How to Make Castile Soap

A History and Tutorial of How to Make Olive Oil Soap

How to Make Castile Soap

Reading Time: 6 minutes

Vegan, sustainable, and often organic, Castile soap is highly sought after. Properties are comparable to soaps made with tallow, lard, or coconut oil but the bar made from olive oil ranks higher. History plus aesthetic esteem have prompted many crafters to learn how to make Castile soap.

A History of Castile Soap

Romans in the first century AD knew how to make soap. Zosimos of Panopolis described the process about 300 AD.

It’s debatable exactly when people in the Levant region learned how to make Castile soap from olive laurel oils. Popular literature claims Queen Zenobia of Syria and Cleopatra of Egypt both used it, though those claims haven’t been verified. But Syrians mastered how to make green soap from both oils by the Crusades.

The Europeans, who had been using tallow soaps at least before the time of Charlemagne, took to the botanical version quickly. Early European soap-makers had little access to laurel oil used in the green Aleppo soap so they dropped it from their formulations.

By 1567, London port books recorded import of “Castile soap” through Antwerp. John Hunt, in 1616, described a process where barilla, an impure sodium carbonate from halophyte plant ashes, was boiled with olive oil instead of tallow. Brine was later added to the boiled liquor so soap-makers could skim off the purer product rising to the surface. This produced the first hard, white soap, a contrast to the green Aleppo product. It was named jabon de Castilla after the city in Spain.


Though true Castile soap is made from olive oil, the term is now used for any soap made from vegetable oils. The name sits on labels of both solid and liquid products which may not contain much olive oil in relationship to other ingredients. Some manufacturers use the term “Castile” simply as a marketing standpoint, touting their plant-based product’s superiority and including essential oils to broaden the claims. Soap aficionados maintain that Castile soap is only from olive oil.

Soapmakers often mix other oils into their Castile soap because a bar made with pure olive oil is initially soft and does not cure quickly. It can be slightly greenish if the oil used is not purified. The creamy, moisturizing properties contribute to soap which turns slimy in the shower instead of lathering into fluffy bubbles. It can also be expensive learning how to make Castile soap because olive oil is costlier than some other products. An economically produced Castile bar contains palm and coconut oils to harden the soap, whiten it, make it last longer and cure faster, and produce a cleansing lather.

The Right Ingredients

You need three main ingredients for Castile soap: olive oil, lye, and distilled water. Other ingredients may include additional base oils, fragrances, colorants, and additives.

When learning how to make organic soap, keep in mind that lye for soap making is not organic. It is made in a chemical factory. But proper saponification consumes all available lye. So, technically, soap made with organic olive oil plus lye is still organic because no lye exists within a finished bar.

Traditionally, alkali was leeched out of ashes from wood or other plants. This process is not recommended for people learning how to make Castile soap because the alkalinity is uncertain and the process is dangerous. Modern soap-makers are instead advised to use lye because of its constant pH.

Extra-virgin olive oil is not necessary for a good bar of soap. The cheapest oils are some of the best. Be sure the product is guaranteed to be 100 percent olive oil. Different fats have different saponification values, which is the exact amount of lye necessary to turn one gram of fat into soap. Mixing oils without knowing the exact amounts of each can result in dangerously lye-heavy soap.

If you wish to mix oils within your soap, purchase pure oils then use a soap calculator to determine how much lye and water you need. Maintain a superfat of at least 5 percent to avoid lye-heavy soap.

Cold vs. Hot Process

The oldest method, hot process, involves cooking the soap for several hours to several days until the lye is inert. This allows for more discrepancies in the alkalinity of the base product, which used to be made with ashes from wood or other plants. Modern soap-makers turn to hot process because the product can be used within a day or two versus several days to several weeks if cold-processed.

