Three-Dimensional Color - Unlocking the Secrets of Interference Pigments - A Tutorial by Carmi Weingrod

Imagine a blue that is rich, vibrant and three-dimensional - a blue that actually changes colors when viewed from different angles. You can obtain this kind of blue by combining an interference pigment with a conventional blue, like phthalo or ultramarine.
By looking at the color chart of D.S. Luminescent Oils in our catalog, you might think interference pigments are only for artists who want a pearlescent or metallic finish. They aren't And you might think they wouldn't show up on any surface other than a black one. But they do.

DANIEL SMITH Luminescent Oils, 37ml

Interference pigments can illuminate and strengthen the palettes of oil and acrylic painters. When exploited to their fullest they can yield a remarkable array of colors - with a potency which is startling and three- dimensional.

The unique qualities of interference pigments are, unfortunately, difficult to depict photographically. Using diagrams, I'll explain how they produce color and I'll describe some simple experiments you can do in your studio to reinforce my descriptions. But until you actually try them, either in oils or acrylics, the true character and potential of interference pigments may remain a mystery.

Color and Light
A knowledge of basic color theory is critical to understanding how the human eye interprets interference pigments. When white light (sun- light) passes through a prism, it separates into its component parts - i.e., the colors of the rainbow. Each color component of the rainbow corresponds to a specific energy. Since light travels in waves, each color is characterized by its specific wavelength. A surface hit by light will reflect certain wavelengths, determined by the nature of the surface, or what we call its color.

1-2-3-Dimensional Pigments
Pigments can be divided into three types: absorption, metallic and interference. Conventional organic and inorganic pigments are considered absorption pigments because they absorb certain wavelengths of the incident (striking) light. The sensation of color is produced by the remaining component of the formerly white light - i.e., the reflected color (the one we see.)

For example, a surface of ultramarine blue pigment reflects that portion of the light which produces a blue sensation and absorbs all the rest. Titanium white reflects all of the light and absorbs none, while carbon black absorbs all and reflects none. Due to their irregular absorption of light, absorption pigments do not display luster and are one-dimensional.

Metallic pigments consist of tiny flat pieces of aluminum, copper, gold, silver, zinc and other metals which reflect light the way a mirror does. These pigments are two-dimensional.

Interference pigments consist of various layers of a metal oxide deposited onto mica, a natural mineral. Light striking the surface of these pigments is refracted, reflected and scattered by the layers that make up the pigment. Through a superimposition (or interference) of the reflected rays of light, a changing play of color is created, with the most intense color seen at the angle of reflection.

The colors produced by interference are dependent on the angle of observation and illumination, and they will alternate with their complementary color as the angle changes. As a result, interference pigments are considered three-dimensional.

Where Does The Color Come From?
Conventional absorption and metallic pigments display their individual colors even in dry powdered form. But interference pigments, made as they are from two nearly colorless substances - a metal oxide and mica - all have a white to gold appearance in dry form, depending on the metal oxide utilized. So, the obvious question arises, where does the color come from in interference pigments?

Again, the answer lies in the way the human eye sees color. One approach is to study the formation of color in its natural counterpart, mother-of-pearl. Natural mother-of-pearl shell consists of alternate layers of lime (CaCO3) and protein. The luster of the pearl is produced by the reflection of light on these thin layers and the superimposition (or interference) of the various reflected rays. The sensation of color results solely from the interference of light rays, and not from any pigments or dyes present in the shell. The irregularity of the shell's layers produce the constantly changing play of colors - its three-dimensional quality.

Now, let's translate this to synthetic interference pigments. The latter are made by coating mica particles with extremely thin layers of either titanium dioxide (TiO2) or iron oxide (Fe2O3) - both of which have high refractive indexes (see box on useful terms.) The color of the reflected light varies, depending on the thickness of the metal oxide layer. By applying increasingly thick coatings of titanium dioxide, a spectrum ranging from silver through yellow, red and blue to green is produced, as diagrammed below. Colors ranging from bronze through copper to red result from increasing the thickness of iron oxide coatings onto mica particles.

When interference pigments based on titanium dioxide are given an additional layer of iron or chrome oxide, or combined with a conventional absorption pigment, additional three-dimensional effects result and the range of colors increases.

By immersing interference pigments in a surrounding vehicle (e.g. oil, acrylic emulsion), the same optical effects result as with the natural mother-of-pearl - except that they are predictable. Since the refractive indexes of all the components are known, the interaction of transmission, refraction and reflection can be calculated in advance by the laws of optics. It can also be determined how light of a given wavelength (i.e., a specific color) will be intensified or distinguished. The layer thicknesses that produce specific colors can likewise be computed.

