I won't get into real life stuff (because who cares, it is a game).
I am utterly convinced, in real world folklore, that people have utterly misinterpreted the phrase "cold iron".
"Cold iron burns faeries" is the basic folklore.
People have thus tried to figure out what "cold iron" means -- is it cold-worked iron, does it include steel, must it be meteoric iron, etc.
I think people are overthinking.
Remember, in folklore, one can thwart the faeries by turning your clothes inside-out. So, "cold iron" is just a reversal. That is, I read it so:
"Cold (room-temperature) iron burns faeries (as if it were red-hot)."
In the Dresdenverse, it's way more expanded than that, but I think starting from the idea that all iron and steel are poison to the fae is a good way to go.