Castile soap is a name used in English-speaking countries for soap made with olive oil originating from the Castile region of Spain.

Olive is a highly valued tree & commodity in Islamic lands to this day. When the Islamic civilization ruled Spain, the highest level of cultural sensitivity & cleanliness was maintained. Muslims believe that ‘cleanliness is part of faith’. In order to worship their Lord, they needed to maintain states of ritual purity at all times. For this, cleansers were made of which the “”Castile Soap”" was one such fine apparatus that is still in much use today.

Importations of “”Castile soap”" through Antwerp appear in the London port books of 1567–68 (Dietz 1972), though the OED has no references to “”Castile soap”" earlier than 1616. In his article “”A small history of soap”"

, John Hunt maintains that barilla (an impure form of sodium carbonate obtained from plant ashes) 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 hard soap, which hardened further as it was aged, without losing its whiteness, forming jabón de Castilla. To an apothecary it was known as sapo hispaniensis or sapo castilliensis.

Another theory is that modern-day castile soaps are conceptually similar to, and most likely derived from the so-called white soap that has traditionally been manufactured in Northern Italy since the early 1600s. From Venice, where the oldest white soap factories were established, olive oil-based soap reached the Greek island of Crete first, and Southern France (Marseille) at a later stage.

The Muslims in Europe maintained a high level of cleanliness. Their cities were clean, well lighted & attracted people from all over Europe to their well-known universities. To maintain this heightened awareness of Clean Living, soaps were invented. Marauding crusaders marveled at the Islamic splendor and soon took these wonderful creations to other parts of Europe. Until 19th Century this was the most taxed item.

Production of soap was widespread in Muslim lands during the Medieval period. Soap is made by mixing oil or stout with an alkaline substance (the Arabic word al-kali is the origin), made from ashes of certain plants. Emulsified, the mixture turns into solid soap. Castile soap from Spain became well-known, and the Crusaders learned soap-making centers in the eastern Mediterranean lands. Olive oil became the preferred type of oil for soap in Al-Andalus and around the Mediterranean. Castile soap is still sold in stores”