Is Lye in Soap?
Share
Is Lye in Soap?
You may have heard that soap is made with lye. But does that mean there is lye in the bar of soap you’re washing with?
The answer is no—not in properly made finished soap as free, active lye.
Lye is used to make soap, but during soapmaking it reacts with oils in a process called saponification. That reaction is what creates soap.
Lye is necessary to make soap
True soap begins with three basic things: fat or oil, water, and an alkali.
For bar soap, that alkali is usually sodium hydroxide, commonly called lye.
Without an alkali, oils remain oils. They may be useful on the skin, but they aren't soap. The reaction between the oils and the alkali is what transforms them into something entirely different.
So where does the lye go?
When lye and oils are combined in the correct proportions, they undergo a chemical reaction called saponification.
During saponification, the fats and the alkali are transformed into soap and glycerin.
Think of it a little like baking a cake. Eggs and flour go into the batter, but after baking, you don't have raw eggs and flour sitting inside the cake. They've taken part in a transformation and become something new.
The chemistry of soapmaking is different, of course, but the idea is similar: the ingredients used to make something aren't necessarily present in their original form in the finished product.
Then why do some soap labels list sodium hydroxide?
This is where things can get confusing.
You may see sodium hydroxide listed among the ingredients of a traditional soap. That doesn't necessarily mean there is active lye remaining in the finished bar.
Soap ingredients can be described in different ways. A maker may list the ingredients that went into the soapmaking process, including the oils and sodium hydroxide. Another label may describe the ingredients after saponification.
That's why you may see names such as sodium olivate, sodium palmate, or sodium cocoate on a bar of soap. These refer to the saponified forms of olive oil, palm oil, and coconut oil.
For liquid or soft soaps made with potassium hydroxide, you may see similar names beginning with potassium, such as potassium olivate or potassium cocoate.
Other labels simply list the original ingredients used to make the soap — for example, olive oil, water, and sodium hydroxide (or lye).
So a label that says sodium olivate and one that lists olive oil and sodium hydroxide may be describing the soap at different stages of the same process.
What did soapmakers use before modern lye?
People were making soap long before sodium hydroxide could be purchased as a standardized ingredient.
Historically, soapmakers obtained alkaline substances from materials such as wood ash and the ashes of certain plants. Those alkaline solutions could then be combined with fats or oils to make soap.
Modern soapmaking allows much more precise control over the type and amount of alkali used, but the basic principle remains the same:
Oil + alkali → soap.
Can soap have too much lye?
Yes. If a soap is formulated or made incorrectly, excess unreacted alkali can remain, making the soap harsh or irritating.
That's why the proportions and the soapmaking process matter.
But in a properly formulated and fully saponified soap, the lye has done its job. It has reacted with the oils to make soap.
So, is lye in soap?
Lye is used to make soap, but properly made finished soap should not contain free, active lye.
In fact, this is one of the most interesting things about traditional soapmaking:
Lye doesn't just go into soap. It helps turn oils into soap.
Soap School Fact
You can't make true soap without an alkali.
Whether that alkali came historically from plant or wood ashes or comes from carefully measured sodium hydroxide today, an alkaline substance is what makes saponification—and therefore soap—possible.
Continue Learning
Previous Lesson: What Is Lye?
Next Lesson: What Do the Ingredients on a Soap Label Mean?