This is reading notes from the accessible potash document I described in this post.
Sage et al (p.1) note that potash smells acidic and like urine. It usually is dark brown in coloration (p.2), when it is in its coarse state. When fired in the reverberatory furnace, it turns slate blue or white and is now called potash (p.2). When potash is calcined well [i.e. turns into pearl ash, RCK], it becomes porous and light, like pumice (p.3).
The most common method for potash production is to burn wood (p.5) and put the ashes into a copper vessel, adding water and boiling the mixture. One lets the lye settle, decants it into a separate boiler, then lets the remaining water evaporate until the potash is dry.
In places of charcoal production, stovepipes can be used to gather up the humidity distilled out of the charring wood pile into basins, which are then evaporated using boilers (p.6). If the salts sticks too strongly to the pans, scissors and mallets are used to detach it (p.7). All woods produce some salts, but some more than others. Older ashes produce more, provided they are kept wet, and the best come from hardwoods (p.7). In the summer, they use cold water; in the winter, they mix hot and cold water half-and-half. Water that is too hot spoils the process, as the grease cannot separate from the salts. Stagnant waters give twice the potassium than clear waters.
One places wooden troughs of oak or pine a finger and a half apart, with a false bottom, where one places a bed of straw, and masses the ashes on top of that, so that the water cannot drain too quickly (p.8). One replaces the straw every six weeks in winter, and in the summer only every two months. Once the liquid stops fuming, one has to stir it with a stick, let the fire go out and cut the salt out with sticks.
The best method is the Swedish method, where one cuts the wood into pieces and piles it up.