Castile soap originated in Spain, though the name came to refer to any hard, white, olive-oil based soap. 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 were considered a luxury item, favored by royalty. Unlike soaps made with animal fat, or tallow, true Castile soap is made with olive oil, which makes the soap very creamy. Originally, Castile soap was made with 100 percent olive oil. This produces a soap that is slow to lather, so variations were developed with up to 80 percent olive oil and other oils, such as coconut oil or palm oil, making up the other 20 percent.
Any grade of olive oil may be used for soap, though some people prefer cold-pressed oil over oils extracted through chemical methods. Chemical extraction will not effect how the oil behaves in the soap, but some people prefer an organic product.
Soap makers use distilled water to avoid minerals and impurities in the water that discolor the soap.
Lye mixed with water and added to the oil causes saponification, a chemical reaction that yields soap. Lye, or sodium hydroxide, is highly corrosive and should be handled with care. Keep lye out of reach of children, and wear eye protection and protective clothing when handling it. Lye is sold at hardware stores as flakes or pellets, in a dry powder or as a liquid.