The Complete Guide to Saponification: How Soap Making Actually Works

What Is Saponification?

At its core, saponification is the chemical reaction between fats or oils and a strong alkali (usually sodium hydroxide for bar soap). Oils are mainly made of triglycerides – a glycerol "backbone" with three fatty acids attached. When you introduce lye and water, bonds are broken and remade, forming soap (salts of fatty acids) and glycerin.

A number of scientific papers describe this process in more detail. For example, a review in the journal Cosmetics on soap properties and formulation explains how triglycerides combine with sodium hydroxide to form fatty acid salts and glycerin, and how this affects hardness, lather and moisturising properties of soap bars.

Another review on the saponification process and soap chemistry from ScienceDirect highlights how the fatty acid profile of your oils (for example, coconut vs olive) influences cleansing strength, solubility and mildness of the final bar.

Even teaching resources from organisations like the Royal Society of Chemistry on soap making describe saponification as heating fats or oils in lye to produce soap and glycerol, reinforcing that same basic picture.

In practice, I'm not standing in the workshop reciting equations – but I am very aware that every batch is a live chemical reaction that needs to be measured, respected and allowed to finish properly.

The Three Key Ingredients: Oils, Lye and Water

Every traditional cold process bar begins with three essentials:

1. Oils or butters

These might include olive, coconut, shea, cocoa, sunflower and more. Each oil has its own fatty acid profile, affecting hardness, lather, conditioning feel and cleanse. If you're curious, you can see how I talk about oil choice in our piece on moisturising bar soap and the top ingredients to look for.

2. Sodium hydroxide (lye)

This is the alkali that drives saponification. It's highly caustic in its pure form, which is why safety is so important, but once fully reacted there's no free lye left in a properly formulated bar.

3. Water

Water dissolves the lye and helps the reaction take place evenly throughout the mixture. Over time, much of that water evaporates during curing, leaving a hard, long-lasting bar.

When I sit down to design a recipe, I'm balancing all three: the oils for performance, the lye for full saponification with a little superfat (we'll come to that), and the water content to support a good pour and proper cure.

How Saponification Unfolds in Cold Process Soap

Here's how the process usually looks in a small artisan studio like mine:

1. Measuring and melting oils

I weigh each oil carefully, melt any solid butters, then blend them into a uniform mixture. The ratios here determine a lot about the bar's character – for example, in our coconut oil soap article I talk about why I keep coconut within a certain range to balance bubbly lather with gentleness.

2. Preparing the lye solution

In a separate container, I add sodium hydroxide to water (never the other way round) and allow it to dissolve and cool. Safety guidance from sources like the UK government’s sodium hydroxide incident management page and the US CDC medical management guidelines emphasise how corrosive lye is in its raw state and why goggles, gloves and ventilation are non-negotiable.

3. Bringing oils and lye together

When both mixtures are at suitable temperatures, I slowly pour the lye solution into the oils and blend. This is when saponification begins in earnest – triglycerides start to break apart and reform as soap and glycerin.

4. Trace and pour

As the reaction progresses, the mixture thickens to "trace". At this point, I can add clays, botanicals and essential oils, then pour into moulds.

5. Gel phase and early saponification

Over the next 24–48 hours, the soap hardens. Some batches go through "gel phase", where the soap briefly heats and becomes more translucent in the centre, then cools again.

6. Curing

Even after the bar is solid enough to unmould and cut, the saponification and drying process continues for 4–6 weeks. Our article on the art of cold process soap making explains why this slow cure is essential for a gentle, long-lasting bar.

During this time, the water content gradually falls, the bar hardens, and the chemistry settles into a stable, mild soap.

What Research Says About Cold Saponification

You don't have to take an artisan's word for it – there's growing research on how different soap-making methods affect the final bar.

A study in Cosmetics looked at cold saponification and found that this method can help preserve more of the unsaponified fatty matter and some plant antioxidants compared with more intensive processing. This helps explain why properly cured cold process soaps often feel more conditioning and "creamy" on the skin.

Other scientific reviews on saponification process and soap chemistry emphasise how the fatty acid profile influences hardness, foam and moisturising ability, reinforcing what soap makers have observed for generations in practice.

I find it reassuring that the traditional methods my hands have learned over the years are now being examined and validated in the lab.

Glycerin: The Quiet Gift of Saponification

One of the loveliest side effects of saponification is natural glycerin. When triglycerides break apart, glycerin is released and remains in the bar if you don't remove it.

Dermatology research shows glycerin to be a powerful humectant that supports hydration and barrier comfort, which is why it appears in so many medical moisturisers. We explore this more thoroughly in our dedicated guide to glycerin in handmade soap and why it matters – but the short version is that saponification doesn't just create soap; it also quietly builds in a skin-friendly hydrator at the same time.

