Mechanical Properties of Hair 230 necessary to measure the diameters of the hair in order to accurately assess the effects of treatments. A disadvantage is that it is necessary to keep track of each individual hair and it may be more difficult to perform realistic treatments. If the cross-sectional areas of the fibers are measured, the Young’s modulus, yield stress, and breaking stress can be determined, significantly improving the results without using each hair as its own control. Combining this approach with use of large numbers of hairs is becoming more common because of the above-mentioned automation. Wolfram et al.36 reported effects of bleaching treatments on the mechanical properties of both wet and dry hair. Treatment was with 3% H 2 O 2 at pH 10 with a temperature of 35°C for 30, 90, and 180 minutes, respectively. Results from Table IX of this study are plotted in Figure 5 after recalculation as percentages of untreated values. Note that even 180 minutes of treatment did not affect the dry yield stress (5A) and dry breaking stress (5B) was unaffected by 30 and 90 minute treatments, while all treatments had significant effects on the wet yield and break stress. This is in complete agreement with this author’s experience and that of many other workers that dry mechanical properties in general are far less Figure 5. Effect of bleach treatment on wet and dry hair mechanical properties calculated from data in Reference 39