If you would like to understand potash, you must realize that Lucifer has oversimplified fire considerably. Reconsidering Equation 1-1, you will notice that all of the products of combustion are gases. Where, then, do the ashes come from? You will recall that these equations are for the combustion of cellulose, and that wood is only mostly cellulose. When wood is heated anaerobically, it turns black as the water is driven off, leaving charcoal, or carbon, behind. When charcoal burns in air, the carbon combines with oxygen, producing the gas, carbon dioxide. But if you have ever used a charcoal grill, you may have noticed that charcoal turns white as it burns. This white ash is what remains of the non-flammable minerals which were present in the wood to begin with. You don't really notice them until the carbon has burned away. These ashes have a composition which varies according to the kind of wood and the soil in which it grew, and it is this variable composition which marks ash as a mixture rather than a pure substance.

You will recall, no doubt, that a mixture can be separated into two or more pure substances by recrystallization, distillation, and chromatography. You will be pleased to learn that we are discussing only recrystallization in this chapter. You have, of course, noticed that some things, like salt and sugar, are soluble in water, while others, like sand and charcoal, are not. Recrystallization separates substances which differ in their solubility. Ash, for example, is mostly insoluble in water. Only a small portion of the ash dissolves in water, and this is the substance we call potash, or potassium carbonate. To make potash, you must add your ash to a quantity of water. Any leftover charcoal will float to the top, while the insoluble minerals will sink to the bottom. The good stuff, the potash, will be dissolved in the water. You must separate the water from the charcoal above and the minerals below. Once you have done this, you will have what looks like clear, clean water. But if you boil the water away, or let it evaporate in the Sun, a white, crystalline residue will remain. This residue is potash.

Table 8-1. Combustion Products of Beech Wood

Now, it is important, if you are to be successful, that your ashes have never been wet. If they have been wet before you started, then, of course, the potash will already have been washed out of them. So you must get your ashes from a fire that has been allowed to burn out, not from one which has been doused with water. But if your ashes were dry, and if you were careful to skim off the charcoal, and if you allowed the minerals to settle completely, and if you were able to collect the water without stirring up the sediment, and if, finally, you boiled away all the water, you will have nice, pure, white, crystalline potash, which is a lovely thing to behold.

This potash will look just like salt or sugar, so how will you know that it is not just salt or sugar? You will give it a taste. If your mother was as strict as mine, the taste will be reminiscent of a day when she caught you saying words you were not supposed to know yet. This is the bitter taste of alkali, or base. It would be irresponsible of me, of course, to suggest that you should go around tasting everything. Chemists have learned the hard way that tasting unknowns can get you into a world of hurt and so they have developed pH test paper to serve as a virtual tongue. Bitter things turn pH test paper blue and sour things turn it red. Salty and sweet things leave pH test paper a neutral yellow color. If you have never used pH test paper before, use a few strips to test materials whose flavors you already know. Good choices are lemon juice, vinegar, baking soda, and soap. From this experience you will be able to use pH test paper to distinguish bitter things from sour things, alkalis from acids, without risking your health.

Before we get too much farther, I should tell you that potash, or potassium carbonate, is not the only soluble component of wood ash. Depending on the soil conditions, sodium carbonate may also be present. As a matter of fact, when the ashes come from burning seaweed, there may be more sodium carbonate than potassium carbonate, and in this case we refer to the product as soda ash. Table 8-1[1] shows what happens to 1000 pounds of Beech wood when it is burned. Most of it is consumed in the fire, of course, producing gaseous water and carbon dioxide. Less than six pounds of ash remain. Most of this ash is not soluble. When the water is boiled from the soluble bit, a little over a pound of crude potash remains. As I have explained, most of this crude potash is potassium carbonate, but some of it will consist of sodium carbonate, potassium sulfate, and other soluble compounds. You may be wondering how you could remove these contaminants. I am happy you asked.

Table 8-2. Solubility of Alkali Sulfates and Carbonates

Adam, I must tell you, has considerably simplified the whole business of solubility. Solubility is not a black-and-white issue; some "soluble" compounds are more soluble than others. Table 8-2 shows that potassium carbonate has a much higher solubility than the other compounds we might expect to be present in wood ashes. If, instead of boiling away all the water, we were to boil away only most of the water, the less soluble compounds would precipitate, that is, they would sink to the bottom of the solution as solids, and the potassium carbonate would stay in solution until the last possible moment. If we were to pour off this solution and boil it to dryness, the resulting solid would have fewer contaminants than the crude potash.

Well, really, we have done the same thing to remove the sodium carbonate and potassium sulfate that we did to remove the insoluble ash. In both cases we are physically separating compounds that differ in their solubility. This process, known as recrystallization, remains the most widely-used technique for purifying solids.

