HAIR VOLUME MEASUREMENT 89 The absolute average value of the fiber diameter can be obtained from the volume of the tress, if the length and the number of fibers in the tress is known. II. EFFECTS OF HUMIDITY ON THE HAIR VOLUME The effect of relative humidity on the volume of virgin hair at 21øC is shown in Table III. The percent weight gain and the density of hair as a function of relative humidity are listed in Tables IV and V, respectively. Since keratinous fibers are not uniform, Table IV % Gain in Hair Weight at Various Relative Humidities a Percent Weight Gain at RH (%) Tress Number 12 44 65 75 85 1 2.86 6.79 12.26 15.20 17.82 2 3.41 7.92 13.13 16.26 18.61 3 2.71 6.00 11.72 15.60 17.33 4 3.07 7.56 12.39 15.86 17.98 5 3.37 8.12 13.20 16.44 18.60 6 3.12 7.35 12.57 15.57 18.07 Average _+ S.D. 3.09 _+ 0.28 7.29 _+ 0.78 12.55 _+ 0.56 15.82 _+ 0.46 18.07 _+ 0.49 aTemperature was 21øC Table V Density (g/cm 3) of Hair in Various Relative Humidities Tress Density of Hair at RH (%) Number 0 12 44 65 75 85 1 1.32 1.32 1.34 1.35 1.32 1.31 2 1.32 1.32 1.35 1.38 1.32 1.33 3 1.32 1.32 1.33 1.35 1.32 1.32 4 1.33 1.33 1.34 1.37 1.33 1.33 5 1.33 1.33 1.34 1.38 1.33 1.33 6 1.33 1.32 1.34 1.37 1.32 1.31 Average _+ S.D. 1.32 + 0.005 1.32 + 0.005 1.34 + 0.006 1.36 + 0.015 1.32 + 0.007 1.32 + 0.01 small differences in the values obtained by others are expected. The absorption isotherm based on weight and volume change are shown in Figure 2. The sigmoidal shape of the isotherm based on the weight gain is in good agreement with the previous study. (1) Figure 3 compares the percentage change of hair and wool fibers' diameters as a function of water up-take. For the hair, the percentage change of diameter was calculated from data in Table III using Eq. 1. For the wool, the data were taken from the literature. (6) Very similar changes were found between these two fibers. To ascertain that the drying process did not alter the water absorption property of the fibers, the experiment at 65% RH Was repeated twice. The results were the same.

90 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 3O 0 / .'" •' lj ß / ...' •. .,,• ,.:...::,,:.: ...... 40 60 •0 100 noet.A-rtvoe •UMtUt'ry (%) Figure 2. Absorption isotherms of virgin hair fiber at 21øC. ß represents weight and ß represents volume. 2O 10 I I I lO 20 3o WEIGHT GAIN (%) Figure 3. Comparison of percentage change of hair and wool fibers' diameters as a function of water up-take. ß represents hair fiber and • represents wool fiber.