There are also commercially available "anti-bumping granules," which are porous granules composed of aluminum oxide (Al2O3). The following "anti-bumping" techniques are recommended by "Practical Skills in Chemistry": 1. Add one or two "boiling stones" or "anti-bumping granules" to the mixture; these can be filtered out later. 2. Increase the heat under the pot until it starts to bubble, then reduce the heat and simmer for another 10 minutes. 3. After cooling down the liquid, filter it through a paper coffee filter or several layers of cotton cloth. Anti-bumping agents are used when you want to make sure that your mixture doesn't foam up during cooking. They work by preventing any active ingredients from coming into contact with the hot metal of the pan, thus avoiding their degradation. Some agents, such as soda ash or sodium bicarbonate, have boiling points lower than water; others, such as cornstarch or potato starch, absorb moisture from the surrounding air and become sticky when they reach 100 degrees Celsius (212 degrees Fahrenheit). When cooking with these agents, it is important not to bring the mixture to a boil before adding the vegetables, since this could result in the loss of some of their protective properties. Anti-bumping agents are usually included in recipes as a suggestion, since there may be variations in heat retention by different pans.