Science and Technology of The Han Dynasty - Metallurgy - Furnaces and Smelting Techniques

Furnaces and Smelting Techniques

A blast furnace converts raw iron ore into pig iron, which can be remelted in a cupola furnace to produce cast iron. The earliest specimens of cast iron found in China date to the 5th century BCE during the late Spring and Autumn Period, yet the oldest discovered blast furnaces date to the 3rd century BCE and the majority date to the period after Emperor Wu of Han (r. 141–87 BCE) established a government monopoly over the iron industry in 117 BCE (most of the discovered iron works sites built before this date were merely foundries which recast iron that had been smelt elsewhere). Iron ore smelted in blast furnaces during the Han was rarely if ever cast directly into permanent molds; instead, the pig iron scraps were remelted in the cupola furnace to make cast iron. Cupola furnaces utilized a cold blast traveling through tuyere pipes from the bottom and over the top where the charge of charcoal and pig iron was introduced. The air traveling through the tuyere pipes thus became a hot blast once it reached the bottom of the furnace.

Although Chinese civilization lacked the bloomery, the Han Chinese were able to make wrought iron when they injected too much oxygen into the cupola furnace, causing decarburization. The Han-era Chinese were also able to convert cast iron and pig iron into wrought iron and steel by using the finery forge and puddling process, the earliest specimens of such dating to the 2nd century BCE and found at Tieshengguo near Mount Song of Henan province. The semisubterranean walls of these furnaces were lined with refractory bricks and had bottoms made of refractory clay. Besides charcoal made of wood, Wang Zhongshu states that another furnace fuel used during the Han were "coal cakes", a mixture of coal powder, clay, and quartz.