Of all the wonders in the natural world, iridescence is one of the most illusive and intriguing. Development of multi-interference pigments and other types of special effect paint attempts to imitate and duplicate this property with ever-increasing success. This essay addresses the scientific and artistic nature of iridescence as a quality occurring in both nature and technology. It details the optical properties of iridescence as diffraction, diffusion, and interference, as well as the physical properties that cause these optical manifestations. It considers how iridescence has been used and developed historically, including the synthesis and characteristics of specific special effect paints and their use in art and industry. The essay includes a limited literature review of current and past research on the subject.
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Observation of the natural world reveals many wonders of science and beauty. One of the most distinctive and illusive is the manifestation of iridescence. This quality brings a level of fascination and even surprise at its discovery in whatever form. How many times has one become distracted by the briefest flash of pink and blue from the oily residue in a parking lot after a rain, or the glint of green and purple on the throat of a hummingbird? This aspect of mere light physics is intriguing to the most casual of observers, and when pursued further lends itself to a complex technology made perfect in the simplicity of nature. It has become a challenge of modern technology to duplicate iridescence in many forms for the enhancement of consumer goods, and the intrigue of the artist to create the embodiment of this most delicate and fascinating element.
My research expands on current exploration and experimentation in the fields of science and art, specifically in the technology and use of special effect paints and pigments, and how their synthetic iridescent qualities compare to nature. As an artist, from the beginning of this process my desire has been to combine the available technology of special effect paints and pigments with my knowledge, understanding, and practice of visual arts to create a work of art that showcases not only the possibilities of technology but also the experience of iridescence. Initially, I intended this work of art to be as scientifically accurate to the property of iridescence as possible. Although I have not strayed from the scientific integrity of the work, I decided that an abstract expression of iridescence was more artistically valid. Now the work is not only as scientifically intricate as possible, but also intentionally complex on the level of artistic representation. Within this context, I have had the pleasure of working with these paints and pigments to explore their importance to the artistic community through my creative interpretation of the manifestation of iridescence.
Iridescence: Culture and Nature
The relevance of special effect paints and pigments is made obvious in our society of consumerism. To say that our culture is visually oriented is an understatement. Almost every choice in modern society—what to eat, what to wear, what to drive—relies on visual stimulation. More than a line or shape, color seems to be linked very strongly with the attractiveness of an object. As a case in point, a recent survey revealed that 40% of new car buyers would change their preferred brand if the color they wanted were not available (Rochard, 2005). Interestingly, the designation of value on an object through color translates into the natural world through mate selection and health description within the animal kingdom. It can serve as a warning of danger and declaration of power and strength. We even separate many inorganic natural objects exclusively based on rarity of color, such as precious stones and gems. To the scientist and artist, the iridescence observed on the skin of a soap bubble is no less fascinating than its simulation on the surface of a plastic cell phone cover.
As an optical property, iridescence is achieved through several different physical mechanisms (Hand, 2005; Vukusic & Sambles, 2004). First, there is dispersion. The typical example of dispersion is white light sent through a prism. This light is broken into separate visible wavelengths and then exhibits the full color spectra as a rainbow. Second and thirdly, there is diffraction and interference. These two words can be used to describe the same physical phenomenon of iridescence because their mechanisms are dependent on each other. Diffraction is simply the bending and reflecting of light waves through the surface of an object. It is this mechanism in conjunction with superposition of objects (layering) and combination of waves (those being bounced back or allowed to pass through) that interference arises. In the occurrence of interference, light is required to pass through different levels of surfaces, much like grates, which allow certain wavelengths through and not others. The waves that are reflected begin to return to the surface but often are bounced back several different times by the different grating surfaces. By the time the wavelength is returned to the exterior surface, it has been bent to the degree that it no longer absorbs at the same level that it did when it entered the object, and thus, one perceives multiple colors.
Diffraction is most noted from so-called thin film interactions of light waves and is a fundament of interference. Also known as thin film iridescence, it occurs when different wavelengths of light are returned from the surface of an object, such as an oil film. We observe different colors from different angles due to varying thicknesses of the surface. True interference, also known as structural iridescence, on the other hand, involves not only the reflection or absorption of light waves at difference wavelengths but also their cancellations and reinforcements through the layers within the object. It is this multilayer trend that has been most usefully translated by modern technology.
