Baker's Ammonia is a leavening ingredient called for in many old world recipes, especially those from Scandinavia. It is also called "hartshorn".
Unlike baking powder or soda, Baker's Ammonia (ammonium carbonate) leaves no unpleasant alkaline off-flavor in baked goods. It is used for cookies, crackers and cream puff-type pastries, items which are small, thin or porous. It is not used for cakes or other large items because the ammonia gas cannot evaporate from these items. You will notice an odor of ammonia while baking, but this will quickly dissipate and the 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.