"Hirschhornsalz", hartshorn, ammonium carbonate or baker's ammonia is a leavening agent that is sometimes used for flat, baked goods such as Lebkuchen, cookies and flat biscuits or crackers. It is not used for cakes since the gaseous ammonia given off during baking cannot escape the thicker, higher batters and would make the baked goods smell bad. It leaves no salty or soapy taste residue as baking powder sometimes does since it completely decomposes into three gases.
Hartshorn lends a distinctive crispness and lightness to the baked good by building large pores, which has caused it to persist in certain recipes, despite the overwhelming use of baking powder and baking soda in modern baked goods. You can substitute baking powder for hartshorn in a pinch, but the final baked product may not have the exact same texture. The designs on molded cookies are also said to keep their shape much better when hartshorn is used.
If ammonium carbonate is not available, baking powder can be substituted in a 1:1, 1:2 or 1:3 ratio. That is: 1 teaspoon baking powder for every 1/2 teaspoon of baker's ammonia (or 1 1/2 teaspoons baking powder for every 1/2 teaspoon baker's ammonia). You can also add some baking soda together with the baking powder (see Barbara Rolek's solution).
Usually, baker's ammonia is mixed in with the liquid before adding to the dry ingredients, so that it dissolves well and mixes thoroughly. It must be stored dry, in a well-sealed container, because it is very hydroscopic and clumps easily. To tell if it is still active, place a small amount in hot water. If it bubbles vigorously, you can use it in your recipes. Buy it here:
"Hirschhornsalz" was made in the Middle Ages by burning (or dry distillation of) keratin-containing materials (keratin is a structural protein found in the animal kingdom). They did this by heating shredded horns, hooves, antlers, skin (and even decomposed primate urine) in lime kilns (Kalköfen), which were ovens built for heating limestone (calcium carbonate), a manufacturing process which goes back to Greek and Roman times to make quicklime, a building material. (source). The residue was collected after the ovens cooled off. Since wood ash was often boiled in water and used as a leavening agent, it is possible that these ashes were used in the same way. Hartshorn was used to treat diarrhea, dysentery, fevers and many different types of bites. More recently, it was a common smelling salt for Victorian ladies.
The name was of particular importance in the Middle Ages, when people believed that the salt collected from burning particular parts of the animal had special, medicinal value. (Dr. Karl A. Hofmann. Lehrbuch der anorganischen Chemie. 2nd ed. Braunschweig Friedr. Vieweg & Sohn. 1919)
Today, the white powder is chemically produced by heating ammonium chloride or ammonium sulfate with chalk (source).
"Hirschhornsalz" is a mixture of three molecules:
- Ammonium carbonate (NH4 )2CO3 → 2NH3 + CO2 + H2 O
- Ammonium bicarbonate NH4 HCO3 → NH3 + CO2 + H2 O
- Ammonium carbonate (second form) NH4 NH2 CO2 → 2NH3 + CO2
These molecules break down into the gases ammonia, carbon dioxide and (water) steam when heated to 60°C or higher (140°F). The gases lift the dough or batter (which has air bubbles in it through creaming) before the batter is set as they rise towards the surface of the cookie. As the dough sets up, the bubbles remain but the ammonia, carbon dioxide and steam are dispersed (throughout the air in the kitchen).
Baker's ammonia can react with certain sugars and amino acids in the dough to make small amounts of acrylamide, a carcinogen. Also, in 2008, melamine was found in ammonium carbonate shipped out of China, which started a recall movement in Germany (source).
Other information about ammonium carbonate:
Recipes with hartshorn:
E503ii (EU food additive)
bicarbonate of ammonia
ammonium hydrogen carbonate
powdered baking ammonia, bicarbonate salt of ammonia
salt of hart’s horn