CRACKING OF HUMAN HAIR CUTICLES 149 2.kx {}81 Figure 6. Typical combing abrasion pattern of hair with cracked cuticles. i.e., 10øC per minute up to 75øC, crack formation was almost nil. In contrast, if hair samples at 25øC were suddenly exposed to a temperature of 75øC, they immediately showed an increase in the number of new thermal cracks. Thus, high temperature rates cause cracks because they produce a rapid contraction of the top part of the cuticles while maintaining in a swollen state both the cuticle layers underneath and the cortex itself. Conversely, low temperature rates do not cause cracking because cuticles and cortex contract in a more synchronous manner. It is worth mentioning here that cracks similar to those produced thermally were also formed in fibers swollen with mixtures of strong swelling solvents and strong dehy- drating solutions. As is well known, formic acid is a strong swelling solvent because it is able to break hydrogen bonds and salt bridges not accessible to water and to cause higher levels of swelling in keratin fibers (19,20). In contrast, solutions of saturated NaCI are known to dehydrate the fibers (21). Experiments with mixtures of formic acid and saturated solutions of NaCI (50/50%), or mixtures of formic acid and glycerin (20/80%), were seen to produce similar cracks on the cuticles (see Figure 8). The cuticle cracks were observed to appear after ten hours of fiber immersion in these solutions. Also, it was observed that if a thin film of gold is deposited onto a clean hair fiber and then allowed to swell in water, that part of the cuticle covered with gold shows also vertical cracks somewhat similar to those produced thermally (see Figure 9). The ex- planation to these phenomena is straightforward, i.e., in both cases, with the swelling experiment and with the gold film, only a portion of the cuticle is restrained to expand during the swelling process of both the cuticle layers underneath and the cortex, and cracking occurs by the already advanced mechanism.

150 JOURNAL OF COSMETIC SCIENCE Average number of short longitudinal cuticle cracks per mm of hair 600 5OO 4OO 300 2OO lOO 20 30 40 50 60 70 80 90 lOO 11o 12o Air temperature (C) Figure 7. Plot of average number of cuticle cracks reproduced in the laboratory vs blow-drying tempera- ture. The number of applied thermal cycles was kept constant at 20 cycles, and the air temperature was measured at the level of the hair surface. Error bars represent one standard deviation about the mean. Figure 8. Cuticle cracks produced after immersing a hair fiber in a 50/50% solution of saturated NaCI and formic acid for ten hours.