Figure 8-1. Recrystallization as a Process

Figure 8-1 illustrates the recrystallization process in schematic form. The first reactor, the lixiviator, is a container in which part of a solid is allowed to dissolve in water. In the next section, we will use our familiar 2-liter soft-drink bottle to lixiviate wood ashes. The second reactor, the furnace, should be familiar from Figure 1-3. A beaker in an oven will serve well for this. Unlike previous processes, there is no chemical reaction here. It is simply a physical process for separating things that differ in solubility. The usual conventions are followed; reactants come in from the left, waste products exit to the top and bottom, and the main product exits to the right.

You will be quite interested to know that Nature does some recrystallizing of her own. When a sea becomes land-locked, soluble minerals wash into it from the rivers and streams that empty into it. Eventually, the Sun dries up the water, and the least-soluble component precipitates, forming a bed of, say, salt. If the climate is more arid, soda ash may begin to precipitate, and if the sea dries up completely, a layer of potash may form on the top, providing that there was any potash in the river water to begin with. So an ancient sea-bed consists of beds of material which differ in their solubility. This makes it quite convenient for mining, since it saves you the trouble of quite a lot of recrystallization.

Now I am quite aware that some readers will be nodding off at this point. If you think this is bad, you've obviously never had Satan poking his big fat nose into your ashes. But I am afraid that I must tell you a bit more about potash if you are to avoid confusion later on. Adam has told you that when inorganic compounds dissolve in water, they fall apart into cations and anions. What he did not tell you is that water itself does the same thing, as shown in Equation 8-1(a). A water molecule may fall apart into a hydrogen cation and a hydroxide anion. Only a tiny portion of the water falls apart in this way, but, it turns out, this tiny portion is extremely important. In pure water, of course, there are an equal number of hydrogen and hydroxide ions, since each water molecule gives one of each. For this reason, we say that pure water is neutral and assign it a pH of 7. Please notice that pH is spelled with a small p and a big H.

Figure 8-2. The pH Scale

Now, potassium carbonate falls apart into two potassium cations and a carbonate anion. The potassium ions float happily about the solution and take no further part in the chemistry for the moment. But if carbonate ion bumps into a water molecule, it may swipe a hydrogen ion from it, leaving a hydroxide ion behind. Alternatively, it may bump into a hydrogen ion, which may stick to it. In the first case, the number of hydroxide ions has increased. In the second, the number of hydrogen ions has decreased. In either case, there are now more hydroxide ions floating around than hydrogen ions and the solution is no longer neutral. We say that the solution is alkaline, or basic, and it gets a pH bigger than 7. The more basic a solution is, the bigger the number. A 10% potash solution, for example, has a pH of 10; a strongly alkaline solution can go as high as 14.

Equation 8-1. Reactions of Potassium Carbonate with Water

The anion HCO3- is called the bicarbonate[2] ion. You may know that baking soda is sodium bicarbonate, and now you are able to write its formula.

WarningMaterial Safety

Locate MSDS's for potassium carbonate (CAS 584-08-7) and sodium carbonate (CAS 497-19-8). Summarize the hazardous properties of these materials in your notebook, including the identity of the company which produced each MSDS and the potential health effects for eye contact, skin contact, inhalation, and ingestion. Also include the LD50 (oral, rat) for each of these materials.[3]

Your most likely exposure is eye or skin contact. If you get some potash in your eyes, you should flush them with cold water and go to the emergency room. If you get some on your skin, wash it off when it is convenient to do so.

You should wear safety glasses while working on this project. Leftover ash can be disposed of in the trash. Your potash can be saved for use in future projects or washed down the drain.

All of this was to tell you why I happened to be sitting in the ashes when we met. All living things are rather picky about their pH, most preferring something close to pH 7. If you should get some potash in your eyes, you will discover this for yourself in a rather painful way. But most of your body is well-protected by a layer of skin. Bacteria, on the other hand, have no such protection. So applying potash to a cut or boil sends the germs right around the twist. I can only wish that it had the same effect on the fellow who gave me the boils in the first place.

NoteResearch and Development

You are probably wondering what you need to know for the quiz. I will tell you.

  • You should know the meanings of all of the words important enough to be included in the index or glossary.

  • You should have studied the Research and Development items from Chapter 1 and Chapter 4.

  • Know several synonyms for potash and soda ash, along with their formulas.

  • Know the hazardous properties of the alkali carbonates.

  • Know how recrystallization can be used to separate compounds that differ in their solubility.

  • Know the equations for the ionization of water and the reactions of carbonate ion with water and hydrogen ion.

  • Know how to test for alkali and be familiar with the pH scale.

  • Know why potash is antiseptic.



Data from Reference [29], p. 123.


The modern name for the bicarbonate ion is the hydrogen carbonate ion. The older name, however, continues to be widely used.


The LD50 was introduced in Section 7.2.