Pentair MiniMax 250 firebox material

Hi all! I recently replaced my old Pentair MiniMax PlusII D 250 heater and now I'm dismantling it to get rid of it (and recover the copper). Can anyone tell me what the material is that the firebox is made out of? I stopped in alarm because it looked like you-know-what, but by the stickers the unit appears to be from the mid-90's. So I'm assuming it is not asbestos, but I'd like to be sure and better yet, know what it is.

Thanks!

Brian