"But Iron -- Cold Iron -- is master of them all."—Rudyard Kipling, "Cold Iron"
Iron may be treated as naturally magic-disrupting or just poisonous for certain creatures. Sometimes it's supposed to suck the magic out of the Fair Folk (similar to the way it sucks heat out of the body), usually accompanied with screams about how "it burns". More recent fictions sometimes say that it's got something to do with ferromagnetism, or related to iron's nuclear stability.
There's no agreement about what "cold" actually means in this context. Sometimes it just mean that the iron, at the moment, isn't hot. Sometimes it's cold-worked iron. Or something more complicated, like the iron that has never been smelted. Or this may be a poetic reference to any iron, just because metals that aren't hot feel cold thanks to heat conductivity. Which may be pulled as "magic vs. technology" symbolism. It may also be a reference to the fact that heating magnets to a certain point causes them to lose their magnetism, so "cold" iron is iron that still has its magnetic (magic) power.
Thunderbolt Iron may or may not be related: meteorite alloys are iron-based and frequently cold-worked because they cannot be tempered like steel anyway. Ironically, they are good in really cold climates, not only because the fuel for a smithy would be a bigger problem, but because they don't become brittle when carbon steels do.
Some anthropologists believe that the tradition of "cold iron" is actually a distorted memory of the conquest of Bronze Age peoples by Iron Age civilizations—the Picts of England, for instance (frequently cited as the basis for British "little people" myths), in the face of Rome.
- In Munchkin Bites, 'Cold Iron' is a trap card that only affects Changeling characters.
- In the The Sandman story Cluracan's Tale, the title character (a faerie) is captured and bound with cold iron chains, in a cell with cold iron bars. He has to call on the Sandman to free himself.
- In Marvel Comics' The Mighty Thor, the dark elves of Svartalfheim are vulnerable to iron. This is explained by iron being "the metal of humans", so it kinda fits with the nature-vs-science thing mentioned above.
- Marvel demons are also often vulnerable to iron. Even Adversary, an Eldritch Abomination aiming to destroy creation and create a new world had to avoid contact with Colossus.
- In the Top Ten spin-off Smax, Gadgeteer Genius Toybox finds herself having to defeat a dragon in a magical realm where her gadgets don't work. Her eventual solution has a big technobabble justification, but essentially it's Cold Iron.
- In the Hellboy story The Corpse, Hellboy exposes a changeling by touching an iron horseshoe to its forehead. Later he tests the real baby the same way, just to make sure. Conversely, in The Iron Shoes (usually published alongside The Corpse, since the latter is not quite long enough to fill up an issue), some folklorists explain that a few fairy creatures don't mind iron and in fact are rather fond of it, including the title character:
Live or die,
Win or lose,
MY IRON SHOES!"
- The possessed in the Night of the Demons remake are vulnerable to rusted iron.
- Putting a horseshoe over the doorway was considered a way to protect the home from intrusion of The Fair Folk- this has allusions to the story of the Exodus and the Passover. Sometimes burying a doornail was used this way too. Although often burying iron was a way to conceal the iron from The Fair Folk, and if you could get them to stand over it they would be trapped and bound until they agreed to your demands.
- There is an Italian wedding tradition that requires the groom to have iron in his back pocket.
- Andre Norton's novel Steel Magic. Cold iron is defined as being any metal "forged by a mortal in the world of mortals", so the three protagonists end up using their stainless steel picnic cutlery as weapons; respectively a spoon, fork and knife. Fortunately the cutlery develops unusual properties in the magical world (such as changing size) and is pretty dramatically lethal to any magical being it touches.
- In Lords and Ladies, elves' primary sense is based on detecting magnetic fields, which iron messes with.
- The horseshoe above the door is explained by saying the shape isn't important, it's simply that horsehoes are the closest available source of iron.
- In "Wee Free Men", Tiffany Aching uses a frying pan to fight the fairies- this shows that she intuitively knew that iron would be dangerous against them.
