Chapter 5 175 Where F re /F 0 = The ratio of the static force after reduction to the static force before stress relaxation and ΔF ro /ΔF 0 = The ratio of the dynamic force after oxidation to the dynamic force before treatment. This equation tells us that set will be proportional to F 0 /F re , the fraction of bonds broken during reduction, and ΔF ro / ΔF 0 , the fraction of bonds reformed during oxidation. Thus oxidation is necessary for permanent set as reported by Albrect and Wolfram.28 Set recovery of hair subjected to the same treatment protocol was measured in extension by determining the new equilibrium length. Bending set recovery was measured by treating the hair on a small cylinder of known diameter D c , then measuring the diameter, D, to which the hair loop opened in water. Recovery, R, was measured according to: R = 1-D c /D. The authors found little set in extension, and set in bending was actually better than predicted by Eq. 8. Wortman and Kure54 used the balanced fiber method of Robbins55 to measure bending stress as it changes during reduction and reoxidation. They found that bending stiffness decays more rapidly than tensile stress in reducing solution and hypothesized Figure 13 Method of Wortman and Souren. After stress relaxation reducing agent is added at time t 1 . Oxidation occurs from t 3 to t 4 . Reprinted with permission of the Society of Cosmetic Chemists.53