In cosmetics and personal-care products, fatty acids and mixtures of fatty acids such as Stearic Acid, is used in a variety of cosmetic creams, And soaps. Stearic acid is a very common amino acid is used in the manufacturing of more than 3,200 skin and hair care products sold All Over the World. Stearic Acid - Traditionally used as a thickening agent in lotion, this vegetable derived waxy substance is also used as a hardening agent in soaps (at a. 5% of your oils as a usage rate). Stearic acid is also used as a hardening agent in candles, vegetable or paraffin based.