From Trow and Holden

WEDGES AND SHIMS
(or plugs and feathers)

Most Stone will split cleanly if the proper breaking technique is used. Wedges and shims have been used for eons in the stone industry, and are still among the most effective tools for splitting stone. Here's how to use them:

1. Drill holes along your split line at least as deep as your wedges are long and about 6" apart. For difficult splits, drill holes deeper and closer together. Your split line may be straight or curved.2. Insert wedges and shims into holes as illustrated. Make sure the "ears" face the direction you want your stone to split. 3. Strike the wedges in sequence, firmly but without forcing. Wait a few minutes. Repeat until stone splits (clear the way!).

I spent the better part of today, doing this wrong.  I didn't drill deep enough and then pounded the hell out of the wedges.  Wish I read this instruction earlier.