16 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS A product having a pH of approx- imately 6.8 when packaged, after six months or a year or more at room temperature, might change to a pH of 8.5 or 9 and definitely show flakes. The smaller the container, the greater is the relative change, for the change depends on the sur- face-to-volume ratio. A 2 oz. bottle will show more alkali extract than will a 32 oz. container, since there is more surface with respect to the contents in a smaller container. Glass with poor or low chemical durability will "weather" or show crystal growth on the inside surface when exposed to damp or humid conditions of storage before use. This crystalline growth has been considered as a mold growth by some observers, though culture tests and examination under a' microscope definitely indicate that it is a condi- tion on the glass surface due to the glass itself, and not due to mold. A dilute acid rinse has been used in the past to reclaim such containers, though glass of good chemical dura- bility, low in alkaline extract, will not show the weathered condition. Acid products do not show any attack and only a slight change in pH when packaged in glass of poor chemical durability, so it is possible to introduce organic acids such as malic, to lower the hydrogen ion concentration, and thus prevent the formation of flakes or p.recipitates due to the glass. Borosilicate glasses are not alka- line in nature, consequently have good chemical durability. They are about five times more expensive than the better commercial glass containers and their use is indicated only for special biological and pharmaceutical use. The three major factors in pack- aging have been discussed in some detail with special reference to the container and the closure. It is apparent that there is a very close relationship of these three fac- tors and each should be considered in relation to the other two. Best results can be obtained by close cooperation between the technical staffs of the customer and the producer of the closures and con- tainers.

ZINC OXIDE IN FACE POWDER* By L. D. GRADY Deve/opment Eninering Division Technca/ Departmen, The New •7ersey Zin Compan (of Pa.), Pa/merton, Pa. INTRODUCTION ACCORDING to definition, face powder fulfils the useful and orna- mental purpose of altering the appearance of the face to conform to the current criterion of beauty. Face powder is sold because it re- places shininess with a delicate sheen and at the same time covers up sur- face imperfections in the skin. In addition, of course, face powder lends a bit of color and fragrance. Ingredients which are to be used for this purpose must fidill certain preliminary requirements. U.S.P. zinc oxide rafters these initial de- mands because it has a bright white color, fine texture, is unaffected by exposure to light and is not toxic or irritating to the skin. COVERING POWER Beyond the above characteristics, satisfactory covering power is prob- ably first in importance. Face pow- der is called upon for a certain de- gree of camouflage. High covering power is not the criterion--it is how well the powder blends in with the * Presented at the May 15, 1946, Meeting, New York City. surroundings rather than how well it covers. Too much covering power is worse than too little because of the unnatural effect which it creates. The measurement of covering power has been a controversial sub- ject for a long time. By varying the test conditions, the pigment concen- tration or the method of measure- ment one can obtain widely different values for the relative covering power of pigments. Tabulations of such covering-power values have been. published in journals of the paint industry, the paper industry, and the cosmetic industry. No ex- perimental data will be presented in this paper but, in order not to avoid the subject completely, values for covering power have been calcu- lated. In order to compare the covering power of several face powder in- gredients, calculations have been made based upon the refractive in- dices of the pigments and various media in which they may be used for cosmetics. Fresnel's law (1) states titat the reflectivity of an absorbing medi•m is proportional to the square of the difference in refractive index between the two components 17