What Is Lye?

If you are a beginner at making cold-process soap, the word "lye" might seem intimidating. You may have heard that it is dangerous or corrosive, and you might even wonder why anyone would want to use it. Especially in a product intended for application on the skin.

In reality, lye is an essential ingredient in traditional soapmaking. Without it, oils and butters cannot be transformed into actual soap. Understanding what lye is, how it works, and how to handle it safely is one of the most important steps in learning the cold-process saponification method.

What exactly is sodium hydroxide?

In the context of cold-process soapmaking, the term "lye" refers to sodium hydroxide (NaOH). Sodium hydroxide is a highly alkaline substance (having a pH above 7). When mixed with water, it forms a solution capable of reacting with oils and butters. This reaction is called saponification.

Simplified version:

Oils + Sodium hydroxide → Soap + Glycerin

Sodium hydroxide does not remain in its original form in the finished soap. Once the recipe has been correctly calculated and the saponification reaction has taken place, the lye reacts with the fatty acids contained in the oils and butters to form soap.

Why is caustic soda necessary for making soap?

Oils and butters are not soap in themselves. They contain triglycerides that must be broken down during the saponification process. Sodium hydroxide provides the elements necessary for this chemical reaction. It is important to note that different oils require varying amounts of sodium hydroxide, as their fatty acid compositions are not identical. For example, coconut oil, olive oil, and castor oil do not all require the same amount of NaOH to be fully saponified. Therefore, one cannot simply replace one oil with another in a soap recipe without recalculating the amount of lye required.

Is lye still present in the finished soap?

When a soap recipe is well-formulated, all of the sodium hydroxide is consumed during the saponification process. This is an important point, as properly made soap is not dangerous. Sodium hydroxide (lye) is used to transform oils into soap, and the final product contains no remaining traces of NaOH. However, this does not mean that lye can be handled without caution. During the manufacturing process, the lye solution is highly caustic (with a pH of 12 to 14) and can cause severe burns. For this reason, safety measures are essential whenever you begin working with sodium hydroxide. This means wearing gloves and protective eyewear, and using a mask to protect your lungs.

Caustic soda is dangerous, but that does not mean that soap is.

One of the most confusing aspects for beginners is the fact that sodium hydroxide is dangerous, yet the soap made from it is perfectly safe for the skin. The explanation lies in the chemical reaction that occurs during saponification. This reaction should be viewed as a simple transformation.

Before saponification:

Oils + sodium hydroxide → a highly reactive mixture

After saponification:

The fatty acids have reacted with the sodium hydroxide to form soap.

This is why soap-making requires precise measurements and a well-tailored formulation. You cannot simply choose arbitrary amounts of sodium hydroxide and assume it will be fine just because it will still result in soap.

What happens when caustic soda is mixed with water?

When caustic soda (sodium hydroxide) is added to water, the mixture releases a significant amount of heat. The solution can heat up very rapidly and emit irritating vapors, especially during the mixing process. Furthermore, you must always add the caustic soda to the water, never the other way around. Pouring water directly into a container holding concentrated caustic soda can cause a rapid temperature spike and lead to splashing. This is a fundamental rule that must always be followed. Therefore, always add the caustic soda to the water, work in a well-ventilated area, and use appropriate protective equipment.

How can caustic soda be handled safely?

When you are starting out with cold-process soap making, safety is the most important consideration. To begin formulating, you must follow specific safety rules and have the necessary protective equipment:

• Protective gloves

• Safety goggles

• Long sleeves and clothing that covers your skin

• Closed-toe shoes

• A well-ventilated workspace

• Heat-resistant containers and utensils that are compatible with lye (caustic soda)

• An accurate digital scale

Keep children and pets away from the workspace. It is also important to avoid using aluminum containers or utensils when working with sodium hydroxide, as the lye can react with the aluminum. Always follow the safety instructions provided on the packaging of your products.

Why is the amount of caustic soda calculated rather than measuring out a fixed quantity?

There is no universal amount of lye that applies to all soap recipes. The required amount depends on:

1. The type of oils and butters used

2. The weight of each oil

3. The desired superfatting level

4. The purity of the sodium hydroxide

Each oil has a specific saponification value (SAP value), which indicates approximately how much sodium hydroxide is needed to saponify a given quantity of that oil. A soap calculator uses these values ​​to determine the amount of NaOH required for your specific recipe. This is why changing even a single ingredient may necessitate recalculating the entire recipe.

What is superfatting?

"Superfatting" refers to the percentage of oils intentionally left unsaponified in the finished soap. For example, a recipe formulated with an 8% superfat level uses less sodium hydroxide than the amount theoretically required to saponify 100% of the oils. The superfat percentage is a key parameter to consider when calculating the amount of lye, as it helps make the soap more pleasant and gentle on the skin. However, a higher superfat percentage does not necessarily guarantee a gentler soap; the overall formulation, as well as the type and proportion of the oils used, also play a role in the mildness of the final product.

Concentration of the sodium hydroxide solution and water

When making soap via the cold-process method, you will often encounter another important calculation: the concentration of the lye solution. This concentration refers to the percentage of sodium hydroxide present in the lye solution. For example, a 30% concentration means the solution contains:

• 30% sodium hydroxide

• 70% water

The concentration of the lye solution is different from "superfatting." Superfatting determines the amount of oil that remains unsaponified, while the lye solution concentration determines the amount of water in relation to the sodium hydroxide within the solution. Understanding this distinction will greatly simplify your soap-making calculations.

What happens if you use too much or too little lye?

Precision is extremely important. If a recipe contains too much sodium hydroxide, there will not be enough oil to react with all of the lye. The finished soap may then be excessively alkaline and potentially irritating or caustic. If a recipe contains too little sodium hydroxide, a larger amount of oil will remain unsaponified. This can affect the characteristics and stability of the finished soap. For this reason, the amount of lye must always be calculated based on the specific oils and the exact quantities you intend to use. Never estimate the amount of sodium hydroxide by eye.

Does the cure help eliminate the sodium hydroxide?

Curing and saponification are two different things. Saponification is the chemical reaction that transforms oils and sodium hydroxide into soap, whereas curing takes place at the end of the manufacturing process. Curing involves letting the soap dry for about 4 to 6 weeks. During this phase, the soap continues to lose water, and its physical characteristics become more refined; the bar of soap becomes harder and gentler to use. Therefore, curing should not be viewed merely as a waiting period intended to make the "lye disappear." The primary chemical transformation occurs during saponification, while curing allows the finished soap to develop its definitive qualities.

We must not be afraid of sodium hydroxide.

Caustic soda (lye) is one of the essential ingredients in traditional soap-making. It is also the component that requires the greatest level of caution regarding safety. Once you understand the role of sodium hydroxide, the manufacturing process becomes much less daunting. You do not simply add raw caustic soda to a soap recipe, hoping it will become harmless over time; rather, you use a precisely calculated amount of sodium hydroxide to trigger a chemical reaction that transforms oils and butters into soap. The key principles are simple:

Measure accurately.

Calculate carefully.

Wear appropriate protective equipment.

Handle caustic soda responsibly.

Never guess the amount of caustic soda to use.