EFFECT OF CURLING IRONS 233 90 85 80 75 70 65 125 • Intermittent [] Continuous 135 145 155 165 Temperature (øC) Figure 5. Comparison of Trp decay for fibers exposed to several curling iron temperatures for 3-min under continuous conditions and intermittent conditions (15-s intervals). the excitation wavelength of 320 nm, the ratio I465/I42 o increased from 0.79 to 0.95, while the corresponding change for the spectra obtained at an excitation wavelength of 450 nm increased from 0.95 to 1.04. These results suggest a shift, in favor of the

234 JOURNAL OF COSMETIC SCIENCE formation of 1-kynurenine, that could also be responsible for the yellow coloration of thermally treated unpigmented hair. Similar variations in emission intensities, as in white hair, can also be observed in Piedmont hair. Somewhat different spectral changes are observed in other types of hair such as bleached (Figure 6), light-brown Caucasian (Figure 7), and Asian (Figure 8). For bleached hair, the Trp emission appears at 345 nm and is reduced as a result of thermal treatment. The longer wavelength emission band is narrower than that corresponding to white hair and appears as a superposition of two peaks with maxima at 433 and 450 nm. The intensity of this band also decreases, without a change in the ratio of 145o/1433, as a result of treatment with curling irons. It should also be mentioned that bleached hair becomes black-yellow colored as a result of thermal treatment, a result discussed in the following section of this paper. As in the case of unpigmented hair (white, Piedmont, or bleached), the Trp emission appearing at 335 nm also decreases significantly for pigmented hair (Figures 7 and 8). However, the longer wavelength emission intensity, corresponding to kynurenines, revealed an increase rather than a decrease. The extent of change is clearly evident for light-brown hair, which has also shown a small increase in the ratio of 145o/1433. Similar trends, with a smaller magnitude of change, can also be seen in the spectra obtained for Asian hair. The difference in behavior of thermally degraded pig- mented and unpigmented hair is similar to that observed in irradiated hair, which has also shown increases in the emission intensities for bands at 420 and 450 nm (26). To explain this, it was suggested that melanin provides a selective photoprotection to kynurenines, or that bleaching results in higher absorption and, consequently, fluores- cence emission by hair chromophores. Both of these explanations do not hold in the case of thermally degraded hair. An additional argument may invoke an antioxidant or free-radical scavenging nature of melanin, which may contribute to the retardation of both the photo and thermal degradation of kynurenine chromophores. COLOR ANALYSIS One of the clear manifestations for the effect of high temperature on unpigmented hair is the resulting change in color. As illustrated by Figure 9, white and Piedmont hair become perceptibly yellowish even after 2 min of treatment at 164øC. Bleached hair acquires both yellow and dark-brown or black coloration after similar treatment. Table II presents the results of Hunter colorimeter measurements (tristimulus L,a,b values) for white, Piedmont, bleached, light-brown Caucasian, and Asian hair treated at 164øC for a total of 30 min. The data are presented as differences in readings between the thermally treated and untreated sections of a hair tress, and also include calculated parameters such as the total color change (AE), chromaticity change (AC), and change in the yellowness index (AYI). The total color change, AE, was the largest for unpigmented fibers, i.e., bleached, Piedmont, and white hair. Pigmented hair, light-brown Caucasian, and Asian hair experienced only small color changes. The largest contributing factors to AE were the variations in lightness, L, especially in the case of bleached hair, which becomes dark- colored after the application of curling irons. The reason for the dark-brown or black color formation in bleached hair may be attributed to thermally induced condensation of melanin fragments, produced during bleaching by reaction with hydrogen peroxide.