The size of the mica platelets used in manufacturing the pigment has a direct effect on its surface finish and covering power. Mica platelets are measured in microns (1 micron {u} = one-millionth of a meter). While all platelets are approximately the same thickness (o.5u, they vary in size from 5u to 250u.

Pigments produced from the smallest (fine) platelets have a velvety luster and the greatest covering power. Those produced from the largest (coarse) platelets yield a sparkling effect and do not have good covering power.

History of Interference Pigments
Early Iridescence

The desire to duplicate the pearl luster found in nature has been expressed by civilizations dating back to ancient times. The shimmering iridescence of shells, pearls, fish scales, feathers, butterflies and other insects inspired the Egyptians, and later the Persians, to produce similar effects in cosmetics and glassware.

Sculptures and paintings depicting Nefertiti, and other women of the high court, show the lavish use of cosmetics as facial adornments. Cosmetic formulas, handed down from the Egyptians, utilized powders made from natural bismuth (known as pearl-white) to produce the lustrous look of pearly iridescence.

The Egyptians were probably also the first to use luster glazes on glassware. Matured at low firing, theses glazes were simply film-like coatings of metal or metallic oxide having nacreous (pearly), metallic and iridescent sheens. Luster glazes spread from Egypt to Persia, where they flourished for some time. Recipes reached Spanish and Italian glassmakers as early as the fourth century where they were integrated into traditional styles and patterns.

From Rome to Tiffany
A resurgence of interest in iridescent pearl glazes came in the nineteenth century, when excavations of Roman sites unearthed examples of glassware with a radiant iridescence. Contrary to appearance, this iridescence was not the result of a luster glaze of the type used by the Egyptians or Persians, but was rather a surface phenomenon caused by centuries of burial in the damp earth.

At right, cinerary glass urn, early Roman. Seattle Art Museum. Normal & Amelia Davis Classic Collection.

Carbon dioxide dissolved in the moisture of the soil formed carbonic acid, which decomposed the glass surfaces resulting in a film with a metallic luster. With time, this film became a thick layer of iridescence that actually separates off the glass (like scales) and can be flaked away. It also gave the glass a certain appeal, and thus the buried Roman glassware was greeted with enthusiasm.

The proponents of the Art Nouveau movement, with their dedication to opulence, were particularly impressed with the beauty and radiance of the iridescent glaze. American designer, Louise Comfort Tiffany (1848-1933), was the first to recognize this inspiration. Attracted to the glow of ancient Roman glassware, he sought to replicate this effect by mechanical means. To produce iridescence, he experimented with weathering and firing conditions, as well as with glazing and aging glass in an acid-charged atmosphere.

At left, vase, c. 1915, Lewis C. Tiffany, The Chrysler Museum, Norfolk, VA.

Some of his pieces, in a style he called "Cypriote glass," were traditionally shaped with the rough, decomposed surface texture of ancient Roman glass. But Tiffany also created a new glass style with his experiments in iridescent glazing. He designed elegant, stylish forms which flowed with the light and color that played on the luxurious surface of the glass. These pieces brought him fame throughout the world and motivated European glassmakers to embrace iridescence as well. Art Nouveau designers in Germany, Austria and Belgium responded to Tiffany's work with their own inspirations. Together, they perfected the technical procedures for obtaining flawless iridescence in glassware.

Natural Pearlescence
In 1650, a French rosary maker named Jaquin made the first imitation pearls from a concentration of the lustrous residue found in the scales of the beak fish. Initially, the substance was called pearl essence; in 1861 it became known as guanine. As methods of refining guanine improved, and more of the luminescence found in its multi-faceted crystal was retained in the process, it became difficult to distinguish imitation pearls from natural ones.

With the onset of the twentieth century, demand for a steady supply of reasonably priced pearl luster increased. Guanine, being a product derived from nature, was expensive to extract and produce. The need for a synthetic equivalent with mechanical, chemical and physical stability became apparent, though some top-of-the-line nail polishes continue to be made from guanine.

Synthetic Pearlescence
From the 1920s to the 1950s, the first synthetic pearlescent pigments appeared in the form of tiny, flat crystals of metallic and bismuth compounds. They began to replace the former methods of achieving pearlescence which had grown costly and laborious.

Lead carbonate
Elegant pearlized buttons, which had once been cut from actual snail shells, were then being made from a variety of synthetics with the addition of lead carbonate as the pearl luster pigment. An inorganic compound, lead carbonate consists of large, perfectly hexagonal crystals with smooth surfaces. Until recently, when the toxicity of lead pigment was determined, it was the primary substance for making pearlized buttons.

