Industrial heat in the middle ages
Across the Yamethin District of central Myanmar, large mounds of a dark, glassy waste material disrupt the landscape. These are not natural formations but immense slag heaps, the solid byproduct of a large-scale iron industry that operated here from approximately the 12th to the 17th century. At one site, Tegyigone, archaeologists estimate the slag pile contains over 60,000 tonnes of material. This volume of industrial waste points to a sustained and organized production of iron, a metal for agriculture, construction, and warfare.
The smelting process used local lateritic iron ores, which were heated with charcoal in furnaces. The byproduct of this operation is slag, which forms when impurities in the ore, such as silica and alumina, melt and separate from the iron. As the slag cooled, it trapped chemical and microscopic evidence of the conditions inside the furnaces. This evidence, preserved for centuries, allows researchers to reconstruct the sophisticated techniques of Myanmar's medieval iron smelters. The work, partly led by archaeologist Dr. Thomas Oliver Pryce of the French National Centre for Scientific Research (CNRS), has revealed a surprising level of technological achievement.
A matter of degrees
Analysis of the slag's microstructure shows that the furnaces in the Yamethin District regularly reached temperatures between 1,350°C and 1,400°C. These temperatures are exceptionally high for the period. For comparison, typical European bloomery furnaces of the same era operated at around 1,100°C to 1,200°C. The Myanmar smelters achieved this heat without using blast furnace technology, which would not be developed in Europe for several hundred years. Instead, they perfected the bloomery method, a process that produces a solid, spongy mass of iron called a bloom rather than liquid metal.
The reason for these high temperatures is found within the slag itself. Microscopic examination reveals dendritic (tree-like) crystals of the mineral wüstite within a fayalitic slag matrix. The formation of these specific structures requires the very high temperatures recorded. It is possible the smelters achieved this by designing their furnaces to use the powerful monsoon winds as a natural bellows, forcing large amounts of air into the furnace to raise the combustion temperature of the charcoal fuel. This technique is an adaptation to the local environment and pyrotechnology that was centuries earlier than similar technology.