There are 2 types of iron:
Wrought Iron: flexible, workable, used in forging
White Iron: Regular MC iron, brittle, unwrokable, refined

Regular furnaces do not take iron ore.

A bloomery can smelt iron ore into wrought iron. (it takes 4x longer and is not
very cost efficent)

Big furnaces(any MF furnace) can smelt iron ore into wrought iron. A bronze
furnace is needed to properly smelt iron.

White iron is made from wrought iron + flux in a crucible