Pig iron is used in the production of steel. It is made by smelting iron ore
in a regular furnace.
What you need:
1. Bronze pickaxe
2.A furnace (anything but a bloomery)
3.Something to burn
4. A firemaking bow
First get a large iron ore with the bronze pickaxe. Then light up the burnable item and put it inside with the iron ore. Keep burning more to keep the fire going. After a few minutes the large iron ore should turn to pig iron.
The main use is for the production of steel.