- The Midnighters series plays with this trope. Any kind of alloyed metal (like steel) hurts darklings, cutting through them like a soft knife through butter. Rex notes that the darklings' real weakness is new ideas (hence the signifigance of names and math when fighting darklings), which means that simple worked metal worked against darklings centuries ago, worked stone arrowheads and spearheads worked against them millennia ago, and he predicts that in the future they will have to use plastics, carbon fiber, or other exotic alloys against them.
- Poul Anderson's Operation Chaos series says the iron is anti-magical, but it's set in an alternate history where one of the famous physicists of the early 20th century found a way of suppressing the effect, allowing mankind to have its cake and eat it too.
- 'Cold iron' is any iron in the October Daye series. Changelings have a better resistance to it because of their human blood, and the Coblynau, who are the only Fae able to work with iron, are a very ugly race, implied to be due to their affinity with iron.
- In Tim Powers's On Stranger Tides, it's said that the presence of iron disrupts magic, and the increasing use of iron-based technologies is why you don't get magic much any more.
- In The Dresden Files, "cold iron" is any iron, even if it's been smelted into steel. Fae creatures generally have zero tolerance for it in any form—injured by mere contact with anything containing Iron. When cut with it, the Iron sets the Faerie blood on fire. It was noted by one Fae that even if the damage is not fatal, it will leave pain that lingers for a long time. Also, bringing iron into the Nevernever (to say nothing of leaving it there) is roughly equivalent to carrying around uncontained nuclear waste.
- In Fletcher Pratt and L Sprague De Camp's novella "The Castle of Iron" Harold Shea attempts to use some gold coins conjured out of sand to pay a blacksmith; however, when he rings the coins onto the anvil, they turn back into sand. (He remembers afterward that the Rudyard Kipling poem that he based his incantation on had made iron "the master of them all.")
- The Wheel of Time has "Iron to bind" to deal with the Elfinn.
- In The SERRAted Edge by Mercedes Lackey, the Elves get around the use of cold iron by making their race cars with fiberglass bodies and titanium engines (with a little magic here and there) -- which has the added benefit of making them lighter and therefore faster.
- In a later SERRAted Edge book, the reason why cold iron affects fae is understood well enough that a defensive perimeter against them is built using hydraulically positioned iron rods, insulated in silk until needed, and electrically energized to maximize the magnetic/conductive properties responsible for the effect.
- In the book Merlin's Godson the title character has to save a tiny Fae civilization from an iron nail that has accidentally fallen into their realm.
- In The Once and Future King the young boys Wart and Kay take iron with them as protection when they visit the fairies' castle.
- In Stardust Dunstan touches some coins to an iron nail to make sure that they are real and not "fairy gold".
- In The Last Unicorn Mommy Fortuna uses cold iron bars to trap the unicorn and the harpy. The unicorn is able to endure being closed in by the iron cage, but feels pain if she touches the bars.
- In Spiderwick Chronicles there is a passing reference to this.
"Steel. It cuts and burns."
- The Saga of Recluce features this with regards to Chaos-mages. Iron, both naturally and when transformed into black iron by Order-mages, is a natural repository of Order. Chaos-mages who have a surplus of Chaos in their body will suffer painful burns when the two energies interact. Consequently, Chaos-mages are fond of white bronze.
- One of the more notable examples was when the Chaos-mage ruling Fairhaven had a fellow mage locked away. Her hands were bound with iron manacles, leaving her screaming in agony; the ruler idly noted that with the amount of Chaos in her body she'd likely be dead before the day was done.
- In Doc Sidhe, the people of the fair world find the touch of iron painful (which makes things interesting for construction workers building 1930s-style steel-framed skyscrapers). Doc and his colleagues are surprised to learn that the human protagonist carries a pocketknife with a steel blade, and more surprised when he demonstrates that he can touch the blade with no ill effect.
- Rudyard Kipling's "Cold Iron", quoted at the page top, uses the reference solely as a metaphor for armed combat (as opposed to gold, silver, and copper, for trade, jewelry and metalcraft). Then it gets overtly Christian.
