DIY 100% Pure Castile Soap Bars

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I’ve started a new adventure in cold-process soapmaking to start the process of replacing nearly all my store-bought bath and body products with high-quality handmade alternatives. As I learn more about the toxicity levels in mass-produced items, I become increasingly compelled to maximize the performance and health-promoting features of said products by making them myself.

First up: Castile soap. It’s one of the gentlest soap types and is safe for even the most sensitive skin. I also hope to use some of the bars in homemade laundry detergent, making this soap a versatile and cost-effective option for the home.

Here’s What You Need

This recipe was adapted from the book Pure Soapmaking by Anne-Marie Fabiola.

Tools

• 1 weight scale

• 1 small (4-cup or 1-quart) glass measuring cup

• 1 large (8-cup or 2-quart) glass measuring cup

• 1 infrared thermometer

• 1 immersion blender

• 2 silicone soap molds (I chose 2, 6x6” slab square molds for this recipe, but feel free to choose whatever mold quantity or configuration desired. For example, you would only need 1, 9-cube mold or 1, 6x12” slab mold.)

• 1 spray bottle

Optional

• 1 silicone spatula

• 1 soap cutter or large knife (If you use a cube mold or another type with individual wells, you won’t need a cutter because the soap will naturally set to the desired size. Slab molds provide more options when cutting the final product—you can get different sizes from one batch—but you will need something suitable to cut the bars from the slab.)

Supplies

• 10.9 ounces distilled water

• 4.2 ounces lye (sodium hydroxide)

• 33 ounces pure olive oil

• 1 ounce essential oil (I chose organic lavender, but feel free to choose whatever scent you like)

• a few spritzes of 99% rubbing (isopropyl) alcohol

Instructions

Note: Practice safety first, and be sure to wear the appropriate attire and safety gear. Typically, this means long-sleeve shirts, full-length pants, toe-covered shoes, protective gloves, and safety goggles. Please also be sure to prepare your soap in a well-ventilated space.

1. Add the distilled water to a small glass measuring cup. Add the lye to the water and stir gently.
   Note: Never add water to lye because this can cause splashing and unnecessary exposure to the lye, a naturally caustic substance. There’s no need to fear soapmaking, but caution is required.

2. Safely set the lye water aside, away from potential spills or accidents. The temperature of this mixture will decrease as it sits, and its color will transition from cloudy to clear.

3. Add the olive oil to a large glass measuring cup.

4. When the lye water has cooled below 135°F (57°C, use your thermometer to confirm the temperature), add it to the olive oil by sticking the end of an immersion blender into the oil and pouring the lye water over the blender to minimize splashing and air bubbles.

5. Gently tap the immersion blender a few times against the bottom of the measuring cup to release any pockets of air trapped in the blades and under the blender’s mini dome-like end piece. Note: Do not turn on the stick blender until fully immersed in the oil and lye water mixture.

6. Blend the oil and lye water mixture for approximately one minute or until a thin trace is achieved.

7. Slowly add any desired essential oils to the soap batter. At this point, you may want to switch to a silicone spatula to gently stir in the essential oil until well combined.

8. Pour the batter into your desired mold(s). If you haven’t already, pour the rubbing alcohol into your spray bottle. Spritz the top surface of your newly-molded soap mixture with the alcohol to prevent the formation of soap ash.

9. Set the soap aside for 48 to 72 hours before removing from the mold. Allow the bars to cure for 6-8 weeks before using; however, 100% pure Castile soap performs best when cured for at least 6 months, with 12 months as the ideal.

Store-Bought vs. Handmade Soap: Cost Breakdown and Comparison

At the time of writing:

• 1 gallon (128 ounces) of distilled water costs approximately $1.39. Only 11 ounces are needed for this recipe, which means $0.12 worth of distilled water is used per batch.

• 1, 32-ounce jar of lye costs approximately $15.69. Only 4.2 ounces are needed for this recipe, which means $2.06 worth of lye is used per batch.

• 1, 33.8-fluid-ounce bottle of organic* extra virgin olive oil costs approximately $12.99. The recipe requires 33 ounces.

• 1, 4-ounce bottle of organic* lavender essential oil costs $17.99. Only one ounce is needed for this recipe, which means $4.50 worth of essential oil is used per batch.

• 1, 16-ounce bottle of 99% rubbing alcohol costs approximately $8.34. Let’s estimate that “a few spritzes” comes to 0.5 ounces. That’s $0.26 worth of isopropyl alcohol used per batch.

Therefore, one batch of soap costs me approximately $19.93 in supplies. I do not count the cost of tools, as these items can be used indefinitely and across multiple projects, making their cost per use negligible. Not to mention, if your handmade products come out cheaper than their store-bought counterparts, your tools will eventually pay for themselves.

As you’ll notice, I chose to use two 6x6” slab molds, because I prefer square bars for body soap, as they are often easier to grip in the shower. After everything is said and done, I can yield eight 3x3” 100% pure Castile soap bars using organic ingredients, totaling approximately $2.49 per 5.2-ounce bar. Three-inch square bars are quite large, so you could stretch this further by cutting the soap slabs into 12, 2x3” 3.4-ounce bars (as seen in my images above), totaling approximately $1.66 per bar. You could also choose a different mold altogether.

I prefer to use organic ingredients whenever possible. Still, you can obviously cut costs by purchasing conventional ingredients, making unscented bars (essential oils are pricey for the amount used!), or buying ingredients in bulk for higher quantity discounts.

The closest comparison to my soap would be another 100% traditionally-made pure Castile soap bar using organic ingredients, when applicable. Technically, one can only call something Castile soap if it is made from 100% olive oil as the fat component. Though many vendors call soaps made with a combination of fats “Castile” soap, this is actually Bastille soap. Of course, there’s nothing wrong with Bastille soap (unless you are allergic to the other oils used); just be aware that Castile and Bastille soap are two different products with different oil compositions, and therefore different properties.

Nevertheless, for a true comparison, I found this organic traditional Castile bar soap on Etsy at $14.00 per 5.5-ounce bar. I’m not a fan of Dr. Bronner’s products, but if you were willing to use Bastille soap (their soaps use a combination of organic oils, though the lavender extract is not organic), this would still cost approximately $4.80 per 5-ounce bar.

Handmade for the win.