How Lye Becomes Soap

When caustic becomes clean.


The saponification process: fat and lye become soap and glycerin.

Brad Anderson

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“Let me get this straight, you want me to wear gloves and eye protection, and then I’ll use what we make on my skin?!?” The first time I made soap, I was a touch wary. The warnings about lye are serious – the words “chemical burn” are not ones I wish to be associated with. But I truly enjoyed watching these two somewhat disgusting substances (rendering tallow is not a fun experience) become something useful (see Easy Homemade Soap) – and, as usual, that led me to find out exactly how this was possible.

In order to understand how lye and grease make soap, you first need to understand how soap works. A soap molecule has a “head” that likes water and a “tail” that likes fat. Soap works by grabbing fat – the oils and soil you want to wash off – with one end and attaching to water with the other. Voila, dirt and soap and water all down the drain (see illustration below).

As for making this grime fighter, let’s start with the components: Chemically, you could think of the tallow (a nice name for beef fat) that you make into soap as three long legs (fatty acids) held together by a glycerol cap (this combination is a “triglyceride”). And lye is made up of sodium, hydrogen and oxygen (sodium hydroxide), which splits apart when dissolved in water. When the tallow is heated with dissolved lye, the cap separates from the legs. The sodium from the lye goes with the legs to form soap, while the hydrogen and oxygen together join with the cap to become glycerin. This process is called saponification (see illustration above). The sodium end of the soap molecule spends its time wishing for water, while the tail is just waiting for more fat to join the party.

After saponification, anything that isn’t soap is a byproduct. Using different oils or fats to make soap will produce different amounts and types of byproducts, which in turn affect the soap’s hardness, lather and moisturizing qualities. The most obvious extra is the glycerin.

Soap manufacturers work very hard to remove the extra glycerin – which is good for your skin, but also good for making other stuff, including lotion, lubricants and even nitroglycerine. It’s “hygroscopic,” which means it likes water so much it will draw moisture from the air. It doesn’t freeze solid and therefore is sometimes used to keep things like hydraulic fluid from freezing.

QUICK CLIP: If you leave pure glycerin open to the air, it would incorporate water from the air and end up being 20 percent water, 80 percent glycerin.

Most soap that is clear has had extra glycerin added. Soap made with the “melt and pour” method most often is this kind of soap. Because glycerin is especially soluble in water, this kind of soap dissolves more quickly. It even “sweats.” If you’ve ever seen little droplets of water appear on your see-through soap, it’s that hygroscopic thing I talked about earlier.

As for the whole “why it lathers” question – that’ll have to wait until I do some more research.  

Web Editor Jenn Nemec likes to keep clean with herbal-smelling soaps. Cinnamon, comfrey and lavender are her favorites.