Liquid Castile Soap Ingredients

When they decided to use olive oil, early soap makers in the Castila region of Spain created a method of producing soap that still is considered the finest of all soap formulations. Unlike other recipes that produced harsh soaps, olive oil soap cleansed, softened and moisturized the skin. Even as modern liquid soap makers have tweaked the recipe for castile soap, they still manufacture it with some of the same basic ingredients as that of its namesake: olive oil, alkali and water.

Olive and Other Oils

The abundance of olive trees in the Mediterranean made olive oil a natural choice to use in early Spanish soap making, according to notes accompanying a Smith College exhibition of castile olive oil soap from 2,000 BCE. Soap makers in other lands had used animal fat or tallow in their formulas. While traditional castile soap makers used olive oil alone, ingredient labels on products marketed as liquid castile soap today indicate that manufacturers use olive oil in combination with coconut, castor and other plant and vegetable oils.

Alkali

In a process called saponification, soap is produced from a chemical reaction between oil, an acid and alkali, an acid neutralizing substance. Early Spanish soap makers used ashes from the barilla plant to produce the reaction, according to the Smith College exhibit. Liquid castile soap makers today use potassium hydroxide as the alkali, Alicia Grosso wrote in "The Everything Soapmaking Book." Rather than listing potassium hydroxide or alkali, many liquid castile soaps list "saponified" oils on the ingredient label.

Essential Oils

For therapeutic benefits and to provide a fragrance some liquid castile soap formulas feature essential oils, which are concentrated plant extracts. Popular essential oils added to liquid castile soap varieties include lavender, peppermint, rosemary and sweet orange.

Water

Distilled, deionized or purified water also is an ingredient. The castile soap making process requires water to dissolve the alkali in solution to ensure a complete chemical reaction with the oils. Soap makers also use water to dilute the finshed soap base to produce the desired liquid consistency, Susan Miller Cavitch wrote in "The Natural Soap Book."

Preservative

Liquid castile soap formulas also feature vitamin E or grapefruit seed extract to prevent the batch from going rancid, as indicated on product ingredient labels and recipes in Cavitch's book.