Chapter 5 159 Figure 2 shows curl set retention from water set and heat set (20 minutes at 115°C) hair. The water set hair retained its set much better than heat set hair, and, as expected, set was lost more quickly at 90% RH than at 65% RH. Swatch hold will be affected by both the single fiber properties and surface interactions between hairs, since hair fibers will have to move past one another for the swatch to uncurl. To study effects on fiber set retention independent of fiber-fiber interactions, single fiber methods must be used to measure set. A useful method for studying the setting behavior of single fibers is the helical coil method described by Robbins.5 This method is based on Bogaty’s observation that the geometry of hair coiled in a curl is closely related to that of a helical spring.6 We have applied the helical coil method by winding hairs from water onto Teflon rods or by winding them directly on a heat curling iron at a fixed angle with the temperature controlled by a rheostat.7,8 The method of winding a helical coil of diameter D, at pitch angle α, and the resulting coil, after removal from the rod, are shown in Figure 3. A small hook is fastened to the root end and attached to the rod with a rubber band (not shown). The weight, W, Figure 2 Swatch set retention for water and heat set hair. Modified from Diaz and Wong2 with permission of the Society of Cosmetic Chemists.