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Pearlescent pigments are well known and have been widely used to add a variety of special effects to color cosmetics and personal care products. A platelet shape is necessary to minimize light-scattering and provide the desired reflection; since the 1970s, the bulk of commercially available pigments of this type have been based on naturally occurring muscovite mica, usually coated with titanium dioxide and/or iron oxides. Other materials such as ferric ferrocyanide or carmine may also be precipitated on to the pigment surface to provide variations in the pearlescent color. The thickness of the metal oxide coating determines the interference effect; the reflected color will generally range from silver-white through gold, copper, red, violet, blue and green. Preparations of such pigments have been summarized in previous work.
The pigments discussed in this article are layered systems consisting of a substrate — such as natural muscovite mica, silica flake or alumina flake — coated with cosmetically-acceptable metal oxides, usually titanium dioxide or iron oxides. The substrate provides the necessary platelet shape; the interference color is determined by the thickness of the metal oxide layer and its refractive index. While mica has been the substrate of choice for many years, new possibilities in substrate choice have allowed for greater flexibility and effect. Mica, since it is a natural product, brings some limitations to its use as a substrate for these pigments. Small amounts of iron oxide may give a somewhat dark mass tone to the mica. Its thickness as a substrate is also limited and its surface is not perfectly smooth; when coated, the resulting pigment is also not smooth. This rough surface will affect the reflected color, causing more light-scattering and consequently less sharpness and brilliance.
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