The product of the blast furnace is pig iron. From this all other forms of iron and steel are now made. Some grades of pig iron are selected, and without further change are merely re-melted in the foundry for making castings, hence it has become common for the designations "pig iron" and "cast iron" to be interchanged.

All pig iron contains an aggregate of about 7 per cent or less of the five substances, carbon, sulphur, phosphorus, silicon and manganese, and the grade or quality of pig iron for various uses is determined by the amounts of each of these ingredients contained

Iron occasionally takes up other substances in smelting, but these are usually negligible.

For high-grade uses demanding an iron of superior ductility and chemical purity, a limited quantity of "charcoal iron " is at present smelted. Ores of exceptional purity are selected (particularly free from sulphur and phosphorus) and are smelted with charcoal fuel. A cold blast of air causes the iron to dissolve less carbon than it dissolves in the higher temperature of the hot blast, also a lower smelting temperature lessens the introduction of other ingredients into the iron, hence the superiority of " cold blast iron."

The name "pig iron" comes from the former way of running iron from the blast furnace along channels and branch channels, in herring-bone shape, in the level sand bed adjacent to the furnace. The iron along the main channel was called the sow, and that in the branch channels was called the pigs.