The clarity required to make a fine stock is dependent upon beginning with cold water, cooking at a slow, gentle simmer and continual, careful skimming of any fat, impurities, or scum that rise to the surface of the barely simmering liquid. This method not only assures that the base ingredients are cooked properly with every bit of flavor extracted, but also that any impurities in them will rise to the top to be removed during the cooking process. At about 82°C (180°F) the soluble proteins (albumins and globulins) in the bones begin to rise to the surface. At this point, if you allowed the liquid to come to a boil, these proteins would be incorporated back into the liquid. Following this slow cooking method precisely eliminates the possibility of a muddy stock. If a stock is allowed to boil or if it is stirred up from the bottom, the impurities in the meat and bones will dissolve into the liquid, making it cloudy, rather than rising to the surface so you can remove them.