Soap making chemistry question

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New Member
Apr 3, 2024
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Chicago, IL
Hi guys! I'm new here and I have a question regarding the chemistry of soap making.

According to common internet sources, you make soap by mixing lye water with oils. What this means is that you mix Sodium Hydroxide with fatty acids to form a salt soap.

Historically, wood ash was used, making Potassium Hydroxide. However, from what I understand, mixing wood ash with water primarily forms Potassium Carbonate, called Potash. At least that's the result when you filter out the initial precipitate from the water and then evaporate out the water, leaving a smaller amount of secondary precipitate.

From what I've read, historically, you had to add quicklime, Calcium Oxide, to the Potash and water solution to form Potassium hydroxide. Then, you combine that with the oils to form soap. However, a lot of instructions go directly from wood ash water and oils to soap, bypassing the addition of lime.

What's going on here? Am I missing something or does the wood ash also directly form Potassium Hydroxide?

Edit: My question was answered in a different forum. Apparently, the historical word lye refers to both oxides and carbonates. And both are alkaline, so the wood ash only method is valid.
Last edited:
Soap can be made with more than one type of alkali. It's just how much work do you want to do to get the job done.

Yes, you can make soap directly from the ash lye which is mostly a potassium carbonate lye.

And, yes, you can convert the ash lye with slaked lime (calcium hydroxide) to form a hydroxide lye and make soap from that.

It's harder to make soap with the first method -- longer heating and cooking time. Also the carbonate lye contains more impurities.

The ash and lime process was in use during the 1700s.

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