Here’s a question that comes up quite often when people discover how traditional soap is made.
“Does your soap contain lye?”
The short answer is that I use lye to make my soap, but a properly formulated and fully saponified bar doesn’t contain any free, caustic lye.
And before anyone starts worrying about what they’ve been washing themselves with, let me explain. Because this is one of those occasions when a little understanding of the chemistry makes everything much less alarming.
First of all, what exactly is lye?
Lye is the name commonly given to a strongly alkaline substance or solution used in soapmaking.
For traditional solid soap, the alkali is sodium hydroxide (NaOH), also known as caustic soda. It’s usually supplied as white pellets or flakes, which are dissolved in water before being combined with oils and butters. Sodium hydroxide isn’t some mysterious soapmaking ingredient. It’s widely used in industry, including paper manufacturing, water treatment, food processing and the production of cleaning products.
It also happens to be essential to making traditional solid soap.
For liquid soap, soapmakers generally use a different alkali called potassium hydroxide, which produces softer, more soluble soaps. But here at The Soap Mine, it’s sodium hydroxide that I use to transform my lovely oils and butters into the solid bars you see on the shelves.
Isn’t lye dangerous?
Yes, absolutely. And I’m not going to pretend otherwise. Sodium hydroxide is a highly caustic substance, which means it can cause serious chemical burns if it comes into contact with skin or eyes.
When it’s dissolved in water, the process generates a significant amount of heat, and the resulting solution needs to be handled carefully.
This is why soapmaking isn’t simply a case of throwing a few ingredients into a mixing bowl and hoping for the best.
I work with appropriate protective equipment, including gloves and eye protection, and follow careful procedures for measuring, mixing and handling my ingredients. The sodium hydroxide is always added to the water, never the other way around (that’s super dangerous) and suitable containers (never glass!) ventilation and safe working practices are essential.
It’s a substance that deserves respect. Not fear, but certainly not complacency either. I’ve been making soap for years, and I still take these precautions every single time.
So why use something so caustic to make soap?
Because without a strong alkali, you simply can’t make traditional soap. I could mix olive oil, coconut oil, shea butter and cocoa butter together until the cows came home, but I’d still just have a greasy mixture of oils and butters. To turn those ingredients into soap, they need to undergo a chemical reaction with sodium hydroxide.
And that reaction has a rather wonderful name: Saponification.
What happens during saponification?
This is where the chemistry gets interesting.
Oils and butters are made up largely of substances called triglycerides. These are molecules made from fatty acids joined together by a substance called glycerol (also known as glycerine).
When I combine my oils and butters with a carefully calculated solution of sodium hydroxide, a chemical reaction begins. The triglycerides are broken down, and the fatty acids react to form something completely new: the sodium salts of fatty acids. Which is a very scientific way of saying soap. Glycerine is also produced naturally during the reaction.
In simple terms:
Oils and butters + sodium hydroxide = soap + glycerine
The starting ingredients are chemically transformed into new substances with different properties.
It’s rather like the difference between flour, eggs and a finished cake. The ingredients you start with aren’t necessarily present in the same form once the process is complete. (Although I wouldn’t recommend tasting the soap to test the analogy).
Does that mean there’s no lye left in the finished bar?
Yes, that’s exactly what should happen in a correctly formulated and properly made bar of cold process soap. The sodium hydroxide reacts with the fatty components of the oils and butters during saponification. By the time that reaction is complete, there should be no free, caustic sodium hydroxide remaining.
And here’s an important point: the lye doesn’t evaporate or disappear during curing.
That’s a misconception I see surprisingly often. Sodium hydroxide doesn’t simply evaporate away while the soap sits on a shelf. It is consumed during the chemical reaction that produces soap. In fact, saponification is largely complete within the first day or two of making a normal cold process batch.
The four-to-six-week curing period that follows is mainly about allowing excess water to evaporate and the internal structure of the soap to develop, producing a harder, longer-lasting bar.
I’ve written a separate blog post about why handmade soap needs to cure if you’d like to know more about that part of the process.
Of course, if soap has been incorrectly formulated or badly mixed, it can contain excess lye. That’s why accurate calculations, good manufacturing procedures and proper quality control matter so much. It’s also why I don’t recommend treating soapmaking as an exercise in guesswork.
What about all those strange ingredient names on soap labels?
If you’ve looked at the ingredients on one of my bars of soap, you might have spotted names such as:
- Sodium Olivate
- Sodium Cocoate
- Sodium Shea Butterate
- Sodium Cocoa Butterate
- Sodium Castorate
They could sound a bit intimidating, but they’re simply the names of the soaps produced when different oils and butters react with sodium hydroxide. For example, Sodium Olivate is the soap produced from olive oil, while Sodium Cocoate is produced from coconut oil.
Remember that chemical reaction we talked about earlier – saponification? This is the result.
The oils and butters have reacted with sodium hydroxide to form new substances, and those new substances have their own names.
Why don’t I just use the familiar ingredient names?
Because cosmetic labelling in the UK is regulated, and I’m required to provide an ingredient list using recognised cosmetic ingredient names.
These standardised names are commonly called INCI names, which stands for International Nomenclature of Cosmetic Ingredients.
So when you see Sodium Olivate or Sodium Cocoate on my soap labels, I’m not choosing complicated names to make my products sound more scientific, and I’m certainly not trying to bamboozle anybody! I’m just using the recognised names for the saponified ingredients, as required by the labelling specifications for my products.
In fact, standardised ingredient names are intended to make things less confusing, not more. They allow ingredients to be identified consistently across different cosmetic products, regardless of the manufacturer’s preferred descriptions.
They’re particularly important for people who need to check ingredients because of allergies or sensitivities.
So although the ingredient list might look like a collection of rather complicated chemical names, there’s nothing mysterious going on. It’s just the chemistry of soapmaking, written in a language that probably makes much more sense to a cosmetic chemist than to the average person standing in their bathroom. 😂
Why do I think it’s important to explain all this?
Because I think customers deserve to know what’s in the products they’re using, and how those products are made.
I’ve occasionally seen soap advertised as “lye-free”, usually with the implication that this somehow makes it safer, purer or more natural than soap made using sodium hydroxide. But if it’s genuine traditional soap, an alkali was involved somewhere in its manufacture, even if the person selling the finished product didn’t handle it themselves.
Melt-and-pour soap bases, for example, have generally already undergone saponification before the soapmaker buys them. There’s nothing wrong with using a premade base, but it’s misleading to suggest that traditional soap can be made simply by leaving the lye out.
And as I’ve said many times before, I’m not a fan of frightening people with scientific ingredient names or pretending that something becomes inherently safer just because it’s described as natural. EVERYTHING is made of chemicals. What matters is which chemicals we’re talking about, how they’re used and what happens to them during manufacture.
So yes, I use sodium hydroxide to make every bar of traditional cold process soap at The Soap Mine.
I handle it carefully, calculate my recipes accurately and allow every batch to undergo the processes needed to become the finished product. The result isn’t a dangerous mixture of caustic soda and oils. It’s soap. And I still think it’s rather amazing that such ordinary ingredients can undergo a chemical reaction and become something so completely different.
Even after all these years, the chemistry hasn’t lost its magic.
Vicki x
