Understanding Castile Soap Recipe and Formulation Guide

Wondering what’s all the fuss about Castile soap?

Castile soap has been one of the most popular soaps of all times (if not THE most popular soap) and has remained so for hundreds of years. Castile soap also stirs up question and confusion in the soapy community. So let’s dive in, answer those questions, resolve the confusion, and even make some castile soap with my castile soap recipe!

What exactly is Castile soap?

Maybe you’ve read about it in social media soapmaking groups or maybe you’ve seen it in grocery stores, and you’ve wondered what the big fuss is all about. And maybe you’ve even wondered how to pronounce it correctly, too!

For starters, Castile is pronounced “Cass-TEEL”; the word comes from a region of Spain that was known all over Europe for its soap that was mild but highly effective, as well as its perfectly pure, white color. Since the 12^th century, the way Castile soap is made has been basically (although not exactly) the same, with the same simple ingredients: olive oil, water, and lye.

In more recent years, Castile soap has been redefined by modern soap manufacturers as soap made with vegetable oil only (as opposed to soap that is made with animal fat). But for some purists, a “true” Castile soap recipe is made with olive oil, water and lye only – no other additives can be included.

However, there is no official definition of a Castile soap recipe, and even artisan soapmakers don’t always agree. Originally, a Castile soap recipe consisted of cooking olive oil with plant ashes, producing a similar lye to potassium hydroxide, and using brine to harden up the soap. So it could be argued that not even cold process or sodium hydroxide could be used to make a “true” Castile soap.

Most soapmakers today aren’t crazy about the idea of making their own lye or cooking soap for days at a time, so let’s agree that unless our goal is to make historically accurate, medieval-period Castile soap, we can use sodium hydroxide bought from a store, either hot process or cold process, and use other lovely additives to make a spectacular bar of soap with a modern day castile soap recipe.

A variation of Castle soap recipes: Bastile soap!

For continuity’s sake, let’s assume that Bastile is pronounced “Bass-TEEL” because that rhymes with Castile. 😊

This is a term that probably came from modern artisan soapmaking culture, Bastile soap being a “bastardized” version of Castile soap: a mostly olive oil soap that has a small percentage of other oils.

Why exactly would a soapmaker want to reduce the percentage of olive oil, since Castile soap is such a great soap? As wonderfully mild and gentle as Castle soap is, it does have some negatives.

1: The lather is dense, creamy, and conditioning… but if you’re a fan of loads of big bubbles, a Castile soap recipe will be a bit disappointing.

Commercially made detergent-based cleansers are always going to be huge in the lather department, since that’s one of the best things that detergents do. Think of it as a trade-off: less lather, but kinder to the skin. Embrace that lotion-y, creamy lather! 

2: Another problem with Castile soap recipes is that it takes much longer for castile soap to become great soap vs. other types of soap recipes.

Typically, most soap cures within 4 to 6 weeks. While a Castile soap recipe will be safe to use after 6 weeks or so, it won’t be too impressive. It’s likely to turn slimy and gelatinous when only a few weeks old. Castile soap needs more time to age, like a fine wine – typically 9 to 12 months. Somewhere around this 9 to 12 month mark is where the magic happens: low lather becomes rich, dense lather that makes your skin feel like silk and Castile soap becomes your new best friend.

Making Castile Soap

Let’s talk about formulating your own Castile soap recipe, now that you’re itching (pun intended!) to make some:

Cold process or hot process for a Castile Soap Recipe?

One advantage of hot processing Castle soap is that it cuts more easily. If you cold process your Castile soap, cut it as soon as it hardens up. If you wait too long, it can become very hard, and the edges can crumble as you cut it.

One advantage of cold processing Castile soap is using room temperature oil and very hot lye solution can be used. As soon as the lye is completely dissolved, it’s ready to make soap. The heat from the lye solution helps the slow-to-trace olive oil get to trace faster.

Water Amount for a Castile Soap Recipe

There are a few factors to consider when calculation the water content of cold process a castile soap recipe.

• Olive oil takes a long time to reach trace based on it’s high oleic acid content.
• Castile soap can be very brittle to cut if it doesn’t go through a gel phase.

Fortunately, a moderate water amount will address both of those concerns!

Here’s what I mean:

First, realize that the default water content in soap calculators is for hot process soap. Typically that’s way too much water for a cold process soap. All of that water will cause your soap to trace very quickly and will take days to evaporate out enough for you to unmold your soap. Check out this water content demonstration on my YouTube for a visual.

For cold process soap, I typically use a 1:1 (equal parts) water:NaOH formulation. But with a castile soap you need more water to avoid it crumbling when cut and to achieve a trace suitable for molding before you stick blender burns out! So how much water is the perfect amount?

A good ratio of water to lye for a cold process Castile soap is 1.5:1 up to 1.8:1 (depending on your additives). To calculate the water, take the amount of lye needed and multiply that number times 1.5 or 1.8 – it really is that easy! If you’re using additives that contain water (milks, purees etc.) be sure to count them as part of the total water content.

Can You Include Additives in Castile Soap Recipes?

It’s your soap and your formula, so you decide! Any of the additives used to enhance lather will be perfect to add to Castile soap: goat milk, coconut milk, honey, or other sugars. Veggie and fruit purees, teas and herbs can also be incorporated in Castile soap to add color, texture, and scent. For more details and inspiration, hop over to my blog post here on adding food stuff to handmade soap.

Once your soap is cut, start trying it out! I recommend using this soap in stages, taking notes along the way, so you can experience the development of this wonderful (eventually wonderful) lather.

• Use one bar after the cure. Yes, it will be slimy and underwhelming later. Use it anyway!
• Then use another bar every 6 weeks or so to see how the lather changes and develops over the course of 9-12 months.

I hope you’ll make a batch of Castile soap soon, and I hope I’ve given you some fresh ideas to try out – if so, please let me know! Comments and pics, please (and pretty please – you know I love seeing your soaps my dear soapy friends!)