Step 1Remove the fat from the top of the chilled stock. Pour the stock into a 3-quart stock pot and slowly heat the stock over low heat until it comes to a very low simmer.
Step 2Simmer the stock very slowly for about 5 to 6 hours, skimming occasionally and monitoring the pot so that it doesn't come to a boil. (If the liquid is still boiling even at the lowest temperature setting, move the pot slightly to the side of the heat source.) As the liquid reduces, strain the stock into a smaller sauce pot. Reduce the stock until there is about 1 cup remaining and it has a nappe consistency, or coats the back of a spoon.
Step 3Strain the glace through a fine-mesh strainer. Store in a covered container in the refrigerator or an ice cube tray in the freezer, making each cube 1 or 2 tablespoons for convenient use in recipes.