What is Castile Soap?

Castile soap is a name used in English-speaking countries for olive oil based soap made in a style similar to that originating in the Castile region of Spain.

The origins of Castile Soap can be traced back to The Levant where Aleppo soap makers have been making olive and laurel oil based hard soaps for millennia.
It is commonly believed that the Crusaders brought Aleppo soap back to Europe with them in the 11th century. Following the Crusades, production of this soap was extended to the whole Mediterranean area. The first European soap-making factories were created by the Muslims in the 12th century in Spain (Alicante, Malaga, Carthagene and Castile) and in Italy (Naples, Savone, Genoa, Bologna and Venice) and then, in the middle of the 15th century, in Marseille France, giving birth to Marseille soap.
However, early soap makers in Europe did not have easy access to laurel oil and therefore dropped it from their formulations thereby creating an olive oil soap now known as Castile soap.
Importations of “Castile soap” through Antwerp appear in the London port books of 1567–68 (Dietz 1972), though the Oxford English Dictionary has no references to “Castile soap” earlier than 1616. In his article “A short history of soap”, John Hunt maintains that barilla (an impure form of sodium carbonate obtained from halophyte plant ashes that were high in sodium) was boiled with locally available olive oil, instead of tallow. By adding brine to the boiled liquor, the soap was made to float to the surface, where it could be skimmed off by the soap-boiler, leaving the excess lye and impurities to settle out. This produced what was probably the first white (Aleppo soap tends to be green) hard soap, which hardened further as it aged, without losing its whiteness, forming jabón de Castilla. Apothecaries knew the product by the Latin names of sapo hispaniensis (Spanish soap) or of sapo castilliensis (Castilian soap).

Soap #1: Basic Olive Oil Soap
Bars made using just olive oil should be left in the mold for at least 48 hours. They will need at least 4 to 6 weeks to cure. You can add fragrance to this soap if you wish. Using an online fragrance calculator, select your fragrance and calculate for cold process soap at 20 ounces to determine how much to use.



  • 20 ounces of olive oil

Lye mixture:

  • 2.55 ounces of sodium hydroxide (NaOH)
  • 6.29 ounces of distilled water

1. Measure the olive oil into your plastic container. Place the container in a larger pot and pour in enough hot tap water that the container begins to float. Set the pot on the stove and turn the heat to warm. Insert a thermometer into the oil.
2. Goggles and gloves on!
3. Measure the water into a heat-safe glass container. Measure the lye crystals into a separate small glass container. Slowly add the lye crystals to the water, stirring with your spatula as you do so. Do not inhale above this container—there will be fumes that can take your breath away! This mixture will heat up quickly. Insert a thermometer into the mixture.
4. Monitor the temperatures of the two containers. Basically, you are heating up the oil while the lye cools down. You want both to reach 110°F. As needed, refresh the hot water bath or turn the stove burner higher to raise the temperature, or use a cold water or ice bath to bring the temperature down.
5. When both the oil and the lye mixture are at 110°F, pour the lye mixture into the plastic container with the oil. Blend until the mixture reaches medium trace (it will be like thick gravy, and drizzled trails will stay on the top).
6. Pour into your chilled mold (or a clean, dry, quart size milk carton with the top cut off) and refrigerate, uncovered, for 30 minutes. Remove from the refrigerator; spray the top with isopropyl alcohol, cover loosely with plastic wrap, and refrigerate overnight.
7. Remove from the refrigerator and let sit at room temperature.
8. Unmold 48 hours after pouring into the mold. Cut into bars and place in your curing area.
Makes 8 3½-oz bars
Mixing lye with a liquid causes an exothermic chemical reaction. This means that lye will heat up any liquid to which it’s added. A room-temperature liquid can heat up above 200°F with the addition of lye.
Always add the lye crystals to whatever you’re using as your liquid. Never add liquid to your lye crystals! Adding liquid to the lye will cause a volcanic reaction—the surest way to get burned. This is a major no-no in soap making!
Always store your lye container tightly closed in a cool, dry place out of the way of animals and small children.
Saponification (the chemical reaction in the soap-making process) uses up the lye, so that there is no lye left in the finished soap. You need to leave your soap to cure in order to make sure all the lye has reacted, and that you have a sturdy finished bar of soap.
It is a good idea to reserve a few tools for soap making, such as an immersion blender, and any containers you use for lye and mixing your soap. You don’t want to contaminate food with lye or fragrance oils (despite how tasty they may smell!).

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