20 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the percentage relative covering power in changing froin an air to water and air to petrolatum med- ium. As face powder absorbs moisture and oil froIn the skin, its covering power decreases. Under these' con- ditions the high-refractive index pig- ments will lose proportionately less opacity than the materials of low- from fihns which were only a few particles thick (.00092 min.) and should be applicable to face powder considerations. Zinc oxide cuts off the ultraviolet more sharply than any other white pigment used in face powders. It is generally recognized that erythema is produced most rapidly by ultra- violet varying in wavelength from TABLE 3--ULTRAVIOLET TRANSMISSION CHARACTERISTICS OF PIGMENTS Per Cent Transmitted of Wavelength Pigment '4358 4047 365• ' 3342 3131 3023' Zinc Oxide 46 40 0 0 0 0 TiO2 35 32 18 6 1/2 0 China Clay 63 61 59 57 55 54 Chalk 87 86 84 82 80 79 Talc 90 90 90 89 88 87 refractive index which are used in face powder primarily for their absorbing characteristics. The ex- act proportion of ingredients used to satisfy the demands of covering power, absorbency, slip and adher- ence is, of course, the decision of the cosmetic chemist. ULTRAVIOLET ABSORPTION Zinc oxide may be of value in face powders because of its protective filtering action in absorbing ultra- violet from sunlight. Its protective action in prolonging the-life of ex- terior paint by delaying breakdown of the organic binder is well known and perhaps it applies here to a more limited extent. Data are availabl• showing the percentage of light transmitted by various pigments at various wavelengths through the ultraviolet spectrum (3). The val- ues shown in Table 3 were obtained 2950-3150 Angstrom units. Zinc oxide and, to a lesser degree, titan- ium dioxide, should be quite useful in preventing severe sunburn. It is pointed out in de Navarre's book (4) that investigators have found longer wavelength ultraviolet pro- duces specific effects upon the skin. Although these longer rays produce effects which are much less severe than sunburn, it seems pos- sible that the greater absorption o• zinc oxid¾ for these longer ray• might be advantageous. This i• hypothesis without proof. MEDICATIVE VALUE One of the intangible values o• zinc oxide for cosmetics may lie iri that property which makes it valu• able in pharmaceuticals (5). It is used in ointments as a mild local sedative and protective agent ant has slight antiseptic and astringent

ZINC OXIDE IN FACE POWDER 21 qualities. Its fungistatic action has been utilized by the paint and tex- tile industries as well as by the cos- metic industry. Zinc oxide is sooth- ing to the skin. On the hardness scale it lies between chalk and titanium &oxide (6). ('HEMICAL ('HARACTERISTiCS Chemically, zinc oxide is amph, -- teric, forming salts with both acids and alkalies. A water slurry of U.S.P. grade zinc oxide has a pH about 7.3 which classifies :t as a mildly alkaline material. Although there is considerable controvers) re- garding the effect of pH upon skin health, it would seem that a mildly alkaline material to neutralize acid exudations would be beneficial• Certainly some such neutralizatiot action must account for the deo& i' izing qualities long associated with zinc oxide in cosmetic usage. Zinc oxide is a highly water in- soluble solid even in its finely divided state only .00,5 grain dis- solves in a liter of water at 25øC. Chemical impurities are kept at a minimum in the U.S.P. grade. The U.S. Pharmacopoeia specifies rela- tiveiv low amounts of impurities in U.S.P. zinc oxide and our product is designed to be appreciably lower than these limits. PHYSICAL STATE In addition to the foregoing it is probably the finely divided state of zinc oxide that makes it the out- standing face powder ingredient. Zinc oxide is an air-born product. Its particles are formed separately and stay separated throughout their processing. It is a dry process-- there is no cementation of particles together on drying as in the case of wet precipitated pigments. Dry milling can never quite bring pre- cipitated pigments to as fine a de- gree of air dispersion as a pigment ,•hich has never been. wetted and dried during the course of its manu- facture. Zinc oxide and other fume products possess characteristics in the dry state which no wet-processed pigmen,• attain. It is this quality of air dispersion that probably accounts for the greater ease of blending and tinting of zinc oxide. REFERENCES (1) R. \V. Wood: "Physical Optics," pages 411, 512 (1934). (2) Stutz a,nd Pfund: Industrial & Engi- neering Chemistry, 1% 1, 51 (1927). (3) Stutz: Journal Franklin Institute 202, 89 (1926). (4) M. G. de Navarre: "Chemistry Manufacture of Cosmetics" (1941). (-3 Merck's Index, 5th Edition (1940). (6) Handbook of Chemistry & Physics, 28th Edition (1944). (7) Hegsted McKibbin and Drinker: "The Bi½logical, Hygienic and Medical Proper- ties ,,f Zinc and Zinc Compounds," Supp. 179 t• the Public Health Reports (1945).