Baker's Ammonia (Ammonium Carbonate) is a leavening ingredient called for in many old fashioned recipes. It is also called "hartshorn".
Baker's Ammonia is used to make extra-crisp cookies or crackers. Unlike baking powder or soda, it does not leave an alkaline off-flavor in baked goods. It is not used for leavening in cakes or other large-volume items as the ammonia gas will not evaporate entirely.
Note: You will notice an odor of ammonia while baking, but this will quickly dissipate and the finished baked product will not have an odor or taste of ammonia.
Because Baker's Ammonia has a tendency to evaporate when exposed to air, it should be stored in a jar with a tight cover. It will not spoil, but will "disappear" if not stored properly.