I have been working on a cookie story for the Holiday Best special issue Canadian Living puts out for Christmas on international cookies. Quite often in Scandanavian countries ammonium bicarbonate is used as a leavening agent in cookies. I was curious to know why this would be the leavening of choice instead of baking soda or powder. I found out a few interesting facts: 

1 - a bitter taste

Ammonium bicarbonate was used in things like cookies because it did not impart a bitter taste the way some early version of baking soda and powder did. 

2 - only used in cookies that are flat

The ammonia gas gets trapped in baked goods like a cake where it cannot escape and will make the cake smell bad, so it is only used in cookies that are flat and porous where the surface area allows to ammonia to disappear. 

3 - storing

If you don't store it in an airtight jar it will evaporate. 

4 - preserved the flour

Baking powders were invented in Germany during a famine when food was scarce and people were dying. Using baking powder instead of yeast preserved the flour in the baking process instead of it being eaten by the yeast. 

5- An other name

Ammonium bicarbonate is also called Hartshorn, or Horn Salts. 

6 - a deer story

Hartshorn literally means ground deer horn which was supposedly used before ammonium bicarbonate went into commercial production - not clear if it has the same properties though, so don't bother trying to grind up antlers. 

7 - smelling salts

Harsthorn, Horn Salts or ammonium bicarbonate were used as smelling salts.