Simmer uncovered for 6 to 8 hours. Strain stock through a fine mesh strainer into another large stockpot or heatproof container discarding the solids. Cool immediately in large cooler of ice or a sink full of ice water to below 4. Place in refrigerator overnight. Remove solidified fat from surface of liquid and store in container with lid in refrigerator for 2 to 3 days or in freezer for up to 3 months. Prior to use, bring to boil for 2 minutes. Use as a base for soups and sauces.