Air Horns!!!!!!!! Insulation or what?

Hey there, i just bought an old air horn from my buddy, he got it off of an old peter built but said he never tried to sound it. So i took off the cover to find two metal plates over each of the horns (it's two horns in one encasement) and under the metal plates a whole bunch of sand/dirt, that almost had a blue color to it. I havn't taken it out because i'm not sure if that is a part of the function. I dont know the mechanics of an air horn. So any help would be appreciated. thanks.