Place the chicken parts, gizzards, and veal shank and marrowbones (if you have them) in a large stockpot. Cover with warm water and quickly bring to a boil over high heat. Boil vigorously for 3 minutes. Drain; rinse the bones and return them to the pot.
Cover with 6 quarts cold water; slowly bring to a boil. Skim off any scum that surfaces until only a small amoun