Stock is not supposed to thicken while it is still cooking, but a well concentrated meat stock should gel on cooling and any fat can then be scraped easily off the top. When the collagen that is contained naturally in the meat bones dissolves during cooking, it forms the gelatine that later sets the stock. To make a really good jellied meat stock, try to get hold of a raw veal knuckle, calf's foot or beef marrow bone, and simmer it in plenty of water with vegetables and flavorings for about 4 hours before straining and cooling. If this stock is then reduced even further, it will form a syrupy glaze called a glace that is often used in classic sauces. Glace can be frozen in ice cube trays for convenience.
** Asian Recipes