Place bones in a large stock pot (12 quarts or larger) with water and apple cider vinegar. Let it rest for about an hour.

Add salt and bring water to a boil.

Skim white foam off the top of the water (if any is present) and discard.

Reduce heat to low, cover, and simmer for 12-24 hours.

Remove pot from heat and let cool.

Once cool, strain liquid into glass jars, leaving about 1-2 inches of space at the top. Finish chilling in refrigerator.

Store the jars, loosely sealed, in the freezer for several months. Defrost one or two quarts at a time.