Posted By:
AlNot for the faint of heart, BUT, works perfectly for me...
I use a vice on my workbench and place the lower 1/3 of the card in the vice with two scrap pieces of wood on both sides of the card. The scrap pieces of wood are about shim size, maybe an inch or so wide, just enough to hold the card in place. Snug the card to the point it doesn't move, over-tightening is counterproductive at this point!
Using a sharp chisel and hammer...I place the chisel in the tiny groove with half the blade exposed to the outside edge of the card. Begin to tap gently with a hammer until you hear or feel the slab crack. Continue to tap, the bevel of the chisel will allow for the slab to open nicely. Sometimes I go from side to side to relieve the pressure equally.
I can crack a slab in less than 30 seconds and have never come close to touching the card until it is safely freed from the slab. Don't get too agreesive and keep the chisel vertical at all times.
I think with knives and screwdrivers a slight goof and you've toasted the card, but this method appears to work for some.