Of course, butterflies exhibit thin film interference through the combination of multiple layers of fine scales on the wings and the grating-like nanostructure of the scales themselves. The male of theMorpho family of butterflies has radical exhibitions of blue iridescence. Figures 1b through 1d are SEM (Scanning Election Microscopy) and TEM (Transmission Election Microscopy) images of the wing scales of the Morpho Rhetentor butterfly at various magnifications. Both SEM and TEM imaging techniques use electrons rather than light waves for imaging, which allows for the internal structure of an object to be viewed and magnified, easily over 350,000 times its normal size. Figure 1a shows a male Morpho Rhetentor with the electric blue iridescence. This color is only experienced from certain angles; otherwise, the observed color is drab brown. Figure 1b is an SEM image of the shingle-like arrangement of the scales on the wing. Figure 1c is the topside of a single scale, and Figure 1d shows the side of a single scale and how each of the parallel rows is made up of individual barbed projections.
a) male M.Rhetentor.
b) SEM image M. Rhetentor scales, magnified
c) SEM image M. Rhetentor single scale, magnified (top)
d) TEM image M. Rhetentor single scale, magnified (side)
Figure 1a-d. Example of Iridescence in Butterflies. (Retrieved from Vukusic & Sambles, 2004. Reprinted with permission.)
Historical Usage and Development
It is these very same physical principles that man has utilized to mimic iridescence and incorporate it into everyday life to bring beauty to the mundane. Its use is documented as early as the Egyptians and Persians in their treatment of glazes on pottery and incorporation of natural materials to create cosmetics (Weingrod, 2002). The perfection of this beautifying practice was paralleled by the Romans, especially in the area of ceramics and pottery, through various glazing techniques on their funeral urns. Louis Comfort Tiffany further detailed these developments in modern time with the insurgence of Art Nouveau.
Incorporation of synthetic flakes of bismuth and fish scales into automotive paints in the late 1950s led to some of the first synthetic special effect pigments incorporating pearlizing effects specifically. In the 21st century, the new technology of synthetic pigments has opened up great possibilities in the realm of paints and pigments to achieve color effects similar to those common in peacocks, butterflies, and beetles. These special effect paints have high degrees of opalescence, iridescence, color shift, and angle-dependent color properties. Nature set the standard within the mechanism of iridescence and modern technology strives to follow its example. Modern synthesis incorporates the multilayer trend, but uses titanium-coated silica flakes or aluminum flakes. Increasing technological advancements have allowed for more complex paints to be developed, such as Coloristics. Figure 2 shows the color range of the pigments on a dark or black basecoat as well as an example showing the green to blue color shift of HELICONE ®HC Scarabeus.
HELICONE ® HC color chart and VW exhibiting Scarabeus color
Figure 2. Example of Multilayer Technique for Creating Complex Colors. From H.W. Sands Corporation, 2005. Reprinted with permission.
The pigments are composed of organic polymers that rely on liquid crystal characteristics. These compounds retain liquid attributes while maintaining their crystalline integrity as a solid. This technology allows for increased manipulation of the molecules in their orientation and therefore similar deposit systems can be achieved for color-shift effects. The crystals are aligned much like vertical cigars and the color shift varies with the angle of pitch to the horizontal of packed structures. The arrangement is slightly different for each consecutive layer, allowing for very striking color changes.
Other effects in the automotive special effect industry include deep sparkle and metallic color effects. The crystalline effect, begun in the early 1950s, eventually included the addition of mica or quartz flakes. These paints were not entirely predictable in their effect properties because the flake size and shape were uneven. This is no longer a problem with the new technology, for not only are the materials themselves being synthesized, which increases uniformity of formation, but the colors themselves are not dependent on a traditional pigment. HELICONE® pigments have a uniform chemical composition, which allows for identical application properties and treatment (Rochard, 2005). Because their color is not due to the absorption of color by the pigment itself, regardless of their color, HELICONE® pigments have the identical properties. This makes the pigments increasingly popular because it removes more limitations of application and usage. Other pigments with mica or bismuth are limited to shimmer or sparkle effects rather than the color-shift properties of the latest technology.
