Iron

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. – Pad of Definitions (1.19 Provenance), Official Website

Iron in "Supernatural"

The following are typical uses of iron to repel or combat ghosts and demons.

• In 1.17 Hell House, Sam and Dean get Harry Spengler and Ed Zeddmore to post on their website, Hell Hound's Lair, that Mordechai Murdoch is killed by special wrought-iron rounds.
• Sam uses an iron poker to repel Melanie Merchant in 1.19 Provenance.
• in 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.
• Sam uses an iron bar to repel the Acheri Demon in 2.21 All Hell Breaks Loose: Part One.
• In 2.22 All Hell Breaks Loose: Part Two a devil's trap made of an iron railroad, is what keeps the Yellow-Eyed Demon from the devil's gate in Wyoming.
• Bobby's Panic Room is made of iron walls coated with Salt and a Devil's Trap thats light is shown down from the ceiling; it is first shown in 4.02 Are You There God? It's Me, Dean Winchester.
• In 4.06 Yellow Fever, Sam and Bobby use an iron chain etched with spell work to trap and drag a buruburu behind the Impala.

Sam and Dean feel cold iron is the best way to repel a spirit.^BoM They usually employ fireplace pokers, and on at least one occasion a wrench.^{4.17 It's A Terrible Life} Their hunting relatives also made use of iron. A house belonging to the Campbells, Mary's family, was outfitted with iron fixtures.^{5.13 The Song Remains The Same} John owned an iron axe that is eventually used by Sam to behead Leshii.^{5.05 Fallen Idols}

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

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. ( Source: Wikipedia)

Cold iron is also used for similiar purposes. However, cold iron is pure iron that is not smelted out of ore, but hammered pure without melting.