Castile soap originated in Spain, although the name was applied to any hard, white soap based on olive oil. Castile is a mild, emollient soap that has been used for everything from bathing to washing hair and teeth and cleaning the house. Castile soaps are moisturizing and have been favored as a luxury item by kings. Unlike soaps made from animal fat or tallow, genuine Castilian soap is made with olive oil, which makes the soap very creamy. Originally, Castile soap was made with 100 percent olive oil. This results in a soap that foams only slowly, so that variations have been developed with up to 80 percent olive oil and other oils, such as coconut oil or palm oil, which make up the other 20 percent.
Any variety of olive oil can be used for soap, although some people prefer cold-pressed oil over oils that are extracted by chemical methods. Chemical extraction does not affect how the oil behaves in the soap, but some people prefer an organic product.
Soapmakers use distilled water to avoid minerals and impurities in the water that discolour the soap.
Lye, which is mixed with water and added to the oil, causes saponification, a chemical reaction that gives soap. Lye or sodium hydroxide is highly corrosive and should be handled with care. Keep out of the reach of children and wear eye protection and protective clothing when handling. Alkalis are sold in hardware stores as flakes or pellets, in a dry powder or as a liquid.