Updated: May 2025
Truth: You cannot make soap without lye.
Sorry folks, but that’s the way it is. Lye is soaps dirty little secret. In his book “Scientific Soapmaking”, Kevin M. Dunn writes: “…the true art of soap making is…to create a bar of soap that is tantalizing and pleasing to the senses – and to do it within the constraints of science and chemistry…”;
…and lye is an indispensable part of that chemistry.
Even though the word “lye” makes people gasp in horror – and no one wants to hear it’s in a skin product! – it plays a crucial role in the soapmaking process: transforming raw ingredients into the luxurious bars we use for cleansing. This scream-worthy substance might make you never want to use soap again, but it’s an essential component that contributes to the alchemy of soap.
In soapmaking, lye acts as a catalyst in a chemical reaction called saponification. This reaction involves the combination of fats or oils with lye to produce soap and glycerin. The result is a product that effectively cleanses while maintaining skin hydration.
The first step in soapmaking is selecting the oils or fats. Different oils bring distinct qualities to the soap, influencing its texture, lather, and moisturizing properties. Lye then comes into play as it reacts with these oils through a carefully measured process. This reaction is critical because it determines the characteristics of the final soap.
Once the saponification reaction is complete (gel phase), the soap mixture needs to cure for about 4-6 weeks…yes, WEEKS not days. This allows any remaining traces of lye to react fully and ensures a milder, gentle final product. During this curing process, the soap hardens and becomes less alkaline, making it safe to use on the skin.
Soap makers often experiment with different oils and additives to create unique blends that cater to specific skin types and preferences. Lye (also known as Sodium Hydroxide) provides the flexibility for us to customize our soap recipes, each with their own distinct properties.
But wait! How come you never see the word “lye” on soap labels?? Manufacturing companies and commercial marketers (including handmade soapmakers) have come up with some creative ways to add lye to their ingredient lists without actually including the “he-who-should-not-be-named” word. Here’s a list of some of those substitutes:
Sodium Hydroxide
Saponified Oils
Caustic Soda
NaOH
Sodium Hydroxide Solution
Alkali Salt
Saponification Agent
Soapmaker’s Catalyst
Sodium Tallowate
Potassium Tallowate
Pot Ash
Soda Lye
Caustic Potash
White Caustic
Sodium Caustic
Sodium Hydrate
Hydroxide of Sodium
Alkaline Hydroxide
Caustic Alkali
Saponified Oils
I could go on and on, but I digress. If you don’t believe me, go ahead and read those ingredients on store-bought soaps. Anytime you see the word “Sodium” as part of the ingredient list it means lye was used.
Just so you know, we at Fair Isle use “Sodium Hydroxide” on our own labels.
In the end, no matter what you call it, it’s still lye. Without it, the oils and fats would remain a gooey mess. Lye helps transform that gooey mess into high-quality solid soap that conditions your skin instead of stripping it – which actually sounds like magic to me. So, the next time you lather up with a beautifully crafted bar, appreciate the role lye plays in transforming a blend of oils and butters into a luxurious cleansing experience.
Essential Lye Safety Tips for Soapmakers:
- Use 100% Sodium Hydroxide (NaOH) – Ensure your lye is pure and free of additives for accurate soap formulations.
- Measure Precisely – Always weigh lye using a digital scale. Never estimate, as even small miscalculations can impact soap quality and safety.
- Add Lye to Liquid, NEVER the Other Way Around – Pour lye into water (or milk) slowly to prevent dangerous splashes and violent reactions. If this is hard to remember, think: “Snow falls on the lake”.
- Ventilation is Key – Mix lye solution in a well-ventilated area or near an open window to avoid inhaling fumes.
- Use Heat-Resistant Containers – Always mix lye in a container made of heavy-duty plastic, stainless steel, or tempered glass. Avoid aluminum, which reacts with lye.
- Cool Lye Safely – The solution heats up rapidly; allow it to cool to your desired temperature before adding to oils.
- Stir Very Gently, Not Vigorously – Mix lye and liquid slowly and steadily to minimize splashing and excessive heat buildup. Imagine you are gently stirring flower petals so as not to break them apart.
- Label Lye Containers Clearly – Keep lye and lye solutions properly labeled and away from children and pets.
- Wear Proper Safety Gear – Use heat-resistant gloves, goggles, and long sleeves to protect against splashes and burns.
- Be Cautious with Milk or Sugar Additives – Adding lye to milk or sugar solutions causes caramelization and increased heat, so work in an ice bath if necessary.
- Neutralize Spills Safely – Clean small spills with white vinegar, but for large spills, use dry absorbent materials before neutralizing.
- Store Lye Properly – Keep lye in an airtight container in a cool, dry place to prevent moisture absorption and clumping.
- Never Use Hot Liquids for Mixing – Always start with cool or room-temperature liquids to prevent rapid heat buildup and possible eruptions.
- Work with Accurate Lye Calculators – Always run recipes through a lye calculator to ensure correct saponification values.
- Dispose of Lye Solution Responsibly – If needed, neutralize excess lye before disposing of it.