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For the DC Comics superhero Pig Iron, see Peter Porkchops.

Two weights used in the theatre and made of pig iron; because of this, they are dubbed 'pig weights.'

Pig iron is raw iron, the immediate product of smelting iron ore with coke and limestone in a blast furnace. Pig iron has a very high carbon content, typically 3.5%, which makes it very brittle and not useful directly as a material except for limited applications.

Pig iron is typically poured directly out of the bottom of the blast furnace through a trough into a ladle car for transfer to the steel plant in liquid form, referred to as "hot metal." The hot metal is then charged into a steel-making vessel to produce steel, typically with an electric arc furnace or basic oxygen furnace, by burning off the excess carbon in a controlled fashion, and adjusting the alloy composition. Earlier processes for this included the Bessemer Process, open hearth furnace, finery forge and the puddling furnace.

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, bearing some similarity 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 a little sand was unimportant compared to the ease of casting and of handling.

Modern steel mills and direct reduction iron plants transfer the molten iron to a ladle for immediate use in the steel making furnaces, or cast it into pigs on a pig casting machine for reuse or resale. Modern pig casting machines produce stick pigs, which break into smaller 4-10 kg pieces at discharge. Cast iron is made by remelting pig iron, often along with substantial quantities of scrap iron, and taking various steps to remove undesirable contaminants, add alloys, and adjust the carbon content.

The Chinese were making pig iron by the later Zhou Dynasty (1122 BC - 256 BC). In Europe, the process did not become common until the 14th century.

• The Lehigh Valley IronPigs, a AAA-level Minor league baseball team based in Allentown, Pennsylvania and affiliated with the Philadelphia Phillies, is named for pig iron. Allentown and its surrounding Lehigh Valley region of Pennsylvania have historically been a major manufacturing site for steel.
• Episode 7 of the spoof science BBC television show Look Around You showed pig iron as being manufactured by specially-trained pigs.

• Steelmaking