Uses of Multi-Interference Pigments
However abundant they are in the automotive, plastic, and print industry, in the artistic world, special effect paints and pigments are generally less popular. Often this is due to the difficulties involved with the nature of application, including the need for multiple layer application, heat firing, or ionic powder-coating systems, as well as their expense. This is not to say, though, that industrial-grade paints are totally absent in the art world. Their use is limited to very specialized groups of artists who have access to industrial-type materials and resources. Similar paints have been developed in artist grade in both oil-based and water-based paints. However, their effects and color-shift are a good deal more limited than the industrial-grade pigments. These are termed interference paints and they do give tonal shifts when layered over different base colors. Figure 3 shows five of the fine-grained Golden Interference Acrylic colors. There are three available particle sizes from fine to coarse and this changes the relative sparkle effect of the pigment. The effect is most noticeable when the colors are applied over a black background. Viewed over a white ground, colors complementary to the ones visible against the black ground appear.
Golden Interference Acrylics: gold, green, orange, red, blue
Figure 3. Examples of Interference Paints. (Golden Interference Acrylics: gold, green, orange, red, and blue.) From Golden Artist Colors. Reprinted with permission.
As a general rule, artist-grade interference pigments exhibit the most obvious color properties when glazed over a dark background. Golden Acrylics has a variation in their interference acrylics that allows for the addition of a texturizing additive that gives higher dimension to the paint, allowing for more layering. My experimentation with artist-grade interference colors has been limited to water-based pigments and student-grade watercolors. An obvious trend was that as the quality of the materials dropped, the color shift properties declined. By the time I had begun experimenting with student-grade paints the effect was limited with simple reflection of a metallic or pearlescent quality. The addition of the special effect pigment to my partial project (Figure 4) provided an interesting reflective property that enhanced the piece quite well.
a) My partial project: Unfinished with no special effect pigment
b) My partial project: Finished with special effect pigment
Figure 4. Le Poisson Bleu, by Bethany Craney, 2005: Example of a Painting Using Golden Acrylics
Necessity is the mother of invention however, and the possibilities of special effect pigments are not restricted to artistic or industrial paints. The cosmetic industry is overflowing with special effect pigments incorporated into makeup and nail lacquer (Gordon, 1993). With the introduction of automotive paint in the 1920s came the beginning of fingernail paint. By 1930, REVLON was created and the ever-growing rainbow of colors began. Automotive paints and nail polishes continued in a parallel evolution to the present day. There are now more striking special effect paints available in the nail polish industry than the automotive. It would follow that this is due to the impracticality and expense of their production and application on a larger scale than the small nail lacquer bottles. Figure 5 shows the various color shift nail lacquers and the eggs painted with different layering techniques to achieve an iridescent quality to them.
a) Estèe nail lacquer, and SpectraFlair® Sally Hanson Nail Prisms
Figure 5. Example of Use of Nail Polishes on Eggs. a) From JDS Uniphase. Reprinted with permission. b) From JDS Uniphase. Reprinted with permission.
Unfortunately, nail lacquers are very difficult to use on an artistic level. They are designed to dry very quickly and this limits their application. Much to the benefit of the consumer but to the detriment to the artist, nail lacquers are being developed for increasingly faster drying times.
The consistency of the pigment itself presents problems for application as well. They tend to be very viscous, and with extended exposure to the air this increases dramatically, almost to the point of rendering the paint useless. Enter the quandary of extenders and thinners. In order to increase the productivity of the paint, it is necessary to decrease its viscosity through the addition of either clear lacquers or thinners, such as small quantities of nail polish remover. The addition of clear lacquers presents the problem of the introduction of air to the paint in the transfer of one or the other to another container. The addition of a thinner could possibly compromise the chemical integrity of the polish and therefore destroy the effect. The most effective method that I have devised so far is to first choose relatively sheer pigments to begin with; secondly, use the smallest amount of thinner possible; and thirdly, attempt to blend and layer colors while the paint is still relatively wet. Other resources that I employed were various pigments in powder form. The fineness of these powders ranged from extremely high quality, highly iridescent eye shadows and shimmer powders available commercially to simple iridescent craft glitter, as shown in Figure 6.
