Changing the Shape of Hair 158 Figure 1. Swatches hanging to test curl retention. hair after the hair either cools or dries.1 Cohesive set is quickly lost in water or even at high humidity.1-3 Permanent setting is achieved by first breaking disulfide bonds with reducing agents and then reforming them with oxidizing agents. Temporary or Cohesive Set Set in hair can be studied using either hair swatches or single fibers. Swatch hold is typically tested by curling swatches of hair either when wet or with heat and drying or cooling before removing from the curlers. The curls are then hung in a humidity chamber and the length is measured. A typical set up is shown in Figure 1. Curl retention is very dependent on relative humidity and heat set curls tend to fall out faster than water set curls.2,4 Diaz and Wong studied set relaxation in wet and heat set hair. The % set retention at time, t, was defined from the lengths of the hanging curls by the following equation: Eq. 1 Where: L = length of the fully extended hair swatch. L 0 = length when the relaxation starts (initial curl length) L t = length at time t.