Castor oil is a common ingredient in most handcrafted soaps. It is pressed from the seed of the castor plant (Ricinus communis), which is native to the Mediterranean basin, East Africa, and India but has spread to tropical regions around the globe. The castor plant is a fast-growing shrub which can reach the size of a small tree (30 feet). Castor oil is clear, colorless and extremely thick. The oil is also used in the food industry, medicine, a commercial lubricant, and in some biodiesel.
When it comes to soap making castor oil is a unique ingredient. In fact, you really can't substitute another oil for it because it is almost entirely composed of ricinoleic acid (85-95%), a monounsaturated, 18-carbon fatty acid. Among fatty acids, ricinoleic acid is unusual in that it has a hydroxyl functional group on the 12th carbon. This functional group causes ricinoleic acid (and castor oil) to be more polar than most fats. The chemical reactivity of the alcohol group also allows chemical derivatization that is not possible with most other seed oils. Say what??? Translation: i
Say what??? Translation: in cold process soap, castor oil contributes to large bubbles and is known for its cleansing properties. Castor oil can also work to draw moisture into your skin. I only use a small amount of it in my soap. If you use more than 10% in soap the resulting bar will have a sticky feel.
Sources: Wikipedia, SoapQueen.com. If you really want to get your soap geek on be sure to check out Kevin Dunn's "Scientific Soapmaking".