If using a pressure cooker:, If you insist on skimming the schmutz, bring it to a boil first and skim away. Then seal it up and bring to 15lbs pressure. Reduce the heat to just hold 15lbs. Cook 4 hours for beef/venison bones, 1 hour for poultry, 20 minutes for fish. Turn off heat, let cool at least 20 minutes and ensure the pressure is gone before attempting to open. Read the directions on your cooker for how to safely open.