At right, titanium dioxide mica pigments as seen through a microscope. Courtesy EM.

Bismuth oxychloride. The same bismuth used by the ancient Egyptians to make Nefertiti's cosmetics found broad applications in the modern cosmetics industry. Used since the beginning of the century as a polishing powder in the manufacture of artificial pearls, bismuth oxychloride was more recently manufactured in crystalline form to produce the basis for pearl luster cosmetics. Unfortunately, bismuth is somewhat light-sensitive and darkens with time, which prevented it from meeting some of the growing demands of the plastics and coatings industries. Although bismuth is still used for some industrial applications, attempts to improve its lightfastness are ongoing.

Metal-oxide mica pigments. The break- through for creating synthetic pearlescence came in the 1960s with the invention of the metal-oxide mica pigment. In this process, titanium dioxide was deposited on thin layers of fine particles of mica-a natural mineral. Color changes were achieved by increasing the thickness of the titanium dioxide layer. This process was able to yield uniformity, even in large scale industrial production, and the manufacture of these synthetic pearl pigments revolutionized the plastic and coatings industries. In addition to introducing a remarkable new palette of colors, the synthetic pearlescent pigments possess many characteristics appreciated by artists. They are heat and acid resistant, display outstanding lightfastness, are non-toxic and non-polluting.

Applications
In order to benefit most from this section, I suggest you have a few interference colors on hand (D.S. Interference Oils or Golden Interference Acrylics), some conventional colors of the same medium, a palette, mixing knife, brushes and some heavy paper or gessoed canvas, black and white. An old painted canvas, which you would otherwise paint over or throw out, makes an excellent testing surface for further experiments with interference colors.

Test 1: Paint out two strips of interference blue (for example), one on a black surface and the other on white. Note the results. The color becomes bluish on the black surface because black absorbs all incident (striking) light and reflects none; on the white surface, it appears as a silvery white, devoid of blue, because the white surface absorbs none of the light and reflects all. Now take the painted white surface and tilt it to the light until the strips appear yellow-the complement of blue. If you used interference green, instead of blue, your strip should read red as you tilt it into the light.

Test 2: On your palette, mix the same interference blue with a touch of black. Paint a strip on each of the two surfaces, black and white. Note the results. The blue is deep and apparent on the black surface. With the addition of the black, the blue color emerges on both surfaces, because more light is absorbed.

Test 3: On your palette, mix the same interference blue with a touch of a conventional blue, such as phthalo. Paint a strip on each of the two surfaces, black and white. Note the results. The blue comes to life with a remarkable brilliance on both surfaces, although it is again more apparent on the black. While the intensity of the blue is increased by the addition of an absorption pigment phthalo blue), the pearly luster is decreased. Still, the mixture maintains the dimensional effects of an interference pigment. Tilt the paper to pick up various angles of light and you will see a different blue with each turn. Now try this same test mixing interference blue with other conventional blue pigments, such as indanthrone, Prussian, cobalt, ultramarine and anthraquinone. Note the results. Try adding reds, yellows and greens.

Test 4: On your palette, mix the same interference blue with a touch of a yellow- its complement. Paint a strip on each of the two surfaces, black and white. Note the results. On black, you can see how you get a play of blue and yellow; on white, you get a silvery yellow. Both surfaces reveal the dimension typical of interference colors.

A Note on Surface Color: These tests show that by combining interference colors with conventional absorption colors, you can use them successfully on both light and dark surfaces. While the effects will be strongest against black, all dark surfaces provide a background for setting interference colors ablaze. Whether you paint by scumbling layers of color or by applying delicate veils of glazes, you can have the three-dimensional quality of interference color without metallic or pearlescent effects. By overpainting an old canvas with combinations of interference, absorption and even metallic colors, you can observe the numerous options possible for bringing the glow of full-bodied color to your work.

References
EM Industries, Inc., USA and E. Merck, Darmstadt, Germany. Selected technical materials on interference pigments, including the reprint of a lecture by Eli M. Ascbner.
Koch, Robert. Loers C. Trifany's Art Glass. New York: Crown Publishers. 1977.
Mearl Corporation, USA. Selected technical materials on interference pigments. including the reprint of a lecture by L. Armamni.
Neuburg, Frederic. Ancient Glass. Toronto: U. of Toronto Press. 1962.
Parmelee, Cullen. Ceramic Glazes. Chicago: Industrial Publications. 1951.
Phillips. Phoebe (ed.). Tire Encyclopedia of Glass. New York: Crown Publishers. 1981.