Gelatin is a substance found in the bones, skins and other connective tissues of animals. It is normally present in soups, stews and gravies, where it occurs naturally from cooking the meat and bones. Commercial gelatin is made from primarily from hides and hooves. It is refined and purified to remove any flavors, leaving only the gelatin protein. Home cooks will not be able to produce a clear and flavorless gelatin from bones, but simple simmering can produce a very strong gel with a mild flavor, suitable for use in soups or sauces.
Place the bones in a large stock pot with twice their weight of cold water. For 10 lbs. of bones, this would be approximately 5 gallons. Depending on the size and shape of the bones, it may be necessary to use slightly more water to keep them covered.
Bring the pot to a simmer. As the surface proteins on the bones coagulate, they will rise to the surface as a grey-brown film. This should be skimmed off regularly, until no more rises to the surface.
Simmer the bones for six to eight hours. If necessary, add small amounts of fresh water to keep the bones submerged until the end of the cooking time.
Remove the bones from the pot, and strain the liquid through several layers of cheesecloth to remove any solids. Skim off any fat that might be floating on the surface.
Return the liquid to the stovetop in a clean pot, and cook it at a low boil until it has reduced by half. Transfer to a smaller pot, and reduce by half again. Repeat twice more, until the quantity of liquid is 1/16th of its original volume.
Refrigerate the resulting liquid in a sterile sealer jar with an airtight lid. The liquid will congeal to a thick, rubbery consistency with a mild meat flavor. Use by melting it in a double boiler, stirring it into a small amount of boiling water, or adding it directly to soups and sauces.
To make the gelatin concentrate more neutral in flavor, "blanch" the bones by boiling them for five minutes, then changing the water and starting over. This washes away much of the loose protein and removes some of the beefy flavor.
Veal bones make stronger gelatin than beef bones, but are harder to find. The best bones to use are hip and "knuckle" bones, which include joints and their connective tissues.
The same procedure is used to make stock for soups and sauces. Add 5 lbs. of beef or veal shank or oxtail, as well as 2 lbs. of coarsely chopped onion and a 1 lb. each of chopped carrot and celery.
If you follow the same procedure to strain and reduce the stock, you'll end up with an intensely flavored beef gel, known in classic cuisine as "glace de viande." A teaspoon of this "glace" will add a distinct beef flavor to any sauce.