- I filled a big stainless steel (not aluminum!) pot with 8lb of the mixture (about 1/3 full)
- I topped it to 2/3 with water.
- I stirred it thoroughly with a wooden spoon
- Immediately large chunks of charcoal floated to the surface, and I picked them out and discarded them
- I left the rest to sit for a while, coming back to stir it occasionally
- After an hour, the top was covered with small unburnt wood & charcoal, which I strained off and discarded.
- I used a ladle to dip out the clear fluid from the top of the container
- I added more water, stirred, and repeated the process after another hour.
- After 2 more repeats of the process, I had about 2L of potash solution
- I discarded the wet ash
- I put the liquid in some glass oven trays and dried it
Since potassium carbonate and sodium carbonate are both white crystals, I assume the brown stuff is some sort of impurity. I tried dissolving the potash in alcohol, to no avail. Neither the potash nor the brown component dissolved in it. Other people seem to have had the same problem:
at Caveman Chemistry
at Alchemy Forums
Some agricultural forums talk about "red" potash, containing iron impurities. Ferric sulfate has a sort of pinkish-brown color, but is "negligibly soluble" in water. Ferric nitrate is a very light pink. Iron carbonate is exactly the right brown color, but is totally insoluble in water (as are most carbonates). Ferric chloride has a variety of interesting characteristics (it's yellow, or green, or pinkish-purple) and it's soluble in water. So the mystery continues.