https://en.wikipedia.org/wiki/Pig_iron
Pig iron is the intermediate product of smelting iron ore. It is the molten iron from the blast furnace, which is a large and cylinder-shaped furnace charged with iron ore, coke, and limestone. Charcoal and anthracite have also been used as fuel. Pig iron has a very high carboncontent, typically 3.5–4.5%,[1] along with silica and other constituents of dross, which makes it very brittle and not useful directly as a material except for limited applications.
The traditional shape of the molds used for pig iron ingots was a branching structure formed in sand, with many individual ingots at right angles[2] to a central channel or runner, resembling a litter of piglets being suckled by a sow. When the metal had cooled and hardened, the smaller ingots (the pigs) were simply broken from the runner (the sow), hence the name pig iron.[3] As pig iron is intended for remelting, the uneven size of the ingots and the inclusion of small amounts of sand caused only insignificant problems considering the ease of casting and handling them.
How many of us even KNOW what Johnny Cash is talking about in this song. Such a great way to learn about metallurgy.