Brush-On: iridescent glitter
Figure 6. Examples of the Use of Iridescent Powder in Craft Glitter. (Brush-On: iridescent glitter.) From National Artcraft, http://www.nationalartcraft.com/images/sub-0298-02.jpg; reprinted. Reprinted with permission.
The application of these powder pigments was periodically problematic. If simply applied to the surface, there was the possibility that they would be rubbed off or simply flake off. Yet, the addition of a binder or fixative such as a clear lacquer or hairspray changed the color reflection by changing the color absorption. I experimented with adding the powder while the original pigment was still wet, combining the special effect pigment with normal water-based pigments. Another alternative was to moisten the powder itself with water and apply it as almost a paste. This worked for application purposes, but may prove to be insufficient in the long-term durability of the art.
Surface material, it seemed, also played a large part in the relative success of the different pigments that were used. A good surface for the nail lacquers needed to have a certain degree of grain and porosity, but not too much to prevent unwanted absorption. The ideal surface was discovered in real blown eggshells. Their organic ovular shape contributed to the design of that particular phase of the project, invoking the presence of natural elements in conjunction with modern technology. They also provided a curved surface, which showcases the angular dependence of the pigments. The other surfaces used were 50- to 100-lb cold press and hot press watercolor paper, artist grade Mylar, wood, and metal. Figure 7 showcases the iridescent eggs as well as the preliminary drawings for the project and a partially completed wood frame.
Figure 7. Example Using Powder Pigment in Original Artwork with Eggs, by Bethany Craney, 2005.
My Research With Multi-Interference Pigments
In the spring of 2004 when I began the preliminary work for this project, I limited my research to what was available through media materials, chemical companies and their self-promotions, and research projects that others were doing and on which they were publishing their findings. I did not intend my project to become the obsession that it has. I had not anticipated the level of personal growth as an artist that would accompany my development of a thesis. When I began the process of designing the art project, I had very little experience in true self-expression through art. Combine that lack of experience with the vast task of artistically representing such an illusive and provocative element as iridescence, and I realized how unprepared I was. Had I not had a faculty mentor through the process, it would not have been as successful as I feel it has been.
This project has not only developed my researching skills and honed my artistic technique, but it has also taught me many valuable habits that I will use throughout my artistic career. One is the utilization of a sketchbook, from which now I am inseparable, and the extensive journaling of my processes within the studio itself. Notes written on failed and successful combinations of materials as well as documentation on the amount of time within the studio have and will serve to provide boundless insight and growth within my artistic self. Scientifically, I have learned to keep extensive notes on my information sources as well as learning to look beyond what one might perceive as technical limitations of a product.
- Golden Artist Color. (2005). Heavy Acryllic interference colors: Heavy body interference color chart. Retrieved August 8, 2005 from http://www.goldenpaints.com/products/color/heavybody/interchart.php
- Gordon, A. (February 1993). “History of Nails.” NAIL Magazine. Retrieved April 2005.
- Hand, R. (2005). The science of iridescence. Retrieved April 2005.
- H.W. Sands Corp. (2005). Helicone R. Retrieved August 8, 2005 from From H.W. Sands Corporation, 2005.
- JDS Uniphase. (2005). Image gallery. Retrieved August 8, 2005.
- Rochard, S. (2005). Using special effect powder pigments. Automotive and Powder Coatings, Engelhard Corporation. Retrieved August 2005.
- Vukusic, P., & Sambles, R. (2004). Iridescence in butterfly wings. Retrieved April 2005.
- Weingrod, C. (2002). Three-dimensional color – Unlocking the secrets of interference pigments.” Text. Daniel Smith Art Suppliers 2002. Accessed March, 2004. www.danielsmith.com.