Lye from Wood Ash, and why Everyone on the Internet is Wrong About It

This has been a long time coming. It’s the perfect example of one fallacy of the link economy.

This has probably been a long time coming. Forgive the lack of pictures because I’m pressed for time tonight.

So, today, I was making some “lye water” from a bucket of ashes that has been sitting around. The process is pretty simple. I drilled some holes in the bottom of a bucket, filled it with ash, and poured water over it. The liquid drips from the bottom very slowly, and a lot of water stays in the bucket, so more water is put in than what you get out. I put in around 10 gallons of rain water to make about 4-1/2 gallons of the liquor.

I may not know my but from a rabbit hole about making soap, but I do know a little something about chemistry, at least enough to call out all these stupid bloggers that keep repeating bad information like a bunch of parrots. I’ve found faulty articles on Mother Earth News and several other high-profile blogs. Most of them claim that the stuff in that bucket is Potassium Hydroxide (KOH), but it isn’t! And anyone familiar with chemistry who bothers to look into the subject can tell you the same.

The only thing I can figure out, is that soap makers use one of two store-bought ingredients, depending on the kind of soap they want to make. One is KOH, for soft and lathery soaps. The other is NaOH (Sodium Hydroxide), for making regular lye soap. They know that the alkali used historically to make soap at home was potash, so somewhere along the line, the connection was made between KOH and ashes. One problem, there’s little to no KOH in potash made from leeching ashes. The liquor in that bucket contains soluble potash, NOT the hydroxide, and they are not the same thing. (I found a medical blog that botched this as well when discussing skin irritation from ashes, claiming that “caustic potash” caused the dermatitis in the patient)

Why is all this mixing of chemical terms important? I dunno, it just irks me that such misinformation can continue to go unchallenged by everyone on the internet playing the telephone game, and we pretend that the link economy filters out bad information. It also glorifies the point that bloggers just copy and rewrite information from other sources without validating it first.

Anyway, why is there no KOH? And what the hell is potash? KOH is known as “caustic potash” for a reason, it’s very basic (meaning high pH) and it can cause irritation and skin burns, much like the NaOH mentioned above. But KOH doesn’t readily occur in nature, and especially not in fire ashes.

NaOH and KOH are typically manufactured, and while they CAN be made from the stuff found in your fire pit, it isn’t as simple as “just add water.” Potash on the other hand is Potassium Carbonate, a very different thing. It’s still basic, and it will make a soap, but it’s different. It’s slightly safer, especially in liquid form, and it isn’t nearly as reactive in its dried form. On the pH scale, the hydroxide is about 12.7 (not 13 as claimed by some bloggers) while the pH of potash is around 10-11, depending on a bunch of factors. May seem like a tiny difference, but the scale only goes to 14, and that’s a huge jump.

If you want to know how to make KOH, I’ll tell you in a moment, but for now, let’s talk about ashes.

Depending on the species and the bark content of your firewood, you will get a couple pounds of ash for every ten pounds of seasoned wood that is burned (fresh wood contains a LOT of water which makes it much heavier, but you don’t get ashes from water). That ash is the leftover stuff that won’t readily burn. Much of the content is silica and calcium carbonate (the silica content is argued depending on the study you read, but don’t get me started on that one). About 90% of ash is made from that alone: sand and chalk. The other 10% (again, rough and depends on everything for the exact amount) is soluble salts (meaning stuff that dissolves readily in water).

Now when it comes to chemistry, there’s more than meets the eye. This small fraction of stuff is what gets leeched into the water when you make “lye from ashes.” Each salt has two parts, a cation and an anion. Don’t worry too much about those terms, that’s just what they are called and that’s a long conversation about chemistry. The cations are mostly light metals. Calcium, Magnesium, Potassium, Sodium, etc. There’s some iron in there too that contributes to the color, and other trace elements, but those are the four big ones. The anions are the other part of the salt that they stick to, mainly Carbonate and Chloride. This makes an array of salts, linking one cation to one anion. So in that water in the picture, there’s Sodium Choloride (table salt), Calcium Chloride (rock salt), Potassium Chloride, and perhaps some Magnesium Chloride. And likewise with the carbonated. Again, there’s other stuff too, but mostly in trace amounts.

The biggest water-soluble components of the ash are Sodium and Potassium Carbonates. Hardwoods (in theory) have more Potassium Carbonate, and softwoods more Sodium Carbonate. This is the bulk of the potash leeched from the ashes. Chemists refer to Potassium Carbonate as potash, so there’s a bit of confusion there, perhaps as well. The stuff from your fire pit isn’t going to be pure without a lot of refining. (Basically boil down until sediment shows up, then boil of half of the remaining solution, filter hot, boil down some more, let cool, filter again, boil all the way down to crystals, resoak and repeat… how do I know that?)

