interference paints
"Luster" pigments are a minor innovation in artists' materials, brought in from decorative applications in crafts and cosmetics, which in turn had adopted them from the sparkly textures first used in consumer packaging and plastics. Mayer sniffs that iridescence effects "have little to do with painting but often occur in nature" — which is half true.
The first interference pigment was guanine, a protein crystal found in the scales, skin and bladder of many species of whitefish (such as sardines or herrings). The first pearlescent (or nacreous) pigment was developed in 1656 by the French rosary manufacturer François Jaquin, who made artificial pearls by painting guanine on round beads. Natural pearl essence is prized for its toughness, and the subtle warmth of its iridescent color; it is primarily used in shampoos and cosmetics.
A considerable amount of research was required before the structure of iridescent pigments was understood and artificially imitated. The breakthrough was the use of mica, a type of hydrated crystal of aluminum, magnesium or potassium silicates that naturally forms into extremely thin, flexible, transparent sheets. The mica used in artists' paints is usually muscovite, obtained either from mining scrap that is ground into powdery flakes, or (more commonly) crystallized synthetically to the required dimensions. Mica based interference pigments were introduced in the 1960's and today make up about 80% of the total interference pigments sold.
These tiny, transparent mica flakes are coated on all sides with a thin layer of metal oxide — either iron oxide or titanium dioxide. These metal oxides are highly refractive (they can bend light) and reflective. Water, for example, has a refraction index of 1.33; mica, 1.5; natural pearl, 1.9; diamond, 2.4; iron oxide, 2.4; and titanium dioxide, 2.7.
The diagram below shows how these pigments work. The coating of metal oxide creates a sandwich of different refractive materials: light is bent and reflected at the boundaries between the paint vehicle, metal oxide, and mica. (Natural pearl creates iridescence from the boundaries between alternating layers of calcium carbonate and protein.)
how iridescent (interference) pigments work
The metal oxide layer reflects light twice, from the outer surface and from the boundary with the mica flake (this double reflection happens again on the other side of the flake). The delay between the first and second reflection slightly phase shifts the wavelengths of light. The shift cancels out some wavelengths of light and reinforces others — these reinforced wavelengths are those of the dominant color of the iridescence.
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The thickness of the metal oxide layer determines the size of this phase shift, and so determines the color of the iridescence. The lower section of the diagram shows, for coatings of titanium dioxide, that layers around 50 nm thick produce a silvery iridescence; increasing the coating changes the iridescence through yellow, red, blue and green. The brilliance of the iridescence declines if the coating is much thicker than 150 nm.
The alternative coating, iron oxide, refracts light in much the same way as titanium dioxide, but adds the reddish tinge characteristic of iron pigments. The same increases in thickness from 50 nm to 150 nm produce colors that appear bronze, copper, red, red-violet or red-green. Gold and brown colors can be produced by applying a layer of iron oxide on top of a layer of titanium oxide.
In addition to these coated mica flakes, the paint often contains a transparent carrier pigment, usually one of the quinacridones or a transparent iron oxide. This provides a background color for the iridescence, which by itself largely disappears on white paper.
The image shows several shades of interference and iridescent paints from Daniel Smith. (Other brands, including Winsor & Newton, Da Vinci and American Journey, now also offer iridescent watercolor paints or watercolor medium.)
luminescent paint samples from Daniel Smith
top row: iridescent antique silver, iridescent blue silver, iridescent bronze, iridescent gold, iridescent goldstone, iridescent jade; bottom row: iridescent russet, iridescent scarab red, iridescent sunstone, iridescent topaz, pearlescent white, interference lilac
Daniel Smith uses the term iridescent colors for mica flakes coated with iron oxide that are mixed with a tinting pigment; this can be either the same hue or a contrasting hue to the iridescent color. Pearlescent or interference colors (lower right) are mica flakes coated with titanium dioxide that are not mixed with another pigment; these appear transparent on white paper. (These names are not standard industry terminology.)
The iridescent paints impart a strong color to white watercolor paper and will retain the color regardless of the angle of viewing; the pearlescent paints look nearly transparent on white paper but impart a nacreous sheen to darkened paper, depending on the angle of view.
Finally, Daniel Smith also makes metallic watercolors, not shown here, made from finely powered metals (mostly copper or bronze) and a dense carrier pigment.
Most artists use all these types of pigments in three ways: as a glazed color on top of a previous coat of paint, as a separate colors applied on top of white watercolor paper, or as a mixed color with other paints.
The pearlescent or interference colors are best used as mixtures or glazes, as they are relatively colorless in themselves. Thinned sufficiently they give a nacreous sheen to any color they are glazed over, though this application can appear obtrusive because the color does not seem to merge with its background.
They will change the hue and reduce the transparency of any paint they are mixed with, and need a relatively dark background to be seen clearly. This makes them especially effective when mixed with mid valued, highly saturated colors, such as pyrrole red or quinacridone rose: the color is dark enough to display the iridescence, and saturated enough to carry a strong color as a mixture.
The luminescent colors contain a carrier pigment that will mix with any color just as a normal paint would; they are also semiopaque, making them less suitable for glazes. These colors are most often used to provide color accents, or in mixtures with other appropriate pigments.
The two basic approaches to using luminescent colors are the subtle and the emphatic. There does not seem to be much ground between the two.
In the subtle approach, for example, you might mix a small amount of iridescent color in the red of a hummingbird's throat, or the yellowish gray of a sea shell, or the dark ultramarine of a twilight sky. The pigment is used literally, to paint areas of natural iridescence in animals or shells, or to poeticize or romanticize a color where the emphasis is on a peculiar quality of light.
In the emphatic approach, you would lay on the luminescent colors heavily, usually with strongly saturated mixtures and coatings of bright acrylic or metallic powders. This approach is unabashedly decorative and extraverted, and goes well with a stylized and ornate design. Gustav Klimt's evocative paintings with gold leaf suggest the general direction, but style ideas can also be drawn from Russian or Japanese lacquered boxes, medieval illuminated manuscripts, and Asian marquetry.
