Cast iron
Cast iron is made from pig iron, which is the product of smelting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron, often along with substantial quantities of iron, steel, limestone, carbon (coke) and taking various steps to remove undesirable contaminants. Phosphorus and sulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced. Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively. If desired, other elements are then added to the melt before the final form is produced by casting.
Science and technology of the Han dynasty
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 smelted 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.