You'll find that single glass tends to be of higher pitch to almost squeeky. Double glass or even glass over slate will give you a deeper, raspier sound. The bottom glass/slate can be mounted at the bottom of the pot and an air chamber between that and the top. You can be creative and leave a hole on the bottom making your call a glass on top and flipped over for a slate.

Again, decaling, painting or eching, you can produce some really unique calls.

<font color=blue>Good Luck and Good Shooting</font id=blue>

<font color=red>Rob</font id=red>