Cold Iron

There is exactly one building in the world made of cold iron, and that is Fireproof at Hilltop Preparatory Academy. It isn't the raw material that makes large amounts of cold iron expensive, but the difficulty in heating and then chilling it properly in a single continuous sheet large enough to build with. Short of building a forge larger than the First Dwarven Dynasty could have dreamed of, the only way to forge and then quench a wall sized chunk of cold iron is through magic, and that is what the formidable maestri of Hilltop have done. In small amounts, cold iron is not actually that expensive, being made of the same iron dwarves and others mine everywhere, and relying simply on the skill of the refinery and smiths that handle it to transform it into the deadly, magic-killing blades it is better known for. 

(It's quench-hardened carbon steel all right?)

Properties

Material Characteristics

Cold iron looks like regular mild steel to the untrained eye...until exposed to acid, when mild steel will stay bright and cold iron will darken to almost black, revealing its nature as a magical sink. A common technique in blade-making is to wrap normal steel around a cold iron core and then grind the normal steel off the edge, creating a more flexible spine and a deadly cutting edge, revealed in an intimidating manner as a wave of darkness. A dagger made entirely of highly polished, new cold iron is an assassin's tool, distinguishable from a normal steel dagger only to close examination by a trained eye.

Physical & Chemical Properties

Cold iron is harder and more brittle than normal steel, holding a sharper edge, but breaking before it bends. The usual test to determine that the chilling process has worked on the iron is to skate a file along the edge of a blade made of the stuff and make sure the metal file doesn't bite in. In other respects its physical properties are similar to normal steel, being more durable than iron or bronze, yet susceptible to rust.
Cold iron's magical properties are what set it apart from other metals. Most martial metals belong to Beadu , and affect all beings equally, according to their natural resistances to damage. Cold iron, however, being a black metal transformed from its original state by skill alone, belongs to Gléawnes , The Bittersweet, Goddess of Knowledge, and it has a devastating effect on beings that live in partial concealment from the natural world, such as fey and ghosts. It also has strange effects on spellwork, generally nullifying spells cast at it, and blocking spells directed past it. However, there are highly skilled wizards who claim that they can manipulate cold iron to store magic, not to use again as one would a spell stored in a blackthorn wand, but to examine and learn from.

Type

Metal