Cold iron

Cold Iron
Cold iron is a substance which dispels all forms of fae magic (except many works of "magical crafting", since these are often dependent of other types of magic or else are simply a matter of superb craftsmanship). It is inimical to some extent or other to most of the magical races, and so banned by "all civilised realms" of the Twilit Worlds.

Cold iron is not difficult to make; many traditional human swordmaking techniques (and much other ironworking) use appropriate alloys. What is not obvious (even, usually, to the craftsman) is that the making of cold iron is itself a magical act – the will of the smith, the intention of the smith to make an effective weapon (or effective shackles, etc) and working the iron by hand are all involved in the ritual. For this reason industrially produced iron, even if chemically identical, will not suffice. On the other hand, many weaponsmiths using traditional techniques may be making cold iron without knowing it.

Cold iron can move through portal created by fae magic, though it could prevent the portal being created in the first place.

Cold iron is obvious to second sight in most contexts: it appears pitch black to it.

Cold-forged Iron
Forged iron is iron or steel that has been shaped using hammers or rollers. Forging provides metal with a grain and increases its strength over casting.

Cold-forged iron is iron or steel that has been forged below 732 degrees Celsius. As cold forging tends to harden the metal (called "work hardening"), most modern manufacturers prefer to hot forge steel and then harden it after final shaping.

There is no non-magical difference between cold iron and cold-forged iron. All cold iron is, by definition, cold-forged. A good analogy is comparing iron bars to iron magnets.