39 on the Rockwell C scale (HRC), significantly higher than modern commercial-grade gray cast irons.      The microstructures and hardness values were representative of a form of white iron known to have been produced by ancient Chinese metal casters. An interesting feature of the microstructure was the presence of a nodular/polygonal dark-gray phase in the grain-boundary regions next to the light-gray, ledeburite-steadite phase. This is attributed to subsequent annealing of the brittle, as-cast white iron at around 800 to 950ºC to improve its ductility.      The annealing treatment resulted in the nucleation and growth of carbon in a nodular/polygonal form. The formation of nodular carbon was also facilitated by a relatively high sulfur content. The annealing treatment appeared to have been only partially effective because substantial portions of ledeburite were not converted into nodular/polygonal carbon.

     The fracture surface (Figure VI) of the core sample exhibited a brittle mode along the hard grain-boundary phase, and carbon/sulphur nodules can be seen on the fracture surface (arrow). This brittle behavior is attributed to an incomplete conversion of the brittle grain-boundary phase due to the incomplete annealing.

     The chemical composition of the cast iron was typical of the compositional range of ancient Chinese cast-iron artifacts, with the exception of a higher sulfur and phosphorous content. The higher sulfur and phosphorous were no doubt intentional, as they promote ductility (through carbon nodule formation) and fluidity of the pour. Another compositional clue to the statue's age was its very low silicon and manganese content, which is distinctly different from modern cast irons, in which these two constituents are essential alloying elements intentionally added in significant amounts.

     The microstructure, hardness, and chemical composition indicated the statue was not of modern cast-iron foundry practices; instead, these attributes were consistent with a form of white iron known to have been widely produced by ancient Chinese metal casters. The carbon-sulfur polygonal nodules at the grain boundaries are indicative of an annealing treatment following the casting process to improve ductility,and the observed microstructure was consistent with the spheroidal cast irons of ancient China.

     Although the modern practices of manufacturing spheroidal cast iron (through the addition of alloying elements like cerium/magnesium) and malleable iron (through annealing) bear certain similarities to the ancient practices in terms of their intended purpose of improving ductility, the statue is distinctly different in terms of its composition, the incomplete microstructural transformation, and its high and relatively nonuniform hardness.

Conclusion

     The ancient Chinese metal casters were clearly masters of their trade, centuries ahead of the West in terms of their casting skills. Although we found a number of minor metallurgical imperfections in the statue, these can be attributed to the lack of sophisticated testing tools available to the ancient craftsmen, rather than an absence of casting sophistication.

     It is our opinion the Avalokitesvara statue is indeed of ancient origin.

TAPPI Digester Corrosion Information Meeting

Rapid Corrosion Thinning of Continuous Digesters

     In February, we sponsored the Spring 2001 meeting of the TAPPI (Technical Association of the Pulp and Paper Industry) Digester Corrosion Task Group, which was held in Portland at the Marriott Courtyard Airport Hotel. The group meets twice a year to discuss corrosion-related problems in digesters.

     For the benefit of our nonpaper-industry readers, a digester is an enormous pressure vessel (some are over 250 feet in height) used for separating the fiber from wood chips by chemically "digesting" the chip. The predecessor to the current Corrosion Task Group was a Cracking Task Group formed in 1983 following the in-service failure of a digester in Pine Hill, Alabama. In that incident, a girth weld near the top of the vessel fractured, causing the top 30 feet or so of the vessel to plummet toward the ground, where it landed on the roof of an adjacent building. (And no, nobody was hurt, but yes, it most definitely got everybody's attention!)

     Following the failure, mills throughout the country started inspecting their digesters for similar cracking and to everyone's surprise, an industry-wide cracking problem was found. This led to the formation of the Digester Cracking Task Group, which then met at least twice a year for the rest of the decade and into the next, until the industry was able to finally get a handle on the cracking problem. Once the threat of cracking and catastrophic failure was under control, the industry turned its attention to corrosion, which, while less spectacular in its potential consequences than cracking, was becoming a serious concern. Thus, the Corrosion Task Group was formed.

     The Spring 2001 meeting was attended by over 75 people and lasted for two days. During the first day, a dozen presentations were made by industry experts on a variety of digester corrosion-related issues, including several potential solutions to the problem. On the second day, the task group worked on an upcoming document, "Guidelines for Thermal Spray Coating for Protection of Pulp & Paper Process Vessels."

 

Visit our booth at the PLANT ENGINEERING & MAINTENANCE SHOW 23 & 24 May 2001 Portland Convention Center Call or email Holly Lungren for complementary tickets! 503-228-9663 hml@meic.com