NACREOUS AND INTERFERENCE PIGMENTS 163 7- Figure 1. Reflection of light by nacreous pigment platelcts in fihn. S, specular reflection T, transmission D, diffuse reflectance ment is determined by the relative magnitudes of specular reflectance, dif- fuse reflectance, and transmittance, and by the color characteristics o[ each. Color, which is very subtle in the "white" nacreous pigments, is the chief property of the interference pigments. S and D can be measured with a goniophotometer, an instrument in which reflectance is measured as a function of angles of illumination and viewing. Hunter (4) has demonstrated the utility of the goniopho- tometer in the measurement of gloss. Color is, of course, measured by a spectrophotometer. An instrument which would be useful for measur- ing both the directional character of nacreous reflection and the color of interference pigments is a combination goniophotometer and spectro- photometer. No commercial instrument is at present completely versa- tile as both a goniophotometer and a spectrophotometer Hemmendinger and Johnston (5) and Billmeyer and Davidson (6) have adopted the term "goniospectrophotometer" to describe a spectrophotometer with gonio- photometric capacity being secondary. (The reverse case is an instru- ment called a "spectrogoniophotometer.") A commercially available instrument combining spectrophotometric and goniophotometric capa- bility is the Leres Trilac spectrophotometer,* a recording goniospectro- photometer which was used in the present work. This instrument is particularly suitable because it obtains spectrophotometer curves at specular reflection as well as at other angles. The measurements reveal many of the characteristic properties of nacreous and interference pig- ments. * Kollmorgen Color Systems, Tatamy, Pa. 18085.

164 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS EXPERIMENTAL The nacreous pigment is dispersed in nitrocellulose drawdown lac- quer which has the following composition: Nitrocellulose, RS type, 15-20 sec 2.9% Nitrocellulose, RS type, 30-40 sec 6.6 Isopropanol 5.1 Amyl acetate 44.8 n-Butyl acetate 37.6 Monobutoxydiethylene glycol 3.0 00.0% In this particular formulation, which has a viscosity of approximately 2000 cP at 25øC, the two grades of nitrocellulose* are used in order to ob- tain the desired combination of solids content and viscosity. The mono- butoxydiethylene glycol is used to prevent "blushing" or clouding of the lacquer film by condensation of water vapor from the atmosphere. Other vehicles can be devised for nacreous pigment preparations in- compatible with nitrocellulose. Drawdowns of the nacreous pigment dispersion are made on a card which has both a black and a white area. A pool of the drawdown sus- pension is poured on the card held firm against a Bird vacuum plate, and is drawn into a fihn by means of a Bird film applicator producing a wet fihn of approximately 0.003 inch thickness.* The sample for the Trilac spectrophotometer is a 7.5 X 7.5 cm square cut from the black portion of the card. For special purposes, a sample may be taken from the white portion of the card, as will be men- tioned below. This square is placed in the sample holder of the Trilac with a specific orientation, i.e., with the drawdown direction perpen- dicular to the plane of the light beam. The use of a constant orientation is an important factor with pigments which have platelets longer than they are vide. The Trilac instrument may be used to measure either reflected or transmitted light, although the present work is confined to reflectance measurements. (Transmission characteristics are inferred from certain * Hercules, Inc., Wilmington, Del. 19899. * The card used in these experiments is Opacity Chart Form 5C of the Leneta Company, Ho-Ho-Kus, N.J. 07423. The Bird items are available from Gardner Laboratory, Inc., Bethesda, Md. 20014. The vacuum plate is Catalog No. AG-3876 and the film applicator is Catalog No. AG-3800-L.