Castile Soaps

As part of my 365 Soap adventure, I'm getting down and detailed into some different types of soap. It's a great way to explore and take you along for some soapy facts :)

What is "Castile Soap"? You've probably heard the name before, but you might not be sure on what it is (or isn't). So let's start with the basic definition:

Castile soaps are traditionally 100% Olive Oil soaps. Named after the region of Spain they originated from, the term has been around for hundreds of years and has been associated with high quality soap for just as long. However, you may see soaps on the market today that call themselves Castile while including oils other than olive. Why is that? Olive oil alone takes a longer time to cure, and doesn't produce big bubbles. Plus it can be more expensive to make. By adding some Coconut and other oils, companies can produce soaps that are more cost effective and give the bubbly lather modern consumers have come to expect. These days lots of companies and people will use the name "Castile" when referring to any soap made with vegetable oils where olive oil is the main oil. Probably because it's already a well recognized name. Is that technically correct? Eh, debatable. Terms can evolve over time, although the dictionary definition remains true to the original.

So what makes olive oil soaps so different? Why the extra long curing time?

That's down to the fatty acids and how they translate into soap!

All oils and butters have their own range of fatty acids, and each one brings certain properties to the soap. Olive oil is composed of a high percentage of Olecic acid.

When fats are combined with sodium hydroxide a chemical reaction occurs, splitting apart and reforming into new structures. Soap is made up of fatty acids linked to sodium ions (soap molecules), glycerin, water, unsaponified oils and any additives. Each type of fatty acid makes a different soap molecule. Since soap molecules are salts, they form crystalline structures (soap crystals!) under a microscope surrounded by a fluid of free floating soap molecules, glycerin, water, and extra goodies like superfat and additives.

Fatty acids and therefore the soap molecules they make vary in size. With lauric (like in coconut oil) being among the smallest, palmitic making a medium size, stearic making a large molecule with a long chain, etc. and then oleic which forms the largest of soap molecules. It has a long chain but also a double carbon bond that creates a bulky U shaped molecule. Basically olive oil makes for chonky soap molecules. These chonky molecules do not pack together easily to form crystalline structures as readily as other soap molecules.

If you've even made soap that's 100% coconut oil, then you've noticed it gets really hard really fast, and produces big fluffy lather even with a short curing time. Where in contrast, an olive oil soap will remain soft and feel slimy instead of sudsy even after the standard 6 to 8 weeks of curing. That's because the smaller soap molecules formed from the fats in coconut oil concentrate together easily, forming tight dense soap crystals.

Since olive oil is super high in oleic acid, and makes soap molecules that are so big, it takes a lot longer to start forming those crystalline structures inside. Over time, as bars of soap lose their water content to evaporation, and the un-crystalized portions become more concentrated, the makeup of those structures inside the soap begin to shift. This helps the chonky oleic soap molecules form those solid crystalline structures that give a better lather to a bar of soap.

Traditionally Castile Soaps are aged a full year before they are deemed ready. This allows time to let water evaporate, and the structures inside the soap to slowly crystalize.


Soap made from olive oil is super gentle on the cleansing scale, non pore clogging, and can be very skin conditioning. That's probably why it became so popular all those years ago as a luxury soap compared to other soaps that would have commonly been made with animal fats.

It's an excellent choice for sensitive, dry, or delicate skin making it popular for facial soaps or soaps made for babies since it doesn't strip skin of natural oils while still providing gentle cleansing.


In the spirit of experimentation, I made some mini batches of 100% Olive Oil soaps! These are stamped with numbers that correspond with my written notes. These are still pretty soft so stamping was a bit messy.

Soap 42: no additives

Soap 43: wildflower honey

Soap 44: goat milk

Soap 45: silk peptides

Soap 46: coconut milk

Soap 47: soap nut tea

Soap 48: oatstraw powder

Soap 49: rose clay


Looking forward to testing these out periodically as they age over the next year to take notes. Will any of these additives help with bigger bubbles, or how the lather feels? Find out in future blogs!









42 views2 comments