- One conversation in Princess of Wands suggests that FBI members who aren't part of the Special Circumstances group could also take part in slaying supernatural beasties, with cold iron bayonets fitted to their rifles specifically mentioned.
- In the Dragon Keeper trilogy, Dragons are hurt by the mere presence of Iron and Dragon Keepers have to use bronze or copper tools around them. This sometimes leads to problems, as in ancient China, Bronze was more expensive than Iron.
- In The Bartimaeus Trilogy all spirits are harmed by iron, but are harmed even more by silver.
- Dungeons & Dragons
- 'Fool's Gold' spell made copper coins look like gold, but it fails when false gold touches iron.
- Depending on the edition, demons that could normally only be harmed by magical weapons could also be harmed by iron weapons.
- 3e+ has Cold Iron as a special material (like mithril or adamantine) for metal weapons. The rule of thumb is that you need this to harm (or at least, do full harm) to Fey or chaotic outsiders. The downside? It's one of the flimsier special metals (although just as strong as steel), and there's a static price that must be payed in order to enchant it, doubling the price of the the lowliest weapon enhancement (at least in 3e based systems). Still, it's arguably one of the best special materials for weapons.
- In Changeling: The Lost, "cold iron" is anything that has a 95% iron content, and it negates any defense wrought by fae magic. The main book emphasizes that in the modern era, you'll rarely get anything like that unless it's a specialty work or from an earlier era. On top of that, you've got hand-forged iron, which is iron that's never been heated by human hands or means. This means most hand-forged iron weapons are rough and blocky, but they do hideous amounts of damage to the True Fae. There are many given accounts on why this is, but the most common one is that the Gentry once had a Contract with Iron; they got power for it in return for making sure it remained unshaped. Then humans discovered smelting, the Contract broke, and Iron is pissed.
- Likewise, in the predecessor game Changeling: The Dreaming, cold iron wounds do aggravated damage to changelings - and if they're killed with it, their fae soul will never reincarnate, effectively becoming a ghost. The only reason steel doesn't screw up the Kithain is because a changeling pulled a Heroic Sacrifice back in the day to ensure that it wouldn't.
- The 4th Edition version of GURPS Fantasy discusses cold iron, and multiple different ways of implementing it. The default is that it's simply a descriptive term for regular iron.
- Faery's Tale allows you to implement cold iron, though it's optional. Under the game's take, cold iron is simply wrought iron (as opposed to cast iron), and although it can't truly kill faeries (nothing can kill faeries), the merest touch of it will send a faery into a deep sleep for anywhere from hours to weeks.
- Tales of the Questor makes extensive use of this trope. Although it claims "cold iron" is a mistranslation of "north iron" ie lodestone.
- Cold iron in Never Never prevents Pookas from using any of their special abilities, including changing size and increased luck.
- Elves in Dragon Mango are extremely vulnerable to iron, but through Training from Hell can learn to resist it. Afterwards they use iron armors as a Power Limiter. Half-Elves are completely immune to it, likely due to their human half's iron-based blood.
- In Final Fantasy IV the Dark Elf is vulnerable to iron, and has enchanted his cave to be heavily magnetic, requiring you to reach him without wielding anything metallic. When you reach him, at first it is a Hopeless Boss Fight but if you talked to Edward in the castle, he gives you a harp which breaks the spell, allowing you to wield metal.
- In Gargoyles, the Oberon's Children were all weak to cold iron, up to and including Oberon himself. This is utilized in a number of ways - iron chains to bind Puck and the Weird Sisters, an iron robot named Coyote to catch the mythical Coyote, and ringing an iron bell to take down Oberon when he agreed to use only as much power as one of his children for a contest (though when he was at his full power an iron harpoon to the chest only slightly injured him).
- In the life cycle of larger stars, when they run out of hydrogen in their core to produce energy, stars star fusing other elements in order to maintain itself. The star keeps on building layer after layer within the core fusing heavier and heavier elements, and getting less and less return. Fusing iron will give no energy return. A few days after it starts to make iron in its core, it will go supernova. So iron is the star killing metal.