No wonder they were given to you. The donor has quite possibly been trying to get those nuts open for months. Black walnut shells are one of the toughest organic substances known, so hard, in fact, that they are industrially ground and used in place of sand to air blast metal, marble, glass and other substances. In fact, Hammons Pantry offers "products, gifts, cake and cookie recipes, and nutshells for soft-grit abrasives," on its Web site. Another site justifies the premium price it charges for shelled black walnuts over shelled English walnuts by stating, "It is possible to crack your own black walnuts, but you should be prepared with a sledgehammer, an anvil, and good aim."
Still doesn’t solve your problem, does it? Lacking a sledgehammer and anvil, you can try two things: Freeze the nuts for 24 hours or cover them in a pan with water, bring the pot to a boil, remove it from the heat and cover it for 15 minutes. Remove the nuts, let them cool, blot them dry, and then have at it. You may still need a hammer and cement floor (and safety glasses!), but you should prevail. And maybe you can give the gift giver the shells in repayment, in case he needs to do some industrial-strength sanding .
Many of our readers have sent in accounts of their efforts to crack open black walnuts since this article was first written - ranging from hilarious to nearly tragic. One says that for years her father has taken them into the basement and cracked them open in a vise on his workbench. "It does the trick but still takes time to pick out the nutmeats," she says.
We found the following black walnut cracker online. The manufacturer claims that the gear applies the correct pressure to crack even the hardest nuts without damaging the meat. It's not inexpensive, but is cheaper than most bench vises we've seen.