Actually, I don't use shot at all (and if you normally use your tumbler for polishing, you'll want a dedicated barrel just for etching) I use abrasive tumbling media and grit, to wear away at the surface of the beads. It takes eight hours or so, but I love the finish, and the lack of chemicals! There's an awesome thread on tumbling, let me see if I can find it...
I think this is the one:
http://www.lampworketc.com/forums/sh...d.php?t=138057