Three-Dimensional Color and 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.

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.

inksmith-three-dimensional-color-pic2Color 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.

inksmith-three-dimensional-color-pic3Where 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.

inksmith-three-dimensional-color-pic4Now, 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.

inksmith-3D1-coloBy 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.

inksmith-3D-coloPigments 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.