700 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ical depilatories. The most commonly used have been metallic sulfide and aliphatic mercapto compounds (6-12). This paper will be concerned with chemical depilatories, rather than epilatories. Evaluation of the efficacy of depilatories has, for the most part, been qualitative or, at best, crudely quantitative. In vitro methods employed at these laboratories in the past were dependent upon the break time o[ mechanically stressed hair during treatment with the prep- aratiou under consideration. These methods suffer from two major dis- advantages: 1. Tests which rely on hair breaking serve merely to indicate the effect of the preparation upon the weakest point along the hair shaft, rather than upon the hair as a whole. Such a point is usually the thin- nest portion of the hair or some previously damaged region. Analysis of both these regions is unrealistic since depilatories are required to remove hair at the skin line, where the hair is usually strongest and thickest. 2. The application of sizable stress to the hair creates an artificial condition. No such stress will be employed when the preparation is actually applied. The choice of stress must therefore, of necessity, be arbitrary. Correlation of break point data for different stress situations will be difficult since no simple relationship exists. A simple and realistic procedure for quantitative determination of depilatory effectiveness is proposed. It will be independent of cuticle condition, break time, and in most cases provide a degree of precision that is at least an order of magnitude higher than that obtainable through use of break time data. PROCEDURE A ll)-cm hair, having a small weight at its bottom, is hung in a stand- ard taper capped test tube containing the solution of interest (Fig. 1) By means of a traveling microscope, the rate of swelling, both longitudi- nal and cross-sectional, is evaluated. The time at which the rate is maximized is taken after normalization as being inversely proportional to the efficacy of the preparation. The weight is nominally about 1 g and serves merely to keep the hair taut, thus facilitating optical measurement. The stress imparted to the hair by this weight is minimal and will not affect longitudinal swelling in nondepilating solutions. The extension observed for hair in distilled water under these conditions is less than 1% in 24 hours. A good corn-

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