If the stock is boiling, these substances are churned back into the stock and become emulsified. The resulting stock is cloudy and has a dull, muddy, greasy flavor, which only worsens if you reduce the stock for a sauce. When the stock comes to a simmer, skim it every 5 to 10 minutes for the first hour to prevent fat and scum from working their way back into the stock. After the first hour, you need to skim the stock only every 30 minutes to an hour. For the same reasons, a stock should be slowly heated to a simmer.
Rapid heating causes the meat to release albumin too quickly into the surrounding liquid.
Don't use too much liquid. The higher the proportion of solid ingredients to liquid, the more flavorful the stock is. Many beginning cooks completely cover the solid ingredients with liquid at the beginning of cooking. Because the solid ingredients in a stock settle during cooking, cooks often find that they have added more liquid than necessary, and the resulting stock is unnecessarily thin. When adding water, add only enough liquid or stock to come three-quarters of the way to the top of the ingredients.