WHITE NACREOUS PIGMENTS EXPERIMENTAL The use of a recording goniospectrophotometer in measuring specular reflectance of nacreous pigment coatings has been described (3). It has been shown that specular reflectance is a measure of nacreous luster, al- though a full description ot• nacreous characteristics requires considera- tion of diffuse reflectance and transmittance as well. The present work utilized the same technique for preparation of drawdowns in nitrocellu- lose and for measuring spectrophotometric curves with the Leres Trilac spectrophotometer. • All specular reflectance curves were made with an angle of incidence of 15 ø to the normal and a viewing angle of 15 ø, i.e., at -- 15 o / 15 ø. Reflectance was measured relative to that of a pressed cake of barium sulfate. Different portions of a mica sample were coated with uniform anatase layers ot• various thicknesses (4, 5). Drawdowns in nitrocellulose lacquer were prepared at such concentrations that the mica content remained un- changed (Table I). In this way, the number of particles in the several drawdown solutions was kept constant. The estimated optical thick- nesses were calculated (see footnote to Table I) from the known compo- sitions of the pigments and the shapes of the spectrophotometric reflec- tance curves, which are shown in Fig. 4. Although a drawdown consists not of a single reflecting film but of numerous platelets throughout the depth of the nitrocellulose coating as in Fig. 1, the spectrophotometric curves of these drawdowns bear a strik- ing resemblance to the calculated curves of Fig. 3. The 0-nm film, as be- fore, is a straight line, which in this case represents the gloss of the draw- down lacquer containing uncoated mica. The thinnest film (36 nm) has the same shape as the 40-nm calculated film of Fig. 3. The 108-nm film is the first one which has a maximum in the visible range, like the 110-nm film of Fig. 3. The curves are not exactly like the calculated curves be- cause in the experimental system the ret•ractive indices no-n•-n2are 1.50- ca. 2.3-1.58 for NC/anatase/mica instead of 1.O-ca. 2.5-1.50 for the theo- retical model. The refractive index of anatase in the pigment is lower than that in the theoretical model because the deposited anatase has sub- microscopic voids which reduce the refractive index of the layer. DISCUSSION The theoretical curves are based on the interference properties of films which are thinner than those which produce interference color. * Kollmorgen Color Systems, Attleboro, Mass. 02703.

84 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I TiO2-Coated Mica Samples Platelets g TiO2/g in Drawdown TiO• (%) Mica (%) mica Lacquer (%)- Estimated TiOa Optical Thickness (nm)• A 0.0 100.0 0.000 2.19 0 B 10.0 90.0 0.111 2.43 36 C 15.6 84.4 0.185 2.59 60 D 20.5 79.5 0.258 2.75 84 E 23.0 77.0 0.299 2.84 96 F 24.8 74.2 0.334 2.91 108 G 27.0 73.0 0.370 3.00 120 I-I 29.0 71.0 0.408 3.08 132 I 30.7 69.3 0.444 3.16 144 $ 32.4 67.6 0.479 3.24 156 K 34.0 66.0 0.515 3.32 168 L 35.6 64.4 0.553 3.40 180 , Per cent platelets (by weight) is inversely proportional to per cent mica to keep mica content constant in each drawdown. Thus, if platelet concentration for G is 3.00%, that for B is 73(3.00%)/90 = 2.43%. v G was designated as 120 nm because it has the same Xm•x as the calculated 120-nm curve of Fig. 3. The optical thicknesses of the other samples were then made proportional to g TiO•/g mica. Thus, for C, optical thickness is 0.185(120 nm)/0.370 = 60 nm. The optical thick- nesses are all based on refractive index for the D line. The similarity of the experimental specular reflectance curves indicates that these nacreous pigments behave in accordance with interference rela- tionships. Optical interference is thus the general principle underlying all nacreous behavior. The "white"-reflecting nacreous pigments are in fact interference pigments even though this term is normally applied only to nacreous pigments which produce color because their optical thickness is in the color range and their thickness is sufficiently uniform. It is apparent that the most lustrous pigment is the one which has high reflectance over the major portion of the visible spectrum, i.e., films with optical thickness from 100 to 140 nm. As a consequence, the most brilliant nacreous pigments fall in this range. The point can be demon- strated more rigorously by multiplying each reflectance value by eye sen- sitivity at that wavelength and averaging over the visible spectrum the conclusion remains the same. Now let us examine spectrophotometric curves ooe natural pearl es- sence crystals from fish. These platelets, although usually referred to as guanine, are composed of guanine and hypoxanthine. They vary in