So that’s pretty well it, there’s a bunch of stuff in there, but the solution is very basic, much like the caustic soda and caustic potash, and you can concentrate it (by boiling it down a bit) and use it to make soap and hominy, and it works great. You should probably still use rubber gloves, especially if you aren’t bright enough to wash your hands and arms thoroughly and immediately if some of the liquid splashes on you. Actually, for hominy, some get away with simply mixing the ash with their corn and adding enough water to make a slurry, then just wash off the hominy after the process is over. Much easier than fussing with the leeching process 😛

Okay, so, if you were wondering, that’s why your homemade soap is a little different than if you had used hydroxide from the store. There isn’t hydroxide in the water. You can zip down and leave a comment now if you don’t care about making KOH and NaOH, because this is about to get a bit sciency.

Making Caustic Potash

How do I know that there is little to zero of the caustic stuff in your ash? Well, I’m glad you asked. Hydroxides don’t readily form on their own. Especially of the Sodium and Potassium species (again metal cation, hydroxide [OH] anion). Those two metals don’t simply bake their way to an oxide, like calcium does, which would enable them to form a hydroxide with the addition of water. They are manufactured from peroxides, nitrates, and other anion families. So, to make them from ashes, you must use a trick, and it lies in that insoluble part that we discussed earlier, specifically, the chalk.

Chalk, or Calcium Carbonate, undergoes this wonderful metamorphosis at high temperature. (Over 600-800C) The carbonate gives off carbon dioxide, and the molecule transforms into an oxide, Calcium Oxide (or CaO for the chemical shortcut). This is known by a bunch of different names, but sometimes simply referred to as “caustic.” It’s street name is burnt lime. When it is added to water, it transforms again, this time into “slaked lime,” which is a safer and more stable form. Slaked lime is a hydroxide (Ca(OH)2, the 2 should be a subscript). And this magical, wonderful chemical has the power to transmute other chemicals, such as Sodium and Potassium Carbonates, into hydroxides as well.

So if you take your leftover chalk after leeching, burn the bejesus out of it in a kiln at ~1000C for a few hours, then transfer rather quickly to mix it with water (be careful as there will be an explosion of steam and bubbles and scalding hot fluid…that’s also caustic), then you can make slaked lime. I may be exaggerating the explosion part, but the powdered form is going to be super reactive with the water, and give off lots of heat…in addition to already being hot from the oven.

The slaked lime will drop into a white putty at the bottom of the water, eventually, and can then be mixed with your potash solution to convert the carbonates to their corresponding hydroxides, and whala! Hydroxide. But what happened to all of those carbonate anions? Well, they got back with calcium and dropped to the bottom of the solution, again turning into (mostly) insoluble chalk, which can of course be cooked again if you like!

“What if you get ashes from a really hot fire?” you ask. Well, here’s the thing. We prefer working with ashes that aren’t scalding hot. Also, while the fire is scalding hot, you’ll get mostly coal and very little ash. Once the temp drops below 500C, due to the tiny particle size, the Calcium Oxide you have created immediately begins soaking up carbon dioxide again. Note that earlier, I said “little to none.” Yes, some slaked lime will probably work it’s way into your liquor leeched from ashes, but it’ll be in trace amounts. The major components of your solution will still be carbonates, which we call potash.

There is a little silver lining here though. You can buy calcium hydroxide from any feed store as a fertilizer, so you can always hydroxide up your potash solution with that, if you like. It’s already slaked (usually, you want the hydroxide, not the oxide which is better for cement mix), and comes as a powder. They call it “hydrated lime” rather than “slaked lime.” It’s also known as “pickling lime,” just FYI. 😉

Anyway, that’s my little rant. I have no hope that this will set the record straight, as I doubt any of those “soap experts” give a crap if they have all of their chemicals mismatched, but that’s what’s going on with potash. It is NOT hydroxide, and many, many terrible mistakes have been made and repeated across the internet as a result.

BTW, if you DO manage to get a pH of 13 in your “lye water,” I want to know the story. I measured mine tonight at somewhere around 11. I’m not sure the pH of all those different things that are in there, but I just use a litmus paper to make sure it’s basic. If it’s purple, we good. But if you have a pH meter and you do this, I want to know what you get, seriously. Let’s stop asking the internet what it should be and post what you actually get. Could be fun. Or someone donate a pH gadget to me and I’ll measure the darn thing. P.S. I’m keeping the gadget if you do that. 😛

And also, stop believing everything you read. Just because “everyone on the internet says the same thing,” or “this newspaper said so” doesn’t mean diddly. Experiment, read, learn, test, question, repeat. Remember, they’re all a bunch of copy-cats playing telephone. If one of them gets it wrong, then they’re all screwed up.

One thought on

  1. Thanks 🙏🏼
    You just saved me from using KOH to nixtamalize some corn into masa. I know it would work but the flavor would be off.

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