Rinse the shrimp heads and shells quickly under cold water, and place them in a stockpot with the remaining ingredients. Bring to a boil over high heat.
Reduce the heat to low and simmer for 10 minutes. Turn the heat up to medium and cook for 30 minutes. Allow to cool thoroughly, strain, and refrigerate or freeze in 2- to 4-cup containers or ice-cube trays for use in individual dishes. Keeps for 1 month.
Yield: about 3 quarts