The walnuts must be of almost maximum size, but they must be gathered before their shells have started to harden. If a nutpick or icepick can be pushed straight through the walnuts, husk and all, they are young and tender enough to pickle.
Put the nuts in a large kettle and cover them with boiling water. Boil until water discolors badly. Pour off the water, add fresh boiling water, and continue cooking. Continue boiling and changing water until the water remains clear.
Drain the nuts and pack them into a quart jar with the dill, walnut leaves, pickling spice, salt and alum. Add enough boiling cider vinegar to fill the jar. Seal. Let stand for one month.