NACREOUS AND INTERFERENCE PIGMENTS 169 I2 •6 oe o 2 4 6 8 10 I2 % P•gment Figure 5. Relative reflectance at specular reflection of several nacreous pigments TiO,• film, in contrast, is a rather diffuse reflector. Specular reflectance is lower than that of the clear lacquer film, and diffuse reflectance is relatively high and essentially constant over the range of viewing angles which can be covered by the instrument. The slopes of the curves are quite gentle between the viewing angles of 15 ø and 20 ø. A glossy surface is expected to give a sharply spiking curve, as was demonstrated by Hunter (4) and Billmeyer and Davidson (6). The broadness of the maxima obtained with the Trilac spectro- photometer is attributable to the rather wide viewing beam, which sub- tends a total angle of 19 ø. This characteristic limits the resolution of the Trilac spectrophotometer but, on the other hand, makes it possible to measure specular reflectance with a fair degree of reproducibility, which would be very difficult at the peak of a spiking curve. The average deviation from the mean of repeated readings is less than 0.1 relative reflectance unit in the 5.0 to 10.0 range. Several other "white"-refiecting nacreous pigments were compared with pearl essence plates for specular reflectance. These were two samples of bismuth oxychloride and two of titanium dioxide-coated mica. In each case the sample of lower quality is designated as I, the higher as II. The reflectance at 550 nm is plotted against concentration in Fig. 5. BiOCI-I and TiO2-mica-I do not approach the specular re- flectance of pearl essence plates no matter what concentration is used. TiO2-mica-II comes closer, and giOCl-II reaches almost the same specular reflectance as pearl essence plates, although at a higher concen- tration. However, because BiOC1 has a much higher specific gravity (7.7) than pearl essence crystals (1.6), its volume concentration at equal specular reflectance is lower than that of pearl essence. Thus, giOCl-II is more reflective than pearl essence plates, no doubt because of its higher refractive index (2.15 and 1.85, respectively). The interesting

170 .JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS relationships between volume concentration in the film, refractive index, and specular reflectance will be considered at another time. Do two nacreous pigment coatings which have the same specular re- flectance have the same appearance? One such pair is made up of the plate pearl essence coating marked d in Fig. 5 and the BiOCI-II coating marked B. In the standard dipping test the two products are not identi- cal in appearance: the pearl essence has decidedly better pearl luster than BiOCI-II. Drawn down adjacent to one another on the same card and examined by specular reflection, BiOCI-II looks very slightly pink relative to the pearl essence, which appears slightly greenish yellow. Viewed by nonspecular reflection on the black card, BiOCI-II definitely scatters more light than the pearl essence and is thus more opaque. Finally, BiOCI-II is also more opaque at specular reflection. The differences seen on the drawdown card are all discerned by the Trilac, and are revealed by the next three figures. Figure 6 compares spectrophotometric curves at specular reflection for the two samples. The two curves meet close to 550 nm, but BiOCI-II is highest at about 660 nm (red), pearl essence highest at 515 nm (green)--hence the slight color difference which was noted. Figure 7 presents the diffuse reflec- tion of the two pigments from the black part of the card and shows that BiOCI-II reflects more than pearl essence at nonspecular angles. Figure 8 compares nonspecular reflectance at --15ø/60 ø against the white part of the card, which means that the reflection is primarily that of the white card as seen through the nacreous pigment. The reflectance through BiOCI-II is less than that through pearl essence because of the higher opacity ot• BiOCI-II. In other words, BLOC1 transmits less, and its lower transmittance more than overcomes the higher diffuse reflectance at • IO •8 oe, 0 •/v = -15•/15 ø 450 550 650 WAVELENGTH (rim) Figure 6. Spectrophotometric curves at specular reflection for plate pearl essence anti BiOCI-II