From Caustic to Clean: How Lye Becomes Gentle
Let’s have a little heart-to-heart about something that often causes raised eyebrows when folks read a soap label or ask what goes into a handmade bar. That something? Lye.
You might have heard things like:
“Wait, isn’t lye dangerous?”
or
“Isn’t that the stuff they use to clean drains?”
And the truth is, yes—lye on its own is caustic. But here’s the thing: you can’t make real soap without it, and by the time your soap is finished and cured, there’s no lye left. That’s the magic of a process called saponification.
So, What
Is
Lye?
Lye is a strong alkaline substance—either sodium hydroxide (NaOH) or potassium hydroxide (KOH)—that’s essential for turning oils and butters into soap. It may sound intimidating, but this is where science steps in to do something pretty incredible.
Through a chemical reaction called saponification, lye combines with fats or oils to create soap and glycerin. It’s just like baking—flour, yeast, and water become bread when heat and time work their magic. Similarly, lye and oil become something entirely new when mixed just right.
By the time the soap has fully gone through this process, the lye is completely used up. What you’re left with is pure, gentle, nourishing soap—and often, a little extra oil added in (called “superfatting”) to make it even more moisturizing and skin-friendly.
The Two Types of Lye (And Why It Matters)
If you’ve ever wondered how some soaps are solid bars while others are creamy or liquid, the type of lye used is key:
1. Sodium Hydroxide (NaOH)
— Used for bar soap
This is what we use to make good, solid soap bars. Whether it’s a rich tallow soap or a gentle Castile bar made with olive oil, NaOH gives you that classic, long-lasting firmness you want in a bar.
2. Potassium Hydroxide (KOH)
— Used for liquid soap
This one creates soft or liquid soap—the kind that’s perfect for face cleansers, body wash, or hand soap. It stays pourable and is great for pump bottles or squeeze tubes. This is what I use to make my liquid Castile soap!
You can’t substitute one for the other, and both require precise calculations to make sure every bit of lye is properly neutralized. That’s why experienced soapmakers like me always use a soap calculator to get the ratios just right. It’s part chemistry, part craft.
But Isn’t Lye Dangerous?
Sure—it can be, if you’re not careful. Raw lye should be respected, just like you’d respect a hot stovetop, a sharp knife, or your trusty cast iron skillet. With gloves, goggles, and a little know-how, it’s absolutely safe to work with.
And again, once the soap is done curing (or fully cooked, in the case of hot process), there is no lye left. It’s all been transformed through saponification into mild, skin-loving soap.
So if you’ve ever worried about soap being “made with lye,” rest easy. If it’s real soap, it had to be made with lye—but that lye isn’t hanging around afterward.
Why I Love This Process
There’s something deeply satisfying about soapmaking. Watching oils, butters, and lye come together to form something entirely new is honestly kind of magical. Whether I’m whipping up a silky goat’s milk bar or a detoxifying tea tree cleanser, I know exactly what’s going into it—and what’s not.
No detergents.
No fillers.
No weird preservatives—just natural, non-chemicalized preservatives if needed.
Just simple, wholesome ingredients and a whole lot of love.
Real soap is honest. It doesn’t hide behind foaming agents or artificial colors. It gets the job done and takes care of your skin at the same time.
So Next Time You Hear “Lye”…
Don’t let the word scare you. It’s not the villain of the story—it’s the reason soap exists. When handled properly and used in the right proportions, lye becomes part of something beautiful: gentle, handcrafted soap made the way it’s meant to be made.g
So go ahead—treat your skin to the real thing.
