Combine the bones and water in a large stockpot. Bring the mixture to a boil. Maintain a low boil for 24 hours, adding water to keep the bones submerged. Remove and discard the bones. Allow mixture to simmer another 21 to 22 hours. Strain the liquid through a fine-mesh colander and return to the stockpot. Bring to a boil and cook until liquid has reduced to about 2 quarts.
Transfer the stock to heat-safe containers; freeze. Once frozen, there should be three easily-discernable layers. Remove and discard the top-most of those three layers. Thaw remaining portion to use.
