- Stearic acid is one of many fatty acids that occur naturally in various plants and animal derivatives. It’s found in such products as animal tallow, cocoa butter and vegetable fats. When it’s used in cosmetic products, stearic acid primarily fulfills the role of a thickener or hardener. Stearic acid is the substance that helps your bar of soap retain its shape — just as it does in products such as candles, oil pastels and hard candies.
Source: health.howstuffworks.com/skin-care/cleansing/products/stearic-acid-in-skin-cleansers.htm