Structural Colors - Engineered Biomimicry - page 291

paint is deposited on a black ground, reflectance
from the background is absent, and only the
interference color reflected from the flakes can
be seen. Micrometer-sized flakes produce a
smooth sheen, but larger flakes give a sparkle.
Other types of nacreous pigments contain
metal-coated silica flakes. They exhibit strong
metallic reflectance, with the color changing
with the viewing angle, and are therefore
called optically variable paints [132] . Should
flakes of sculptured thin films [79] be added to
paints, polarization-dependent color can be
expected. Nacreous pigments have found a
broad range of applications in cosmetics,
paints, and decorations.

because of the health hazards they may pose,
especially if used in too high a concentration.
For example, flakes, are pigments manufactured
by precipitating a soluble dye with a metallic
salt (calcium salt, barium salt, aluminum salt,
and sodium salt) because the different salts pro-
vide different colors.
Color additives can be combined with tita-
nium dioxide and mica, bismuth oxychloride, or
guanine. When combined with the titanium-
dioxide-coated mica, they create very attractive
dual-color pearls. Bismuth oxychloride is a soft
material used as the frost in lipsticks. Guanine
is derived from herring scales and is used pri-
marily in nail polishes. In general, the mineral
powders in the commercially available cosmetic
products are based on titanium dioxide and zinc
oxide, which, in addition to other technological
reasons for their use, may also provide anti-
inflammatory and calming effects.

11.6.4 Structural Colors in Cosmetics
11.6.4.1 Cosmetic-Grade Color Additives
Cosmetic products must not only promote
healthy skin but also provide a beautiful tapes-
try of colors for every complexion. The state of
the art in classical cosmetics is the use of finely
crushed minerals and botanicals of the best
quality. To suit even the most sensitive skin,
these ingredients should be 100% pure and
completely natural. However, most color addi-
tives to mass-produced cosmetic products are
synthetic: indeed, unless specifically approved
(such as annatto seed), all natural color addi-
tives are banned in the United States by the Food
and Drug Administration to avoid irritation and
hypoallergenic reactions. Cosmetic-grade color
additives, either inorganic (pigments) or organic
(dyes and pigments), are safe for use on humans
and do not contain heavy metals such as lead,
cadmium, and hexavalent chromium. The inor-
ganic pigments include ultramarines, iron oxide,
manganese, chromium oxide, titanium dioxide,
and ferric ferrocyanide. Both organic and inor-
ganic cosmetic-grade pigments are insoluble in
both water and oil; however, they can be micro-
scopically dispersed in a liquid host and made
to appear as though they are dissolved. Only a
few soluble dyes are used in cosmetic products

11.6.4.2 Colored Nacreous Pigments
in Cosmetics
Structural colors have also found a place in the
world of cosmetic products--as glittering mate-
rials. Most of the nacreous materials exhibiting
interference colors and a high degree of opacity
that are used for cosmetics are titanium-dioxide-
coated mica flakes. Colored nacreous pigments
are obtained when colored compounds such as
iron oxide (yellow); ferric ammonium ferrocya-
nide, known as Prussian blue (blue); chromium
oxide (green); or carmine (red) have been added
to those flakes [132, 133] . For example, a coating
of iron oxide alone produces metallic effects simi-
lar to bronzes and coppers. The thickness of the
flakes, typically between 15 and 150 μ m, dictates
the interference between light reflected from the
substrate and that reflected from the flake and
therefore controls the luster of pearlescents. Low
luster--with thin flakes--gives the effect of a
pearl with a smooth sheen; medium thickness cre-
ates a silky or satiny effect. The larger the flakes,
the more sparkle, although they are all shiny.