On a hill beside the Santubong River in Sarawak lies more than 40,000 tons of iron slag, the waste left behind by centuries of metal smelting. The sheer volume points to something significant: this was not a small workshop producing machetes for the daily needs of a handful of local residents.
It was a large-scale industry, and its products crossed the sea to China long before colonial ships arrived in the region.
Roots Far Older Than the Coast
Long before Santubong became an export hub, ironworking traditions had already existed in Borneo for thousands of years, deep in the interior.
Archaeological research in the Meratus Mountains of South Kalimantan has uncovered evidence of iron smelting at Gua Cililin, Gua Larung, and Gua Liang Kale. Similar finds have also been documented in Karang Besi Village in Central Kalimantan and Maridan Village in East Kalimantan.
Radiocarbon dating at Cililin Cave 1 indicates that the earliest iron ore smelting activity in the area dates back around 1,700 to 2,000 years. This predates any written records that mention Borneo.
Over the centuries that followed, the technology spread toward the eastern coast and along various river systems.
When the Coast Turned It into an Industry
The turning point came at Santubong, near present-day Kuching. Between the 7th and 14th centuries, the area developed into both a port and a major iron-smelting center, with activity peaking between the 10th and 13th centuries.
Archaeological excavations led by Tom Harrisson, curator of the Sarawak Museum, from the late 1940s to 1966 identified five major sites: Sungai Jaong, Sungai Buah, Bongkisam, Bukit Maras, and Kampung Ayer.
A single excavation season in 1955 yielded more than 22 tons of iron slag. Evidence of slag was also found across roughly 800 meters along the Santubong River.
More recent excavations at Sungai Jaong, carried out in stages in 2018, 2019, and 2022, provide an even clearer picture.
Across just five test trenches covering a combined area of 43 square meters, researchers uncovered around 5 tons of iron slag, nearly 2,400 fragments of tuyeres, or air-blowing pipes, and 13 workshop floors made from fired clay mixed with smelting waste.
Radiocarbon dating of the floors places their use between 925 and 1278 CE. This suggests that iron production continued with little interruption for nearly three centuries.
One circular smelting furnace discovered at the site is estimated to date from the late 12th to the 13th century. It has also been identified as the first scientifically dated iron-smelting furnace found anywhere on the island of Borneo.
The Secret Behind Its Quality
What made Santubong iron remarkable was not just its scale, but its quality. The bog iron ore used as raw material contained only about 16 percent iron.
Yet the smelting process could produce iron with a purity of up to 96 percent. The ore was smelted in clay pits about 30 centimeters deep. Air was supplied through bamboo cylinders fitted with feather-based pistons.
The process involved two heating cycles, reaching temperatures of around 1,150°C. A small amount of phosphorus was also introduced from the wood ash used as fuel.
According to a chemical analysis by F.E. Treloar in 1975, this phosphorus content made Santubong iron particularly well suited for weapons. Around one to two kilograms of iron was enough to produce a single machete blade.
Abundant Chinese ceramics dating from the Tang to Yuan dynasties have also been found at the site. The finds suggest that Santubong served as a port of call along a maritime trade network linking China, India, and various kingdoms across Southeast Asia.
Iron and iron products were traded there in exchange for goods from overseas.
The industry came to an end around 1380 CE. This coincided with what ceramic researchers call the “Ming Gap,” a period when Chinese exports to Southeast Asia declined sharply following trade restrictions imposed by the new dynasty.
The timing suggests just how closely Santubong’s iron industry was tied to wider maritime trade networks.
Traces of this industry can still be seen by the public today. On October 4, 2025, the Santubong Archaeological Park officially opened with a budget of RM36.6 million.
The park includes three galleries showcasing finds from Sungai Jaong, Bongkisam, and Bukit Maras. It also features the Wallace Centre, which honors naturalist Alfred Russel Wallace.
The park itself was expanded from 20 to 36 hectares after additional ceramic artifacts and iron-smelting remains were discovered during preparations for the site.
