Castile, or olive oil soap, is traditionally a hard white soap made from pure olive oil and originated in the medieval province of Castile in what is today known as Spain. In comparison to other available soaps, olive oil soap became an important export for medieval Castile and was considered to be a great luxury item by 13th-, 14th- and 15th-century European nobility. The key ingredient of Castile soap is pure olive oil that is boiled with sodium carbonate made from the ashes of local Castilian plants instead of the then more common tallow or lard. Olive oil soap is repudiated to be soft and gentile on skin and hair and less likely to irritate those who are sensitive to perfumes and dyes. In addition, olive oil soap is believed to have less of an environmental impact than conventional soaps due to its ingredients and manufacturing process.
Soap made from pure olive oil is a possible option for people with sensitive skin or allergies to perfumes and dyes. Olive oil is believed to be an effective moisturizer for any skin type, and although some manufacturers add perfumes, honey or other oils traditional Castile soap bars are made with few ingredients. These include olive oil, water and an alkali like sodium chloride. Since traditional Castile soap does not contain animal ingredients or synthetic perfumes or dyes, it is often considered to be a sustainable, cruelty-free and environmentally friendly product.
Castile soap bars are sometimes balanced by including palm or coconut oil in addition to olive oil. Palm and coconut oils can produce a lather that is bubblier than the traditional creamy lather produced by bars made exclusively with olive oil. Fragrances or dyes may also be added. Liquid or bar olive oil soap can be made at home and has multiple uses beyond cleansing skin. Olive oil soap can be used as a shampoo while the liquid version is useful as a laundry detergent.
While soap today is commonly made from any animal or vegetable fat that is treated with an alkali-like sodium hydroxide, early medieval soaps were often made with ash as a makeshift alkali and lard resulting in what was called black soap or speckled soap. Olive oil soap produced in Castile was sold throughout medieval Europe at prices at least three times as high as those for common versions. Only a few families controlled olive oil soap production in medieval Castile and the profits bought them privileged social status in some cases. Castile’s monopoly on olive oil soap was eventually broken as the manufacturing process became known and spread throughout Europe beginning in Italy.