Making Potash

"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:

Iron oxides
silicon oxides

aluminum oxide

calcium oxide

magnesium oxide

manganese oxide

phosphorus oxides

unburned charcoal

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:

Creating Homemade Refractory Bricks

"Refractory" is just a word that means "very resistant to high temperatures." In the context of pottery and glassmaking, I need refractory material for the lining of my kiln(s) I am going to make. Regular, pure clay doesn't cut it. I can also use refractory for making bricks to hold up pieces in the kiln or to use for making the actual walls of a kiln, if I want to make one out of entirely my own materials.

For either of these purposes, I need a substance that is actually not only refractory, but also insulating. That is, it should neither break down at high temperatures nor should it let heat get past it. Tungsten metal is highly refractory but a poor insulator. Cotton is not refractory (will burst into flames at maybe 400 degrees F) but is a great insulator.

Pure earthenware clay by itself is mediocre at both of these things by itself--it will slump and melt at moderately high temperatures, and it isn't actually that great of an insulator (Think about how quickly your mug gets too hot to touch on the outside when you pour coffee in it). So we want to improve these qualities for furnace materials.

Refractory recipes can get very complicated, but for mine, I want only locally available ingredients. I am going to have 3 ingredients: dry grass, quartz sand, and some of the clay I prepared earlier here.

• The clay forms the main body of the material and holds everything together. It is also, of course, reasonably heat resistant.
• The sand raises the melting point of the material significantly, since quartz melts about 500 degrees celsius higher than earthenware clay does (note: sand is locally available, but this particular batch used hardware store sand just for now). 
• The oven-dried grass helps hold everything together at first, since the clay mixed with sand is crumblier than pure clay (see here [link]). Later, the grass will burn away in the kiln and leave lots of tiny air pockets that will help insulate against heat transfer.
• ...another ingredient I could have added is wood ash. WASHED wood ash. I.e. the stuff left over after you have soaked new ashes in water and drained off the potash a few times (this will be the subject of an upcoming blog post, since potash is used for glass making and pottery glazes). Washed ash stuff is much more heat resistant than quartz, so replace a little or a lot of the sand with it, if it's available. It must be washed ash, though: raw ash contains potash, which is a chemical that actually lowers the melting point of quartz, and thus is counterproductive.

The ratio I'm using is about 50/50 clay and sand by volume and then about another equal part as the sum of the first two ingredients in dry grass by volume, loose (not packed!). In reality, it's probably something like 70/27/3 by mass for clay/sand/grass, but I don't know. It's easier for me to measure volume, so I'm just using that. Mix it all together, and you get something like this:

This is essentially adobe in it's raw form. You can build some pretty strong buildings out of this in dry climates if you let it bake in the sun for awhile, although you'd probably want more sand and grass in it.

The adobe is much harder to work with than pure clay, but making things like bricks is still easy. Below are some examples of bricks that I made out of this material and then fired (I'll cover firing in a later post). Click for a larger size. As you can see, the grass has burned away and left them porous. These things are super lightweight and feel like pumice, and their insulation is probably excellent. Unlike the raw adobe, though, the fired bricks are also extremely brittle and easy to break, since the binding strength of the grass has been removed and now actually hurts the strength of the material by leaving air behind instead. 

The examples below actually used more like 60/65% sand, 35-40% clay by volume, and the fact that they are so brittle is why I'm now suggesting a 50/50 mixture instead (I also toned down the grass slightly from these bricks):

Notice the gray spots all over -- there was not enough oxygen and/or not enough time during firing. This is the same issue as that cheap hardware store pot I showed in an earlier post. It's not the end of the world, but it does increase brittleness even further, and I need to fix it. I'll discuss this more in my post(s) about firing clay.

You can also see a crack in the middle brick that formed just from roughly piling them in this stack! That's how brittle these are with the sand-heavy recipe... Again, use more clay than this (which I will be doing in the future).

HELPFUL LINKS

Probably the most popular recipe for homemade refractory online is this one:
A version using perlite for insulation and cement as well.
It does not use local materials though. Even less local and not homemade are commercial versions:
An example of hardware store refractory.

My own version is adapted from commercial versions and the backyard metal casting website, but substituting locally plausible materials (the sand and clay are literally locally gathered) to fill all the same roles in about the same ratios.