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Last modified: 26 Mar 2001 (text)

Iron on historic shipwrecks in Southeast Asia

by Michael Flecker

Extracts from PhD thesis on the 10th century Intan wreck, that of an Indonesian lash-lugged vessel voyaging between Srivijayan Sumatra and central or eastern Java. The iron artefacts on this site included a concretion containing a bundle of blades, originally wrapped in woven material, other concretions containing iron bars and interesting cargo items, and cast iron cooking pots.

Iron on ancient shipwrecks is particularly problematic. Over time it graphitises and eventually becomes an odorous black slurry. The redeeming factor is that a concretion almost always forms around the iron object. This is a thick, hard layer formed by iron corrosion products combined with sediments and calcareous marine organisms. Even if the iron has completely disappeared the concretion layer often retains a detailed mould of the original iron artefact. Sometimes other artefacts are trapped within a concretion, and if they are a less active metal, such as bronze, they may be magnificently preserved by the sacrificial anode action of the iron. Where some base metal does remain, conservation is a difficult, expensive and time consuming task, and is not always successful.

Large deposits of high grade iron ore are found in Luzon, the Malay peninsula and along a belt running from northern Thailand through Laos to northern Vietnam and China. India was a major exporter of iron to the Middle East through to the 13th century, and may also have supplied Java.⁽¹⁾ Small, low grade deposits occur throughout Southeast Asia and were suitable for a competent smelter to make sufficient iron for local use. While the quality of iron from these smelters was on a par with that produced by larger smelters in rich ore areas, the iron was more expensive, requiring more fuel and labour to produce a unit of finished metal.⁽²⁾ Small mines in Sulawesi are thought to have produced a nickel-rich iron that was used in manufacturing krises prized in Java, but archaeological evidence is yet to be found for production during the Srivijayan period. Java is known to have large deposits of magnetite sands along its southern coast, which were perfectly suitable for early smelting, and yet there is no indication that Java ever produced iron. It was all imported.

China produced vast quantities of iron and, through economies of scale, it seems that they could readily compete with local producers. This was achieved through advanced technology rather than access to high grade ore deposits. China produced wrought iron by a two stage process whereby ore and charcoal were heated in a blast furnace to a temperature high enough to liquefy the iron (~1350° C). The metal could therefore be tapped from the furnace which permitted continuous production, thereby saving fuel and increasing output. The molten iron cooled quickly into a brittle cast iron with a high carbon content and significant inclusions of phosphorous, sulphur, and silicon. This was then put into a second furnace known as a finery. The iron was again melted and exposed to oxygen long enough to burn out the carbon, making it far more malleable. It was then formed into bars for export and local use. The second stage of the process used more fuel, but the higher production rate made the two stage process more economical than the direct process that was used by other Asian countries.

In the direct process a mixture of ore and charcoal was heated at a relatively low temperature (1200 to 1250° C) in a low furnace. Impurities were drained off in a liquid slag until a solid lump or "bloom" of iron formed at the bottom of the furnace. The furnace was then cooled until the bloom could be extracted. It was then hammered to squeeze out remaining slag. The resultant product was a low carbon iron of high chemical purity, ready for immediate use by a blacksmith.

The Chinese wrought iron was inferior to the iron produced by the direct process. The sulphur inclusions tended to make the metal susceptible to shattering under the blacksmith's hammer. But the low cost seemed to have overcome this shortcoming. Some 200 tonnes of iron was stowed on board the 13th century Java Sea wreck⁽³⁾, and most of it was in the form of wrought iron bars in bundles. Element analysis of the Java Sea wrought iron showed a high sulphur content, which, combined with the fact that there was a large amount of Chinese cast iron and up to 100,000 Chinese ceramic items on the wreck, proved that the wrought iron was manufactured in China.⁽⁴⁾ That shipment was also bound for Java.

Iron is a common find on shipwrecks in Southeast Asia. This is particularly true for cast iron in the form of woks and cauldrons. Although cast iron is extremely brittle, it is ideal for manufacturing relatively thin walled vessels that are not subject to impact loads. China had a monopoly on the trade of this almost indispensable item. Cast iron cooking vessels have been found on the 12th to 13th century Pulau Buaya wreck⁽⁵⁾, along with bundles of iron blades. They were found on several of the Thai wrecks in the Gulf of Thailand, a 14th century wreck near Belitung⁽⁶⁾, and on the 1690 Vung Tau wreck⁽⁷⁾ which was bound for Batavia. Raw and finished iron was commonly exported from China to Southeast Asia during the Song dynasty.⁽⁸⁾ Dutch East India Company records confirm that large quantities of cast iron were shipped from China to Java. For example, in the decade from 1673 to 1682, 9 ships from southern Fujian province brought 55,214 iron pans, 22 hampers of pans, 92 iron shovels, and 6 iron cannon to Batavia.⁽⁹⁾

Javanese 9th century gift list inscriptions mention several types of iron tools, as well as iron bars, presented either in bundles or singly.⁽¹⁰⁾ Javanese blacksmiths set up shop in port areas or nodal points in the overland trade network, where they had ready access to imported iron.⁽¹¹⁾ Rather than competing with the finished products exported by China, they seem to have concentrated on specialised tools and small iron goods made to order.

© M. Flecker.