Extracting Potash

After a sweltering hiatus due to warm weather on and after Thanksgiving, winter is finally back to the old school, so this means that we can continue our winter chemistry experiments over the wood stove. We like extracting chemicals from things around us, so we’ll start with potash, which is derived from wood ashes (1).

Potash is a mixture of chemicals, and is obtained by drying the liquor which results from leaching wood ashes in water (2). The raw potash we will extract in this article, shown to the right, contains a mixture of potassium and sodium carbonates and potassium and sodium chlorides, plus some other minor constituents. If the ashes came from a wood stove that cools during the day, as ours does, then the raw potash will be contaminated with a small amount of brown, soluble tars. We’ll show later how to roast these away. If the ashes are derived from an open fire or fireplace, then they will probably be relatively pure and the first step will produce a whiter product. The amount shown above, about three-fourths of a loosely-packed cup, came from the first gallon of liquor.

In this article, we’ll show how to extract the raw potash. In future articles, we’ll refine this potash into more pure fractions, as well as show some applications. These fractions, and their close derivatives soda lye and caustic potash, are useful for many things:

Soap and glass making.
Baking and leavening agents.
Wood pulping to obtain cellulose fibers and lignin compounds used in filtering, paper making and precursors to a range of organic chemicals.
Buffering agent in mead- or wine-making to reduce bitterness.
Neutralizing solutions for titrations, including the final steps in organic nitrations.
Cleaning and wetting agent in soldering and welding fluxes.
Animal feed supplement.
Precursors for nitrates and gunpowder.
Intermediate products for chlorate and perchlorates, including ammonium perchlorate rocket fuel (3).

For this extraction process, we’ll use plastic milk jugs (or better, distilled water jugs) for the bulk separation, and then taller, thinner containers, such as large peanut butter or pasta sauce jars, for the fine separation of the liquor from the ashes. We’ll also use a plastic funnel and coffee filters for the filtration itself.

Before starting, wash the milk jugs with soap and water, shaking vigorously. For a true caveman feel, use some ashes instead of soap. Not only are ashes and fats the precursors of soap anyway, the ashes also help scrub the little giblets off. Although the ashes will rinse out easily, it is OK if you don’t get the last of the ashes out, the filter will take care of that. Make sure one jug is very clean, this will collect our best liquor before drying.

Next, add cool ashes to one jug, filling it about a third of the way. Add water, put the cap on, and shake vigorously. Then, fill the jug up almost all the way, which will let us easily remove the charcoal later. Let the ashes settle to the bottom overnight. You will notice that the liquor will be fairly clear although most of the charcoal will probably have floated to the top. Take the jug outside, and carefully squeeze it to wash the charcoal out of the neck while only loosing a little of the liquor. Rinse the outside of the jug, and your hands, with a garden hose before bringing the jug back inside.
At this point, you may wish to test the pH of the liquor. This first liquor will be well above 10, possibly as high as 12. For years, we have used Hydrion pH paper, as shown to the right. A quarter-inch strip of this, held in some tweezers, will be enough for each test. We have a roll of this that we’ve been gnawing off a bit here and there for twenty years, and it still works fine. We have another roll around somewhere that has over three decades on it, so one roll will be great for a lot of experiments (4). We wouldn’t worry too much about the expiration date printed on it.

Next, prepare the funnel with a coffee filter by folding it in half, and then half again to make a triangular wedge. Put this wedge into the filter, then open up so that one layer is on one side, and three on the other, as shown below:

You can get more life out of the filter this way, especially for the rough filtrations; when done on one side, flip the middle two layers to the other side. Now, carefully pour the clean(ish) liquor off the top into the filter atop one of your other clean jugs, saving the cleanest for later. If you are careful and take your time, you can get most of the liquor out before the ash flows too much.

When the ash begins to reach the filter, pour the rest of the dirty liquor into one of your tall but thin settling jars, leaving the bulk ashes behind. Each day, add more water to the ashes in the settling jug, shake vigorously, and repeat the process. Also, each day run most of the liquor in the settling jar through the filter, returning its sediment back to the settling jug after a good swirling to clean it out somewhat. The settling jar lets you get more liquor out each day if you want to make the most of your ashes.

Continue extracting a batch of liquor, one batch per day, until the pH drops below 9. At this point, you will have extacted most of the potassium and sodium carbonates and a lot of the chlorides. Keep extracting if you want more of the chlorides (5).

Now, your jugs of filtered liquor will begin to show sediment also, the fines that made it through the filter. Filter these settled jugs again into your clean jug, and swirl the remnants out into a separate, clean settling jar. Use liquor from the final, clean jug in a saucepan (6) to dry into the raw potash product:

The picture above shows the result for the second gallon of liquor, dried to completeness for illustrative purposes. While there is still some discoloration, the product in this batch is much whiter. If you heat with wood heat, and humidify your room with a pan of water anyway, this is a great way to do something interesting with that pan of water (7).

When you are done, you will have extracted about two cups of loosely packed material from about a half gallon of ashes. There is no need to keep the portions separate, or for that matter to get them entirely dry. We’re going to process them further in later articles anyway.

Many of the projects we’re going to do on this site will generate a large amount of wood ashes. We often use these ashes directly in the garden (8). In addition, by using the process shown in this article, we can extract useful materials from these ashes to do interesting things.


(1) For those of you who took our advice to get a copy of the Caveman Chemistry book, read through the potash chapter in that book. Also, refer to the old Caveman Chemistry potash lecture on Dr. Dunn’s site for more information.

(2) What remains after the potash has been leached is mostly calcium carbonate with some silicon compounds (including from dirt, clay or sand that was on the wood or at the bottom of the fire) along with some unburnt char. This is an interesting fraction as well, we will show in a later article how to process this fraction into useful calcium compounds.

(3) Chlorates and perchlorates must be kept separate from sulfur and phosphorus, and any of their compounds, lest spontaneous explosions occur. Unfortunately, wood ashes have enough of these to be dangerous to use as a chlorate or perchlorate precursor. These can be removed by additional steps and materials, but the risk of mistakes isn’t worth it for our purposes here. When we get around to making rocket fuel (similar to that used in the space shuttle boosters), we’ll start with plain table salt.

(4) We’re also evaluating some of those inexpensive electronic pH meters. These need periodic calibration with a buffer solution, and we’re not sure how long they will last, either. We’ll report on these in a future article.

(5) If you don’t have pH paper, you can still tell when the extraction is nearing completion by looking at how light passes through the liquor. At first, it will look “thick”, bending light in a strange way. Later, it will look like ordinary water.

(6) Do not use an aluminum or a non-stick pan, both of these will be ruined. We’re using a stainless steel pan we bought at Goodwill for about a dollar. This pan, and its porcelain counterpart used for the first stage of drying a liquor, is a veteran of numerous chemistry experiments.

(7) For those not using wood heat, an alternative is to leave the pans outside on a sunny day to evaporate the bulk of the water. At some point, the water will no longer evaporate. Save this concentrate (with a large amount of crystallized salts) for later steps.

(8) Or the residue from this process in a controlled way depending on the amount of alkalinity desired.


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