Let’s talk about how can we make a lye solution with ashes and water.


“Historically, lye used in the cold process was made from scratch using rainwater and ashes. Soapmakers deemed the lye solution ready for use when an egg would float in it. Homemade lye making for this process was unpredictable and therefore eventually led to the discovery of the sodium hydroxide by English chemist Sir Humphry Davy in the early 1800s.”


So basically making an aqueous ‘lye’ solution with ashes is making a solution mainly composed of potassium hydroxyde (K+,HO-). The reason why it is K-OH instead of sodium hydroxyde (Na-OH) when we use wood ashes is simply because the potassium is more present in plants than sodium.

Remember the main component we need for making the saponification is OH-.

I tried to find a clear explanation of why the combustion of wood, leading to ashes, and then mixing them with water, would give us a K-OH solution. The explanations I found in different blogs and science books where quite confusing, because some of them give different chemical formulas and explanations.

HOWEVER I think I finally understood why people on the forum and blogs seem confused: basically there are 2 chemical reactions that can happen when making ashes and mixing them with water.

BUT: the recipes are mostly the same. It consists on mixing a certain amount of ashes with rainwater (or distilled water) for a while, and filter the solution.

  • Chemistry

Wood is mainly composed of 99.5% combustible material (mostly cellulose = main component of plant cell wall). The combustion of wood will basically ‘burn away’ most of the hydrocarbon molecules that compose it, and turn it into ashes. Most of the ash obtained (80%) is not soluble in water. The remaining water–soluble components of ash are all the non-flammable, non-volatile minerals which remain after the wood and charcoal have burned away – including our potassium-containing molecules that can be used to produce lye.

During combustion 2 different kind of products are generated, that potentially will lead to formation of lye (K+, HO-):

  1. During burning (which is an oxidation reaction) the heat converts the potassium of the plants into potassium oxides (K2O). Adding water to the ashes converts the oxides to potassium hydroxide (lye!)           K2O+H2O→2KOH      Apparently K2O formation is optimal when the wood is burned at high temperature = this oxidation reaction requires heat.
  2. During combustion of the wood, CO2 is produced. This CO2 can also react with the potassium of the plant to form potassium carbonate (K2CO3) which is also soluble in water. When potassium carbonate dissolves in water it splits into two potassium ions (K+) and one carbonate ion. The carbonate anion in solution can react in two different ways. First, it could reaction with the water to form bicarbonate and hydroxide (lye!):


Additionally, the carbonate can reaction with any hydrogen ions in solution. This decreases the amount of H in solution, which also raises the pH. Therefore the carbonate makes the water more basic (better for saponification):sapo8



  • Recipes

I let you have a look by your own at the recipes. There are different versions but basically the process is simple: mixing the ashes with water and let it stand for a while. Then we need to recover the solution by filtering it. It is important to use gloves during the process. I’ll ask Martin for its own recipe.

The reason why they advise to use rainwater (or spring water) instead of tap water is pretty simple: tap water contains already a lot of different ions, therefore the water is less likely to dissolve the potassium hydroxide and our solution will not be very concentrated in lye. Logically the best water we can use is therefore distilled water. But rain water is very poetic!