Black walnuts are an excellent addition to many baked goods, including cookies, breads and cakes. Black walnuts are ready to harvest, usually in late summer or early fall and are about 2 inches in diameter. In general, it is not easy to crack black walnuts; however, there are special crackers that are efficient at cracking them. Before going on a search for a walnut cracker, according to the North Dakota University Extension, the following method should also work.
Freeze your black walnuts for 72 hours.
Place the walnuts in a vice if they are not fully cracked from the freezing process. Only tighten the vise just enough to hold the walnut in place. Tighten the vise by turning the crank, usually in a clockwise direction. Any type of vise that will hold your walnut, even one as small as 2 inches, will suffice.
Wedge a flathead screwdriver into the crack of the walnut. Then, twist the screwdriver back and forth all along the walnut's crack line. Twist about three or four times in each spot as you move along the walnut.
Undo the vice by cranking the handle in the opposite direction that you tightened it in. The walnut should be open now once it is free from the vise.