THERMAL TREATMENTS WITH A CURLING IRON 19 cells that had been in direct contact with the hot curling iron. This mutilation or disintegration of the surface cuticle cells becomes especially pronounced at the scale edges of hair fibers suspended under the 30-g load (Figure 4d). However, it may be argued that the debris deposited at the scale edges, seen in Figure 4d, may be a combination of extraneous deposits, scale edge fragments, and endocuticular material squeezed from underneath the cuticle cell during the prolonged thermal treatment under considerable pressure. Severe radial cracking (Figure 46, c) is observed as well. This study suggests that the contact time of ten minutes between the hair fiber and the heated metal surface of the curling iron is much too severe a condition for thermal treatments. Such conditions lead to modification of the cuticula (embrittlement), re­ sulting in severe radial cracking of the cuticular sheath during gentle uncurling of the hair fiber in the dry state. There is an important feature of this experiment that should be pointed out. The thermal treatments (up to 200) of this study were inflicted upon the same set of surface cuticle cells, which is an unlikely scenario in standard grooming practices. During everyday grooming, the hair is shampooed/conditioned, dried, combed, and brushed between thermal treatments with the curling iron. This distributes the damage over many fibers, whereas in our experiment, damage is concentrated in the same cuticle cells. REPEATED, CYCLIC CURLING OF WET HAIR Wet hair fibers exposed to 20, 50, and 100 cycles of heating (ten seconds), followed by 30 seconds of immersion in water and blotting, consistently showed the same type of damage, irrespective of the numbers of cycles. This "typical" damage is shown in Figure Sa-d, e-f, and g-j, respectively, for 20, 50, and 100 cycles of wetting/curling. The cuticular damage observed in the curling of wet hair was characteristically different from that observed in the case of dry hair. This new damage phenomenon is seen in the form of bulges or bumps in the scale faces and ripples or "half-domes" at the scale edges. These cuticle distortions are caused by the hygrothermal "fatiguing" of the wet cuticle. Both the bulges/bumps in the scale faces and the ripples at the scale edges are most likely created by the explosive release of steam trapped in the wet cuticula during contact with the heated metal of the curling iron. The ripples at the scale edges appear like scale lifting however, they are very localized and restricted to the delamination of very small areas at the scale edge. These bulges/bumps and ripples occur all around the hair fiber and are not confined to the fiber/metal interface. They are often accompanied by axial cracking of the cuticula (Figure Se, f). The fiber/metal interface is primarily character­ ized by various levels of cuticular compression and disintegration. Except for a few deeper radial cracks, axial cuticular cracking phenomena observed in these fibers appear to be confined to the individual surface cuticle cell without propa­ gating into the underlying layers of the cuticular sheath, or spreading axially. We would like to emphasize that an increased number of cycles of wetting/curling slightly increases the level of damage, but not the type of damage. PROLONGED, HIGH-LOAD CURLING OF WET HAIR Thermal damage from prolonged (ten-minute) curling of the wet fiber under different

20 JOURNAL OF COSMETIC SCIENCE Figure 5. Hair fibers exposed to 20 cycles (a-d), 50 cycles (e, f), and 100 cycles (g-j) of wetting (30 seconds) and thermal treatment (10 seconds). loads produced damage significantly different from that observed in the case of the dry fiber. In addition to compression, disintegration, radial cuticular cracking, and scale edge fusion of the surface cuticle cell observed earlier at the f/m interface (Figure 6a, b), we also observed fine-line cracking scalloped around the fused scale edges (Figure 6c). This