Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux. Charcoal and anthracite have also been used as fuel. Pig iron has a very high carbon content, typically 3.5–4.5%, which makes it very brittle and not useful directly as a material except for limited applications.
The traditional shape of the molds used for these ingots was a branching structure formed in sand, with many individual ingots at right angles to a central channel or runner. Such a configuration is similar in appearance to a litter of piglets suckling on a sow. When the metal had cooled and hardened, the smaller ingots (the pigs) were simply broken from the much thinner runner (the sow), hence the name pig iron. As pig iron is intended for remelting, the uneven size of the ingots and inclusion of small amounts of sand was insignificant compared to the ease of casting and of handling.
The Chinese were making pig iron by the later Zhou Dynasty (1122–256 BC). In Europe, the process was not invented until the Late Middle Ages (1350-1500). Actually the phase transition of the iron into liquid phase in the furnace was an avoided phenomenon, as decarburizing the pig iron into steel was an extremely tedious process with medieval technology.
Traditionally pig iron would be worked into wrought iron in finery forges, and later puddling furnaces, more recently into steel. In these processes, pig iron is melted and a strong current of air is directed over it while it is being stirred or agitated. This causes the dissolved impurities (such as silicon) to be thoroughly oxidized. An intermediate product of puddling is known as refined pig iron, finers metal, or refined iron.
Staring between bars,
My knothead keeps getting stuck.
Wanting to be free,
Over and over I try.
Then he comes, tells me, "Become
The cage, little friend."
Turning into pig iron,
I begin rusting.
Written December 4, 2008
First Posted December 4, 2008