EFFECT OF BILATERAL STRUCTURE ON KERATIN FIBERS 363 solubility test. Without going into the details, it may be stated that the alkali-solubility test is a reliable indicator of damage, when the fiber has been exposed only to a sulfuric acid treatment. If, however, the wool has been given a formaldehyde treatment preceding or following acid exposure, the alkali solubility of the wool is markedly decreased. This is shown in Fig. 5, where, for reasons that will be shortly apparent, the logarithm of alkali insolubility is plotted as ordinate versus time of exposure to sulfuric acid as abscissa. The acid concentration used was similar to one which would be used in commercial practice to apply an acid dyestuff to wool. All the acid treatments discussed here, are done at the boil. It is apparent from looking at the curves that a formaldehyde treatment, at least so far as alkali solubility is concerned, appears to prevent or repair damage, depending on whether the formaldehyde treatment precedes or follows acid exposure. Unfortunately, the effect is a spurious one, because the prevention or repair of damage is not borne out by the results of physical property measurements. That is, the flex-abrasion resistance of the treated fabrics and the properties of single fibers taken from the fabrics are not improved by formaldehyde treatment. The line drawn through the "H2SO4" points shown in Fig. 5 is a calculated one, obtained by a modified least-means-squares analysis of the experi- mental data, and represents an equation which is the sum of two exponential terms. The fitting of such an equation to the experimental data implies that the reaction process--the removal of protein from wool by alkali after a prior "sensitizing" by sulfuric acid--is a first-order one with respect to each of the halves of the cortex. The extrapolation of the latter, straight- line portion of the curve intercepts the ordinate axis at a value near 50 per cent, implying an approximately 50-50 split of the cortex between the ortho- and paracortex portions. The "rate constants" appearing in the equation that fits this curve are in the ratio of about 6 to 1, implying that the orthocortex is "sensitized" by the acid for subsequent removal by alkali at a rate about six times greater than that for the paracortex. This is verified by the photomicrographs shown in Fig. 6, which shows a 64's wool fiber (U.S. Rambouillet) after the fiber had been in acid for six hours and was subsequently exposed to alkali for twenty minutes under the conditions of the alkali-solubility test. A six hour exposure, it will be remembered, leads to an alkali solubility of about 50 per cent. From the photomicrograph of the fiber taken with ordinary light (Fig. 6a), it will be seen that approximately one half of the fiber has been removed by the alkali. This represents loss of orthocortex. The remaining paracortex is shown crossing from one side of the fiber to the other. The photomicro- graph of the same fiber taken with polarized light (Fig. 6b) confirms this observation and shows how the birefringence still exhibited by the fiber is a property only of the remaining paracortex.

364 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 6.--Photomicrograph of a single wool fiber (64's Rambouillet) treated 6 hours with H_oSO4, followed by 20 minutes in alkali (a) viewed in normal light, (b) in polarized light. It has already been discussed how curling in alkali, after an exposure to acid, is another manifestation of an asymmetric structure in the cortex. OTHER TYPES OF KERATIl• FIBERS Because other types of keratin fibers are known to have particular characteristics, such as cystine content and resistance to alkali, it was decided to investigate them to determine how these characteristics might be related to the ortho-paracortex model. To do this, it was decided to examine three other fibers in addition to wool. These were (1) a Buenos Aires fleece wool, (2) a sample of kid mohair and (3) a sample of human hair which supposedly had not been exposed to any permanent waving. The B. A. fleece wool was considerably larger in fiber diameter and had much less crimp than an apparel wool. The kid mohair sample was also somewhat larger in diameter than an apparel wool, and generally exhibited low crimp. As is well-known, human hair has a diameter much greater than wool and essentially no crimp. Being of such a nature that it usually does not curl readily, human hair is an excellent raw material for attempting reactions that will make it curl and, because of this, a profitable field exists for exploitation by people in the cosmetic industry. We have already seen how dye staining provides an indication that a wool fiber has ortho-para differentiation. Using such a procedure it was found, by examining cross sections of dyed fibers, that a B. A. fleece wool appears to show no ortho-para differentiation, when stained with either