Hi Paul,
I think you're just supposed to get the large majority of the scum. I would think the first 30 minutes but I've never timed it.
I always cover my chicken broth. It's supposed to be more concentrated if you don't cover because it reduces the quantity of liquid. But since she didn't say to cover the pot, I am assuming she meant for you not to.
The definition of simmer is to barely see little bubbles moving.
I have never made NT broth though. I just got my book a week ago.

Susie

I make wonderful beef, turkey, chicken and veggie broths that I use at home and also to make organic doggie biscuits (a small home business). I buy the beef bones at Whole Foods market, getting organic bones. For beef stock, brown the bones well in your oven first. I put them on a rack over a pan and cook them at about 450 for an hour or so, turning, till they are really colored. Then cool a bit, and into the pot. The browning first makes a big difference in the flavor of the broth. You can also brown veggies first (not as long) with similar results.

Cover with filtered water. I always keep organic veggie scraps in gallon-sized ziplock bags in the freezer, and add them to the pan. Add 4-6 peppercorns and 2 bay leaves. Bring to boil, covered. Lower the heat to simmer. Then you can start getting the scum off. You'll have to keep checking for scum. Keep the pot covered if you want stock, unless you added too much water. I simmer for 6 hours. Check it once in a while; as the water reduces, you may have to lower the flame.

Cool, strain, then refrigerate overnight. The next day you can remove the solidified fat on the top. Then the broth is ready to use. Can freeze.

best to you, Barb

Some hints on the scum thing - I took a cooking course, and here's what we were taught:

1. Start the bones off alone (no veggies, no spices) in COLD water
2. Bring to the boil (I use one notch below MAX)
3. Let boil for 5 - 10 mins
4. Drain and rinse the bones. This is a hard step, because the broth smells very flavorful and you might think "No, i'm dumping all that wonderful yummy goodness!" but the fact is you have extracted practically nothing at this point.
5. Start the rinsed bones again in COLD water, with your mirepoix (carrot, onion, celery). Mirepoix should be cut to same thickness as bones (i.e. smaller for chicken stock than for beef stock) for maximum flavor. Once a boil is achieved, reduce to a simmer and add spices (1 or 2 bay leafs, 8 peppercorns, small pinch thyme, per litre of water)

You will find that there is very little need to skim of the scum if you do this... most of it comes out in that first boil/rinse.

Keep in mind that although you are just simmering, water is still evaporating so it is possible for the pot to run dry... best to do this during the day, when you can check in every half hour or so. I cover mine almost completely, leaving a very small crack.

Also, about browning the bones - there are white stocks and there are brown stocks. White stocks are where you do not brown the bones, brown stocks are where you do. Therefore, it is entirely possible to have a brown chicken stock or a white beef stock.

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!