What is soap? Understanding the basics before making it.

When we think of soap, we often picture a solid bar that lathers up under water and gets us clean. Yet, behind this everyday object lies a fascinating little chemical phenomenon : Saponification.

Understanding what soap is, how it forms, and why different oils yield different soaps is an essential first step if you want to learn how to formulate your own recipes. You don’t need to be a chemist to get started; you simply need to grasp a few fundamental concepts.

What exactly is soap?

Soap is a product obtained through a chemical reaction between a fatty substance and an alkaline substance. This reaction is known as saponification.

For solid soap made using the cold-process method, vegetable oils or butters, such as coconut oil, sunflower oil, or shea butter, are used alongside caustic soda, also known as sodium hydroxide (NaOH).

When these ingredients react in the right proportions, they primarily produce two things: fatty acid salts (the molecules that make up the soap) and glycerin (which forms naturally during the reaction).

Glycerin is not, therefore, an ingredient added to the recipe; in cold-process saponification, it is produced naturally and remains in the soap.

It is also important to note that a properly formulated soap is not simply oil mixed with soda. Saponification chemically transforms a large portion of the raw materials into new substances.

How does soap work?

To understand why soap cleans, you need to look at the structure of its molecules. A soap molecule has two parts with different affinities: a hydrophilic part (water-loving), which interacts with water, and a lipophilic part (fat-loving), which has an affinity for fats. This dual attraction allows soap to help remove greasy dirt from the skin or other surfaces.

When you wash your hands, the lipophilic parts embed themselves in the grease and residue, while the hydrophilic parts remain on the outside of the grease spot, facing the water. Soap thus helps disperse these substances into the water so they can be washed away during rinsing.

This process relies on the formation of tiny structures called micelles, which trap and disperse fats within the water.

And what about the lather? While it can make washing more pleasant, it does not guarantee cleaning effectiveness. A soap that produces a lot of lather is not necessarily better than one that produces very little.

Why are different oils and butters used?

If all soaps result from a similar reaction, why are there so many different recipes? The answer lies in the composition of the fats. Each oil and butter has a specific fatty acid profile. During saponification, these fatty acids contribute to the properties of the finished soap.

These properties depend on the proportions used, the superfatting level, and the manufacturing conditions. Formulating a natural soap therefore involves selecting and combining fats based on the desired properties. Oils are chosen not for their names or their benefits for the skin, but for how they behave after saponification.

What is the role of caustic soda?

Caustic soda, or sodium hydroxide (NaOH), is an essential ingredient in the making of natural soap. Its role is to react with fats, transforming them into soap and glycerin.

The amount of caustic soda required depends on the type and weight of the oils used. Each oil has a specific saponification value used to calculate the amount of caustic soda needed to transform it. This is why two recipes containing the same total amount of oil may require different amounts of caustic soda.

Caustic soda is corrosive and must be handled using protective equipment and in a well-ventilated area. A recipe should never be prepared by estimating quantities by eye; calculating the amount of caustic soda is a crucial step in the formulation process.

What is superfatting?

In soapmaking, this term refers to formulating a recipe with less sodium hydroxide (lye) than is normally required to saponify all the fats. For example, a soap formulated with a theoretical superfat level of 8% uses less lye than the amount calculated for the complete saponification of the oils.

This choice influences the soap's final composition. However, it does not guarantee that exactly 8% of the original oil will remain unsaponified. Oils have varying compositions, and the chemical reactions involved are not simply a matter of a specific percentage of oil remaining intact.

Superfatting is a parameter to be mastered, not an automatic guarantee of gentleness. The choice of oils, their proportions, the soap's cleansing power, and the characteristics of the final product also play a role.

Conclusion: A chemical reaction at the service of cleaning

Soap is far more than just a mixture of oils and lye. It is the result of a chemical transformation that creates molecules capable of facilitating the removal of grease and dirt.

Understanding how it works helps clarify why oils are not interchangeable, why the amount of lye must be calculated precisely, and why two different recipes can yield very different soaps.

The next step is to explore one of the most important concepts in formulation: the saponification value. This figure allows you to determine the amount of lye needed based on the chosen oils and to start creating your own recipes.