What Is Saponification? How Oils Become Handcrafted Soap

A finished bar of soap looks very different from the oils and butters that went into making it. The transformation happens through saponification—the chemical reaction at the heart of traditional soapmaking.

At Potager Soap Co., we make our handcrafted bar soaps in small batches using the cold-process method. Each batch brings together carefully selected ingredients, precise measurements, and hands-on craftsmanship.

Here’s how those ingredients become the finished bar beside your sink.

A Brief History of Soapmaking

Soapmaking has a history stretching back thousands of years. Some of the earliest written evidence comes from clay tablets in ancient Mesopotamia, dating to the third millennium BCE.

Early soapmakers combined fats with alkaline materials derived from plant ashes, long before they understood the chemistry behind the transformation. Through observation and experience, they learned how to create useful cleansing materials.

A Roman Legend About Soap’s Discovery

One often-told legend places soap’s discovery on a Roman hillside, where animal sacrifices were made above a river. According to the story, rain washed rendered animal fat and ashes down the slope. Women washing clothes in the river noticed that the mixture helped clean their laundry.

It is a legend rather than a documented account of soap’s origins. Still, it offers a memorable illustration of how fats and alkaline materials could come together to create a cleansing substance.

From Traditional Craft to Scientific Understanding

In the nineteenth century, French chemist Michel Eugène Chevreul helped explain the chemistry of fats and soapmaking, bringing scientific understanding to an established craft.

Later in that century, Louis Pasteur’s discoveries helped establish that microorganisms could cause disease. His work strengthened the scientific case for hygiene and careful handwashing, helping explain why keeping our hands clean matters. Soap had been used for centuries before these discoveries, but understanding germs gave cleanliness a new importance in preventing infection.

Today, soapmakers can calculate their recipes with far greater precision. At Potager, we combine that knowledge with the creativity and care of small-batch soapmaking.

What Is Saponification?

Saponification is the reaction that occurs when fats or oils combine with an alkali, such as sodium hydroxide, to produce soap and glycerin.

Plant oils and butters contain molecules called triglycerides. During saponification, these molecules react with the alkali to form fatty acid salts—the substances we call soap—and glycerin.

Put simply:

Oils and butters + sodium hydroxide → soap + glycerin

Sodium hydroxide, also called lye, is essential to making traditional bar soap from scratch. It transforms the oils rather than simply mixing into them.

How We Make Cold-Process Soap at Potager

Our recipes begin with carefully selected plant oils and butters. Depending on the formula, these include organic high-oleic sunflower oil and/or organic olive oil, organic coconut oil, organic RSPO palm oil, and organic shea butter.

Each contributes to the character of the finished bar, from its firmness to its lather and feel.

1. Measuring the Ingredients

Precision matters. Different oils require different amounts of sodium hydroxide to become soap.

Soapmakers use each oil’s saponification value, often called its SAP value, to calculate the amount of alkali needed for a recipe. The calculation must account for the particular alkali being used and any planned superfat—the portion of oil intended to remain unsaponified.

2. Combining the Oils and Sodium Hydroxide Solution

We combine our prepared oils with sodium hydroxide solution and blend them into soap batter.

The reaction releases heat as it progresses. “Cold process” means we don’t cook the batter to complete saponification, although oils and solid butters may be warmed during preparation.

3. Watching for Trace

As the batter thickens, it reaches a stage called trace. A little batter drizzled across the surface leaves a visible trail before sinking back in.

Trace helps us judge the batter’s consistency. It does not mean saponification is complete.

Depending on the recipe, we incorporate essential oils, botanicals, clays, or other ingredients before pouring the batter into molds.

4. Unmolding, Cutting, and Curing

Saponification continues after the soap is poured. Once the batch is firm enough, we unmold it and cut it into bars.

The bars then cure. During curing, water evaporates and the soap’s structure develops, helping the bars become firmer and perform better in use.

Saponification creates the soap; curing helps develop the finished bar.

Does Lye Remain in Finished Soap?

In properly formulated and fully saponified soap, no unreacted sodium hydroxide remains. It has been consumed by the reaction.

That is why you may see sodium hydroxide solution listed among the ingredients used to make a bar, even though the finished soap is a different substance.

For help understanding these terms, read How to Read a Soap Ingredient Label.

From Thoughtful Ingredients to Everyday Rituals

Making soap brings together chemistry, careful formulation, and hands-on craftsmanship. At Potager, it also gives us room to work with botanical scents, textures, and colors that make each recipe distinctive.

Explore our handcrafted bar soaps, or read more about Potager’s story.