Iron used to repel spirits
In theory, if ghosts do exist and they do use electromagnetic energy to manifest, then hypothetically one could use a conductor of electricity, such as iron to drain or ground out the energy from the spirit. Therefore, touching a grounded, iron object to the spirit itself would drain all usable energy from that spirit. Based on this theory, many cemeteries are enclosed by iron fencing to keep the spirits in. Also, it has been said than an iron horseshoe placed at your front door will bring good luck by keeping evil spirits out.
Iron in "Supernatural"
The following are typical uses of iron to repel or combat ghosts, monsters, and demons.
- 1.17 Hell House - Sam and Dean get Harry Spengler and Ed Zeddmore to post on their website, Hell Hound's Lair, that the tulpa Mordechai Murdoch can be killed by special wrought-iron rounds.
- 1.18 Something Wicked - Dean learned from John that a shtriga can be killed by consecrated wrought-iron rounds.
- 1.19 Provenance - Sam uses an iron poker to repel Melanie Merchant.
- 2.06 No Exit - Jo uses a knife with an iron blade, that used to belong to her father, to repel the spirit of Dr. H.H. Holmes.
- 2.21 All Hell Breaks Loose: Part One - Sam uses an iron bar to repel the Acheri Demon.
- 2.22 All Hell Breaks Loose: Part Two - A devil's trap constructed from iron railroad lines by Samuel Colt keeps the Yellow-Eyed Demon from approaching the Devil's Gate in Wyoming until it is broken when the gate opens.
- 4.02 Are You There, God? It's Me, Dean Winchester - Bobby's Panic Room, first shown in this episode, is made of iron walls coated with Salt and has an iron Devil's Trap over the light source and fan in the ceiling.
- 4.06 Yellow Fever - Sam and Bobby use an iron chain etched with spell work to trap and drag a buruburu behind the Impala.
- 5.10 Abandon All Hope... - Sam, Dean, and Ellen build a bomb from propane, salt, and iron nails at Jo's insistence to destroy the hellhounds surrounding them.
- 6.09 Clap Your Hands If You Believe - Sam shoots the Leprechaun with iron buckshot, weakening him.
- 6.18 Frontierland - The phoenix was unable to escape from the jail or handcuffs because they were made of iron.
Sam and Dean feel cold iron is the best way to repel a spirit. They usually employ fireplace pokers, and on at least one occasion a wrench. Their hunting relatives also made use of iron. A house belonging to the Campbells, Mary's family, was outfitted with iron fixtures. John owned an iron axe that is eventually used by Sam to behead Leshii.
Sam and Dean gain firsthand knowledge of the effect of iron on spirits when they astral project with the assistance of Pamela Barnes in 4.15 Death Takes a Holiday. They find themselves trapped when a demon, despite being burned by the metal itself, encircles them with an iron chain.
Wrought Iron Lore
Wrought iron is sometimes asserted to repel, contain, or harm ghosts, fairies, witches, and/or other malevolent supernatural creatures. This belief continued into later superstitions in a number of forms:
- Nailing an iron horseshoe to a door was said to repel evil spirits or later, to bring good luck.
- Surrounding a cemetery with an iron fence was thought to contain the souls of the dead.
- Burying an iron knife under the entrance to one's home was alleged to keep witches from entering. 
Cold iron is also used for similar purposes. However, cold iron is pure iron that is not smelted out of ore, but hammered pure without melting.