In the part 1 of this series, I talked about the different ingredients that are used to create a pearlescent effect in cosmetics. In this part, I’ll talk about the science of how these ingredients produce different colors.
Two types of colors
Pearls can be described in terms of mass tone (absorption color) and interference color. In order to explain how these effects are achieved, we need to consider the physics of light. Visible light consists of a continuous range of wavelengths that compose the color spectrum. When visible light encounters an object like a particle of mica it can be reflected, refracted or transmitted. If the entire visible spectrum is reflected the particle appears silver white. If only part of the spectrum is reflected while others are absorbed, the pigment color is determined by the wavelength of the reflected light. A classic example is a red apple. The apple appears red because it is reflecting wavelengths in the red (700 — 635 nm) range.
Coating mica and other substrates with thin, precise layers of titanium dioxide allows the wavelength of the reflected light to be tuned to produce a specific interference color (Figure 1). The overall mass tone of the mica is white, but with a change in viewing angle the interference color is visible. Black iron oxides and can also be used to create black pearls.
Deposition of other cosmetic colorants on top of interference pigments can create even more interesting effects. An absorption color can be matched with the interference color, for example iron blue combined with a blue interference color, to provide intensely colored pearls. Selecting an interference color that is complementary to the absorption color can also create two-tone effects. Combining carmine (absorption color) with yellow interference produces a pearl with a red mass tone and gold shimmer that is visible when viewed at different angles. A veritable rainbow of pearls is available from special effect pigment suppliers and they will often sample the cosmetic chemist a kit showcasing all of the options available. And, as with any color additives used in cosmetics you should refer to FDA and other global guidelines to ensure safety and compliance.
The next time you are out shopping, walk down the cosmetics isle and see what examples of pearl effects you can find. Or if you like to treat yourself to mani-pedi treatments as much as I do, check out OPI’s Pompeii Purple polish. It is a fuchsia polish with blue interference pigment.
Physics and nail polish, I do love my job.
Have you used pearls in your cosmetics? What have you learned about formulating with them? Leave a comment below.