PERMANENT WAVING AND PERM CHEMISTRY 111 A quick indication of the reaction rate can be obtained by borrowing a concept from the fi eld of heterogeneous reaction kinetics (25), that is, we evaluate a halftime (t0.5) which equates to the period during which the initial stress is reduced by 50%. Figure 6 illus- trates a halftime of around 3 min for this specifi c sample under these particular condi- tions. Figure 7 shows a Box and Whisker plot for halftimes associated with the reaction of single-source hair fi bers with 0.42 M, pH 9.2 solutions of both ATG and cysteamine. Results indicate the presence of signifi cantly faster reaction rates (i.e., shorter halftimes) for reduction with thioglycolate. This outcome is consistent with the aforementioned theory (and indeed real-life observations), whereby thioglycolate is recognized to be a stronger reducing agent. Continuing with the theories outlined earlier, manipulating the pH of a perm solu- tion changes the concentration of the active thiolate ion species [RS-] and would therefore also be expected to infl uence the rate of transformation. Figure 8 shows SFTK results for single-source hair in contact with 0.42 M cysteamine solutions of varying pH and indicates the presence of faster rates with increasing solution pH. These fi ndings are again in line with expectations and consequently help build con- fi dence in the method. Similar experiments involving changes in the thiol concentra- tion also gave rise to predicted responses. As such, despite various concerns outlined earlier pertaining to questionable underlying assumptions, the SFTK method does indeed hold up to validation studies, in that predicted outcomes are obtained from systematic variations in perm solution properties. Figure 7. Halftimes for hair exposed to 0.42 M, pH 9.2 ATG and cysteamine.

JOURNAL OF COSMETIC SCIENCE 112 S IGNIFICANCE OF HAIR TYPE H air samples and tresses provided by various suppliers for use in in vitro experiments usually consist of blended hair that is obtained from a selection of individuals. Initial SFTK testing using this hair source yielded a high standard deviation and poor reproducibility. However, this issue was overcome by switching to single-source hair. Specifi cally, the results shown ear- lier were obtained using hair donated by an Asian female who cut off a relatively long ponytail. I n retrospect, this fi nding is not necessarily surprising because differences in perming ability as a function of hair type have been noted since the advent of the so-called cold wave process (26). Initially, these differences were speculated to be a function of hair di- mensions (i.e., fi ne, medium, and course) however, this appears to be an oversimplifi cation. Nonetheless, Table I shows comparisons involving this Asian hair and a second single source of fi ne Caucasian hair on reaction with comparable thioglycolate and cysteamine solutions. Clearly, the fi ne hair (which traditionally might be expected to be more resistant) Figure 8. Effect of pH on the reaction rates of a 0.42 M cysteamine solution with hair. Table I Halftimes for Two Single-Source Hair Types Tested with 0.42 M, pH 9.2 Ammonium Thioglycolate and Cysteamine ATG (min) Cysteamine (min) Asian hair 3.3 5.4 Fine Caucasian hair 8.3 16.8