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Formulating with Castile Soap: Why It’s Not Ideal for Custom Cosmetics

Foaming, bubbly products are a favorite for many of us! When starting out as a cosmetic

formulator, it’s common to want to make everything at once—especially those fun foaming

products like body washes or shampoos. However, creating foaming products is far from simple to get it right, you need a solid understanding of surfactant charges and how to balance cleansing with skin-moisturizing ingredients.


One common misconception for beginners is that Castile Soap can be used to

formulate shampoos, body washes, or face washes. Unfortunately, this isn’t the case. Castile

Soap is a finished product, not a raw material suitable for cosmetic formulation.


What is Castile Soap?


Castile Soap is a liquid soap made through saponification, a chemical reaction where oils or

fats are combined with an alkali (like sodium or potassium hydroxide) to create soap and

glycerin. While this process produces a functional, natural soap, the product is not versatile for cosmetic formulations.


Why Can’t You Use Castile Soap as a Foaming Agent?


1. High pH: Castile Soap is highly alkaline, with a pH ranging from 8.9 to 9.1.

  • Alkaline Definition: Alkaline substances have a pH above 7, meaning they are

    basic rather than acidic.

  • Impact on Skin & Hair: Our skin and hair thrive in a slightly acidic environment.

    Most skincare products have a pH of 5.0–5.6, while shampoos sit at around 5.5,

    and conditioners are even more acidic at 4.5. Using an alkaline product like

    Castile Soap can disrupt the skin’s natural barrier, leading to dryness, irritation, or

    damage to hair cuticles.


2. pH Adjustment Issues: Many people think they can simply lower the pH of Castile Soap to make it work for skin or hair. Unfortunately, this won’t work because adjusting the pH

will destabilize the soap. The saponification process makes it impossible to lower the pH

without the product breaking apart, leaving you with a separated, unusable mess.


What Should You Use Instead?


To create custom foaming products like shampoos or body washes, you need to work with a

blend of surfactants:

  • Anionic surfactants: These are strong cleansers that provide rich lather.

  • Amphoteric surfactants: These balance cleansing power with mildness.

  • Non-ionic surfactants: These are gentle and often add conditioning properties.


These ingredients allow you to control the product’s performance, pH, and skin feel, resulting in a balanced, professional-quality formulation.


Want to Learn More?


In a future post or video, we’ll dive deeper into surfactant systems and how to formulate foaming products the right way. For now, ditch the Castile Soap and explore the raw materials that professional formulators use!


Don’t forget to check out our YouTube channel for tutorials and more in-depth discussions on formulation science.

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