Why Accurate Lye Calculation Matters

Because sodium hydroxide is so strong, it's crucial to use only the amount needed to fully saponify the oils plus a small safety margin of extra oil – the superfat. Too much lye and you risk a harsh, irritating bar. Too much unreacted oil and the bar can be soft, sticky or prone to rancidity.

Soap makers rely on lye calculators that use published "saponification values" for each oil (how much lye is needed to react with a given weight of that oil). Educational pieces like Chagrin Valley's explanation of the chemistry of soap making and formulation notes in academic reviews of saponification highlight how these values underpin safe, repeatable recipes.

As a maker, I treat this step with respect. I double-check my measurements, keep detailed batch records and never skip safety gear – the goal is always a fully saponified, gentle bar with no free lye left behind.

Saponification and Skin pH: What's Really Going On?

One common worry I hear is: "If soap is alkaline, will it damage my skin?" It's an important question.

Healthy skin typically sits in a slightly acidic range below pH 5.5, sometimes called the "acid mantle". Research on skin pH and cleansers shows that traditional soaps are usually more alkaline than synthetic detergent bars, and they can temporarily raise skin pH after washing.

More recent work comparing natural soaps made by cold and hot processes suggests that while all bars are alkaline, the way they're formulated and cured can influence how they affect skin surface pH in the short term.

My approach as a maker is to keep formulations simple and gentle, avoiding unnecessary irritants, ensure bars are fully cured which allows excess water and alkali to dissipate, and encourage people with very dry or sensitive skin to follow up with emollients, as recommended in guidance such as the NHS overview of emollients.

In other words: yes, soap is alkaline – but well-formulated, fully cured bars used with good moisturising habits can still sit comfortably inside a healthy routine for many people.

How Saponification Shapes the Feel of Your Soap

Once you understand the basics of saponification, a lot of soap "mysteries" start to make sense:

• Hard vs soft bars – Oils rich in saturated fats (like coconut or butters) give harder bars, while softer oils (like olive or sunflower) produce a gentler, creamier feel.

• Bubbly vs creamy lather – Lauric and myristic acids (often from coconut oil) boost big bubbles; oleic and linoleic-rich oils tend to create a creamier, denser foam.

• Cleansing vs conditioning – High-cleansing formulas can feel too stripping for dry skin; balanced recipes with thoughtful superfat feel more comforting.

If you're curious about how this plays out in finished products, our posts on building a simple daily skincare routine with natural products and creating your own spa experience at home show how different bars and ingredients fit into real-life rituals.

Bringing It Back to Your Bathroom Shelf

You don't need to remember the reaction equations to enjoy your evening shower. But a basic grasp of saponification can help you read ingredient lists with more confidence, appreciate why true soap needs time to cure, and understand why handmade bars feel so different from "mystery" supermarket blocks.

If you'd like to keep exploring, the blog home is full of posts on natural ingredients, eco-friendly routines and gentle skincare. And if you're keen to experience the result of careful saponification in your own bathroom, you can sign up to our waiting list on the Shop Now page so you're the first to hear when fresh small batches are ready.

Frequently Asked Questions

1. Is there any lye left in finished soap?

In a correctly formulated and cured bar, there should be no free lye left. The sodium hydroxide is fully consumed during saponification, forming soap and glycerin. This is why accurate measurements and proper curing time are so important.

2. Is handmade soap safer than commercial soap?

“Safer” depends on context. Properly made handmade soap offers transparency and gentle ingredients, but it still needs to be formulated and cured correctly. Many commercial products are synthetic detergents rather than true soap. If you have very sensitive skin, you may find a simple, artisan bar easier to tolerate than a heavily fragranced mass-market product, especially when paired with emollients recommended in dermatology guidance on skin hydration.

3. Does cold process saponification make soap more natural?

Cold process doesn't automatically make something "better", but it does allow the reaction to take place more slowly and gently, which can help preserve more of the oils' natural character. Studies on cold saponified soaps suggest that this method can retain more unsaponified fatty acids and plant components that support a conditioning feel.

4. How long should soap cure after saponification?

Most traditional recipes benefit from 4–6 weeks of curing. During this time, excess water evaporates and the bar becomes milder and longer-lasting. Some simple formulas may be usable sooner, but I rarely rush this stage – patience gives you a better bar.

If you'd like to follow the journey from raw oils to finished, cured bars, you're warmly invited to join the waiting list via our Shop Now page. That way, when a new batch completes its slow, quiet saponification and cure, you'll be one of the first to know.