When making soap, sodium hydroxide (lye) chemically reacts with oils or fats and is transformed into soap through a process called saponification.
The important thing is that properly made soap does not contain the original sodium hydroxide as free lye once the reaction is complete.
What happens chemically?
When you combine lye (sodium hydroxide, NaOH) with oils or fats, three things happen:
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Lye breaks apart the fats. Oils contain triglycerides, which are molecules made of glycerol and three fatty acids.
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The fatty acids react with sodium hydroxide. They form sodium salts of fatty acids, which are what we call soap.
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Glycerin is released. Glycerin (glycerol) is a natural byproduct that remains in traditionally made soap.
The simplified chemical reaction
In other words:
|
Ingredient |
What happens |
|---|---|
|
Sodium hydroxide (lye) |
Gets consumed in the reaction |
|
Oils and fats |
Are converted into soap |
|
Glycerin |
Is naturally produced |
|
Water |
Helps dissolve the lye and facilitate the reaction; some evaporates during curing |
Is there lye left in finished soap?
Ideally, no free sodium hydroxide remains in properly formulated and fully reacted soap.
However, if:
-
Too much lye is used; excess caustic lye can remain.
-
Too little lye is used, some oils remain unsaponified, creating what's called superfatting.
-
The soap is properly formulated; the lye is consumed in the chemical reaction.
Curing helps the soap harden and excess water evaporate, although curing alone does not correct a lye-heavy recipe, that's why I always double check my recipes and test my soap before selling.
Does this mean soap is safe even though lye is dangerous?
Yes! This is a great example of chemistry transforming substances into something with completely different properties. It's literally a daily science experiment in my home daily and is so fascinating to preform!
-
Sodium hydroxide by itself is highly caustic and can burn skin. This is why if you ever see me "suit up" with a long-sleeved shirt, goofy looking goggles and gloves, it is to protect myself from getting burned by the lye. After 24-48 hours, I no longer need to wear gloves when unmolding the soap because the chemical reaction has taken place and no lye remains in the bars.
-
Oils by themselves don't clean particularly well.
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Once chemically reacted, they form soap, which can safely be used for washing when properly made and tested.
The fascinating part: Lye is essential to making traditional bar soap, but it doesn't remain as lye in properly made finished soap. It's transformed into an entirely different substance.