The most preferred hot process method for home soap-makers occurs within a crock pot and is called CPHP or “crock pot hot process.” This involves adding oils to a slow-cooker and heating them until they are liquid, a step which is unnecessary if only olive oil is used. Lye is added to water and dissolved then the mixture is carefully added to the oil. Using a stainless steel whisk or immersion blender, the soap-maker agitates the mixture until it reaches “trace,” a consistency resembling thin pudding. The mixture is then cooked until the texture “gels” and looks like petroleum jelly. The soap is fragranced and colored then scooped into a mold. If all ingredients and steps are done correctly, the soap is ready to use as soon as it cools and hardens.

Cold process involves mixing lye with oil then allowing the “gel” stage to occur within the soap molds. This method allows for smoother, more artistic finished products. Some soapmakers heat the oils until they are a consistent temperature with the lye mixture and others soap at room temperature. Lye is added to water and dissolved then the lye mixture is carefully poured into the liquid oils. The soap-maker stirs the mixture until it reaches “trace” then adds fragrances and colors before pouring into molds. After the soap is set aside in a warm location, it reaches the “gel” phase then cools. Though the soap is safe to use within a few days, curing it for weeks to a few months creates a milder bar that lathers better and lasts longer in the shower.

Whether you choose to hot process or cold process, fully educate yourself on how to make Castile soap with your desired method before beginning. Learn from a soap-making resource what to do if you spill chemicals or calculate the wrong ingredients. Always follow correct soap-making precautions.


Also called hand-milling, rebatching is how to make soap without lye or glycerin while still creating a natural product. This is safer because the process involving lye has already been done for you. Purchase a bar from someone who knows how to make Castile soap. Grate this bar into fine shreds or pulse in a food processor. Combine grated soap with just enough liquid, such as water or goat milk, within a crock pot. Cook just long enough to melt the product, stirring frequently so it doesn’t burn. The mixture will never fully liquefy but will be thick, sticky, and textured. Stir fragrance and other additives in as well as possible then spoon the soap into molds. Press down on the soap or tap the molds against the counter to avoid air bubbles. Let it cool completely before unmolding. Curing rebatched soap is unnecessary because the initial product was already cured.

Melt and Pour

Can you make soap without lye and also involve children in the process? Yes, but only if you use a melt and pour base. Melt and pour soap is not organic; it’s not even natural since petroleum-based products are necessary to facilitate frequent melting and solidifying.

Purchase a melt and pour base from a craft store, soap-making supply site, or an online retailer. The product may say it is a “Castile” or “olive oil” soap even if the olive oil quantity is minuscule. Place the desired amount of base within a microwaveable container and heat just until the product is melted, stirring occasionally. Mix in colorant and fragrance then pour into molds. Cool completely before unmolding.

Melt and pour is suitable for children because the product only needs to melt at about 120 degrees or lower. It will only burn if you overheat. Since the process involving lye was completed before you purchased the base, you can craft melt and pour projects and still use kitchen utensils afterward for food.

Storing Castile Soap

After learning how to make Castile soap, be sure you store it correctly.

Any product made with oil can turn rancid. This is especially true with soaps calculated to a higher superfat, which is the percentage of oils used over what is needed to use up the lye. Heat and moisture can exacerbate or speed up rancidity.

Photo by Althaea Soaps

While the soap is curing, store it in paper bags or cardboard boxes. Do not encase it in plastic at this time. Keep it out of sunlight and in a cool, dry location. Once it is cured, you may wish to wrap it in plastic to preserve fragrance but many professional soap-makers still choose to allow the soap to breathe. Cardboard boxes or brown paper wrap your product for transport, storage, or sale without holding in extra moisture.

The first indication of rancidity is DOS, or dreaded orange spot. It begins with a brownish-orange dot which increases in diameter as the soap ages. If your soap develops DOS, it is still safe for personal use but is not good enough for sale. Seasoned, professional soap-makers plead with new crafters to avoid selling soap prone to DOS because it sullies the reputation of an artisan industry. The best advice for people learning how to make Castile soap is that they allow at least a year to refine their art before offering the product for sale.

Educate yourself while learning how to make Castile soap for personal use or potential sale. Observe safety precautions but have fun. You’re carrying on a piece of history.

Do you know how to make Castile soap? If so, is it part of your soap-making routine?

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