"Potash" is a vague term referring to any of a variety of soluble compounds of alkali metals, mostly potassium carbonate, potassium chloride, sodium carbonate, or sodium chloride. All of these compounds can be found in burned plant ashes as a portion of the leftover non-flammable, non-volatile material left after burning.
I desire potash for one reason: it acts as a flux for silica compounds. I will use this in two different other projects. First, as a glaze for pottery that can melt and seal ceramic items without the underlying item melting, thus allowing me to waterproof earthenware mugs and pots. Second, to lower the melting point of quartz for making glass at reasonable temperatures.
When you burn things to create ashes, though, a lot of other substances also end up in ashes:
The ratios of these and potash depend on the species burned. Willow wood yields a whopping 50% potash, and very little silicon or aluminum. English oak yields 23% sodium and potassium compounds. Rice husks only provide 1.3% potash, etc. I am using leftover ashes left over after other clay project firings, and I don't know the species. This is bad for achieving a consistent pottery glaze. Later, I will want to source a consistent supply of ash, but right now I'm happy with ANY result.
The best way to separate our desired potash from the other substances in raw ashes is to exploit potash's solubility. So first, we shovel some ash into a container, (I strained through a cheap spaghetti colander first to get rid of large chunks), flood it with water, and let it sit for a couple of days. It will look like this:
In the photo, the liquid on top is full of potassium, sodium, hydroxide, and carbonate ions, which we can now pour off into another container for further processing. The solids at the bottom are the non-soluble oxides that would ruin the melting point-lowering properties of the potash.
Don't get greedy. Don't pour off the liquid before the solids have settled, and stop pouring when you start getting any ashes pouring out. The non-soluble ashes will be counter-productive, because they are highly refractory and will raise the melting temperature of whatever you're making.
Careful, the poured liquid is highly caustic. It is not a cellular poison in the sense that something like cyanide is, but it will chemically "burn" you (turn your skin into soap) if you get it on you and don't wash it off quickly. Wear eye protection and gloves when pouring, and flush any splashed skin with large amounts of water. Treat this liquid as essentially Drano.
By the way, save the byproduct solid ashes. For two reasons: One, you can refill with water and repeat the process again for more liquid from these several times. Two, once you're all done, the remaining washed ashes are wonderful for making fire bricks for furnace linings, because all the remaining materials are very resistant to heat and have very high melting temperatures.
Anyway, back to the liquid: letting this air dry is ineffective, because crystals will start covering the water surface and prevent evaporation. Boiling the water off works better. The amount of water you see at the top of the jar poured into a pan and boiled dry yielded this:
Again, these crystals are caustic. They will re-dissolve and saponify your skin if you get them on you with any moisture (like sweat). Be careful. That said, I unceremoniously scraped mine off with a spatula. The potash won't ruin your utensils or pot, and you can use them for food later, as long as you VERY thoroughly rinse and wash the potash off first. Tiny trace remnants that remain after going through a dishwasher or a good scrubbing will not hurt you, they are not toxic beyond being strong bases. Think of it like having Drano or bleach in a pot temporarily -- you wouldn't throw out the pot, but you would wash it well before cooking with it.
At the end of the day I disappointingly ended up with this much potash from one pouring of the container liquid:
These aren't exactly fluffy white crystals, but that's okay. It might be just that there are several compounds mixed here, so they inhibit one another's crystal formation. Since all of the hydroxides and carbonates are useful, though, this is not a problem. We don't need to separate or purify them individually (also, the crystals are still damp in this photo).
Don't put the potash on aluminum foil. It will react.
This is only one pour off the jar of ashes. You should be able to continue getting potash for many pours in a row, leave a day or two between each one, until you stop getting justifiable returns. My second pour-off a couple of days later yielded about 75% as much as the first, for example.
The potash will lower the melting point of clay it is mixed with. So when mixed with crumbled dry clay, ground up, mixed with water, painted onto a piece of ceramic, and left to dry, the surface mixture will melt sooner than the underlying clay piece, creating a glaze. I will cover this topic in more detail in a future blog post, but I have already achieved early success in using my potash to make glaze. The black corner of the right side piece in this photo is vitreous glaze made with the above crystals and clay powder. If/when I can cover a whole piece with successful glazing like this, I will be able to make waterproof items: