When making NT or any good stock, the secret is to remember to soak your bones in water with 1/2 cup vineger. This releases the minerals that are trapped in the bones. Also on the 1st boil that is to remove the scum, this boil should be done on medium heat and it needs to be a small, small boil-more than a simmer but not a rolling boil. You only have one chance to remove any scum as subsequent boils will not bring up the impurities. Make sure your veggies are in the pot to bring out any impurities in them. Once you have skimmed the scum, turn heat down to a simmer and let it go for 24-72 hrs,stirring occassionally.If the water is dropping, your heat is too high, add back until you are with in 1-2 inches of where you started. Ten minutes before you turn off the heat,drop a large bunch of fresh parsley in and let it simmer 10-15 minutes. Then turn off the heat . Finish as you will,straining out the meat and veggies. Dogs love the meat and veggies, see The book's selection on making dog bisquits!