"Pottasche," potash or pearlash is a baking aid used in some German baking recipes, especially gingerbread (Lebkuchen) recipes. It is often used in conjunction with hartshorn or baker's ammonia. In modern baking it has been all but replaced by baking soda ("Natron" or sodium bicarbonate).
"Pottasche" or pearlash is also known as potassium carbonate (K2CO3). It is an alkaline salt (white powder) which reacts with water or an acid (sour milk or fruit juice, for example) to create carbon dioxide, which gives baked goods lift.
As a substitute, use 1/2 teaspoon baking soda for every teaspoon of pearlash or potash. The taste of the final product may be different from the original.
When dissolved in water, pearlash decomposes to potassium and carbonate ions; the carbonate becomes carbonic acid, which bubbles out as carbon dioxide. The potassium recombines with the water’s hydroxide ions to produce potassium hydroxide (KOH), which is also alkaline. In order to prevent it giving a bitter or soapy taste to the food, you have to add an acid to neutralize the potassium hydroxide, but you can get all of the leavening power without acidic ingredients.
From the King Arthur website:
To make pearlash, you first have to make potash which itself is made from lye. To make lye, you pass water through a barrel of hardwood ashes over and over until an egg can float on the residue. (To make soap you boil this “lye water” with lard or other fat until it is thick, pour it into molds and harden it into cakes.) To make potash, you evaporate lye water until you have a solid.
Pearlash is a purified version of potash....Pearlash was used primarily in the seventeenth and eighteenth centuries but because of its bitter aftertaste, it not only did not replace yeast but was eventually replaced by [baking soda].
Other German leavening agents:
salts of tartar
carbonate of potash