Chapter 5 161 hairs tend to take a set better. Robbins reported that initial curl was related to fiber thickness with thicker fibers giving better curl,5 and we also found that the initial set in single fiber curls depends on fiber diameter, unless setting times are extended to 24 hours or more.8 The likely cause of this effect is that for set to occur stress relaxation must occur,9 and the rate of stress relaxation depends on the level of stress.10,11 As can be seen in Figure 9 of Chapter 7 of this text, a thicker hair will have a higher level of stress in the outer and inner parts (zones of extension and compression) of the fiber when wrapped around a curler compared to a thinner fiber. This is true of both bending and torsional strains. The strength of the resulting curl is inversely proportional to the diameter of the curl (Eq. 2). Thus, a thicker hair will produce a stronger coil for two reasons: the strength at a given geometry will be greater and the set will better produce a tighter, stronger coil. This effect holds for curls made from swatches of hair as well. Yin et al.12 made swatches from hair of different average diameters and found that swatches made from fine fibers (average diameter = 65µ) had less initial set and lost set much faster compared with swatches made from coarser fibers (average diameter = 76µ or 89µ). This may explain why fine hair tends to be straight and have little “body.” Set in helical coils can be evaluated by several different methods,5 including directly measuring their spring constants with an electrobalance. Perhaps the most straightforward method is to hang the coils and measure their geometry after removing them from the curling rod.7 Coil length can be measured similarly to swatch length (Figure 1) or alternatively both bending and torsional set can be determined from geometrical measurements. The coil angle α (Figure 3B) at any time t, can be found from the relationship l(t) = Lsin(α(t)) where L is the length of the hair. The total angles of bending and torsion per unit length of hair at time, t, can be shown to be: