Effective Against
GhostIron is salt's close companion in the hunter's toolkit. While salt is primarily defensive, iron is actively disruptive — a swing of an iron crowbar through a ghost's manifestation will scatter it immediately. Iron also has protective properties, with iron horseshoes and iron thresholds appearing in folk traditions worldwide.
Usage
Any iron object can disrupt a ghost's manifestation on physical contact. Iron chains, crowbars, fireplace pokers, and iron knives are all effective. Pure iron is best — steel (an alloy containing iron) has some effect but is less reliable. Iron also affects certain fairy-like creatures and some demons in specific contexts.
Hunters improvise constantly with iron. Bobby Singer famously kept an iron crowbar in his desk and iron rounds for his shotgun alongside the salt rounds. In a pinch, cast iron pans, iron railings, and iron nails can all serve.
Real-World Basis
The belief that iron repels supernatural beings is one of the most widespread folk beliefs in the world. In Celtic and broader European tradition, fairies are unable to touch or cross iron. This belief may have originated during the transition from the Bronze Age to the Iron Age, when iron-working peoples displaced or conquered bronze-using cultures — the "old people" and their traditions became the fairies, and the new metal that defeated them became their weakness. Alternatively, iron's association with blood (iron gives blood its red color) and its role in tools and weapons may have given it a symbolic protective power.