Materials of a fog horn?

From:  "Brad"

If a tin whistle is made of tin, what's a fog horn made out of?

So I'm pretty sure that this was just a comical question we put up for kicks and giggles but we actually got for real answers!!

TB was the first to send us this: 

Mostly brass. There are dozens if not hundreds of design for fog horns. Bellows or pump style, electric or air powered. Fog horns are designed for maritime use and need to be able to resist the corrosive effects of oceanic environments.

C++gUrU then concluded with this: 

A fog horn is made out of liquid nitrogen, air compressed into a tin or aluminum bottle, with a plastic horn used to channel the sound. Its called a fog horn because when you turn it upside down and press down on the part you use to make it sound, the liquid nitrogen comes out looking like fog.