Pearlash is a wood-based product which is used primarily in the manufacture of clear and crystal glass.
 Producing pearlash
Pearlash can be created at a kiln or magma kiln by a dwarf with the furnace operator labor enabled. Doing so requires one unit of potash, as well as whatever fuel is necessary to run the kiln. Pearlash is actually one of the most labor intensive products in the game to produce, requiring you to burn one unit of wood to ash at a wood furnace, then convert the ash to potash at an ashery just to acquire the input material used in its creation.
Pearlash is produced in bars with no quality level. These bars can be stored in bar stockpiles, and are viewable in the stocks screen under "bars."
Pearlash's primary use is to allow the production of clear glass (also requires sand) and crystal glass (also requires rock crystal) at a glass furnace or magma glass furnace.
Alternatively, pearlash is a magma-safe (though eccentric) building material, and can be used in any constructions.
 Real world
Pearlash (or salt of tartar) is a more refined form of potash. Historically, this fine white powder was produced by burning away potash's impurities in a kiln. It was used in glassmaking, china, and soap production, and has many widespread uses today.
[MATERIAL:PEARLASH] - reconstructed from data extracted from memory