Curing black walnuts removes excess moisture and enhances their flavor. When the walnut grows on the tree, it has a green outer husk that yellows as it ripens. Remove the husk immediately after harvest and prior to curing. If left on the walnut, the husk will adversely affect the walnut's flavor and color. After hulling, wash the walnuts before curing. Black walnuts will mold unless properly cured.
Place the walnuts in an even layer along the bottom of the box. If possible, use a box with a screen or wire bottom, to allow proper ventilation. Don't stack more than three walnuts deep.
Set the box in a location that is out of direct sunlight, is cool, dry and has plenty of ventilation. You'll also want a place that is odor-free; walnuts can absorb odors, which taint their flavor.
Break open one of the walnuts after two weeks. If the nut breaks with a snap it is sufficiently cured. If it is soft or pliable, allow the black walnuts to cure a little longer. Test again in about a week.