QUANTITATIVE STUDY OF DEPILATORIES UPON }fAIR 7111 . :: .:..x.x.:...: ... Figure 1. Apparatus for the determination of hair swelling mercial depilatory will exhibit approximately 50% extension in under ten minutes. If a differential plot of swelling per unit time rs. time is made, then the times of rate maximum Td and Tz are clearly defined (Fig. 2). They are seven minutes for Td, the time at which the rate of cross-sectional swelling is a maximum, and ten minutes for Tz, the time at which the longitudinal swelling is maximum. DISCUSSION Inherent in the above procedure are several points of potential diffi- culty. Consideration and compensation for these factors will appre- ciably facilitate the analysis. In the determination of Ta, the question arises: Which point along the length of the hair, and in what orientation, should the rate of swdling be determined? Figure 3 shows the rate of swelling of four equally- spaced positions along a length of hair. No attempt was made to orient

7O2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2. CROSS SECTIONAL / / ! i _..•-,--'/[ 2 4 I0 12 14 16 IS 20 22 24 26 28 TIME, MIN. Cross-sectional and longitudinal swelling in depilatory solution .014 • .013 z _ • .Ol2 ,,, .011 0 • .010 .009 ------J. o o. 5 1.0 1.5 Z.O Z.5 3,0 3.5 4.0 4.5 5.0 TIME, HRS. Figs/re ,2. Cross-sectional swelling at different positions along a hair length the hair. The orientation, however, remained consistent throughout the determinations. The swelling behavior is typical of randomly chosen hair segments, varying according to the conditions of the hair at the particular points being observed. From these curves it is evident that 2',• to be meaningful must be an average value. The hair shaft resembles a tapered ellipsoidal column. Its diameter will consequently be a function of position, as will also be T• and Tt. and 2'• in order to be meaningful must each therefore be "normalized" to make them independent of hair diameter. The normalization process may be simplified if the following precautions are taken: