Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/LEON_a_00114 by guest on 25 September 2021 Technical ar T i c l e ce, Tech N e Iridescent Color: From Nature I , Sc to the Painter’s Palette T a b s T r a c T o: Ar nan The shifting rainbow hues of Franziska Schenk iridescence have, until recently, remained exclusive to nature. and Andrew Parker Now, the latest advances in nanotechnology enable the introduction of novel, bio- inspired color-shifting flakes into painting—thereby affording artists potential access to the full spectacle of iridescence. Unfortunately, existing rules rtists have never captured a color as dazzling selves of two distinct types: those of easel painting do not apply A that are stable attributes of material to the new medium; but, as and dynamic as the metallic blue of the Morpho butterfly, which nature inspired the technol- is visible for up to a quarter of a mile. Now, with rapid ad- substances, and those that are “acci- dental,” such as the evanescent col- ogy, an exploration of natural vances in nanoscience and technology, we are beginning to ors of the rainbow and the colors of phenomena can best inform unravel nature’s ingenious manipulation of the flow of light. some birds’ feathers, which change how to overcome this hurdle. Scientific research into natural nanoscale architectures, ca- according to the viewpoint of the Thus, by adopting a biomimetic pable of producing eye-catching optical effects, has led to the spectator [2]. approach, this paper outlines the optical principles underly- development of an ever-expanding range of comparable syn- ing iridescence and provides thetic structures. Particularly notable are the latest iridescent The “stable” colors, associated technical ground rules for its flakes. Although industry has exploited the novel properties with chemical pigments, have pre- incorporation into painting. of these flakes for almost a decade, fine-art painting has been occupied painters for millennia. slower to assimilate them. The major apparent hindrance is The rainbow, by contrast, remained the incompatibility with—and resulting confusion caused by mysterious until the 17th century, the material’s non-adherence to—color theory as applied in when Newton famously united light and color through his painting. Iridescent flakes, in themselves colorless, are “opti- prism experiment that proved white light consists of all the cal devices”—entirely different from the chemical pigments colors of the spectrum. The changeable hues of bird feathers traditionally found in paint. Consequently their application kept their secrets much longer. It was only in the mid-20th presents major challenges for artists. However, since the flakes century that science verified what the ancients had intuitively mimic nature’s technology, systematic analysis of the mecha- believed: that the colors of the rainbow and iridescence (a nisms that cause iridescent effects in animals can inspire analo- term evoking Iris, personification of the rainbow) are inextri- gous artistic methods. We can therefore establish, via studying cably linked. Both phenomena are caused by light interacting nature, some ground rules for adapting color-variable technol- with transparent, colorless matter. A rainbow is created when ogy to fine art painting. water droplets, like Newton’s prism, split white light into its As demonstrated by Schenk [1], the latest iridescent tech- components—the colors of the spectrum. Newton concluded nology offers manifold innovative artistic possibilities, together that the angle-dependent colors of birds’ feathers must equally with a unique potential aesthetics. result from light splitters (i.e. thin films) but did not compre- hend the precise color-producing mechanism [3]. In the 1950s electron microscopy, enabling nanoscale observations, finally wo isTincTly ifferenT ypes of olor T D D T c ascertained that iridescence in hummingbirds, for example, Gage’s Color in Art opens insightfully: is indeed produced by what effectively equates to a stack of thin films [4]. Here spectral colors are made visible via the Any European account of color in art must begin with the be- optical phenomenon of constructive interference, resulting lief, which dominated Western culture for many centuries, that light and color are distinct entities, and that colors are them- in color that changes with the direction of illumination and viewing angle. In the last decade, alternative iridescence-inducing ar- chitectures have been discovered in animals, and initial Franziska Schenk (artist, educator, researcher), Bournville Centre for Visual Arts, attempts to replicate them have been made by manufactur- Birmingham City University, Linden Road, Bournville, Birmingham, B30 1JX, U.K. ers [5,6]. Thin-film interference is the most common form E-mail: <[email protected]>. URL: <http://www.biosciences.bham.ac.uk/ artists_in_residence/>. of iridescence in nature and currently prevails in industry. Andrew Parker (research leader), Department of Zoology, The Natural History Museum, This optical phenomenon is, therefore, the focus of the Cromwell Road, London, SW7 5BD, U.K. E-mail: <[email protected]>. URL: <www.gtc. article. ox.ac.uk/andrew-parker>. Nature has retained a “monopoly on this metallic-like, col- This paper is presented as part of the Leonardo special section Nanotechnology, Nanoscale Science and Art, guest edited by Tami I. Spector. ored effect” [7]. Artistic attempts to capture iridescence have See <www.mitpressjournals.org/toc/leon/44/2> for supplemental files related to this been thwarted by restricted materials that appear much too article. dull in comparison. Chemical pigments absorb particular wave- lengths of incoming white light. The color impression, the re- article Frontispiece. cuttlefish, iridescent medium on board, a4 maining part of the light, changes neither hue nor brightness, size, 2007. (© F. schenk) even when viewed from different angles. Moreover, the color ©2011 ISAST LEONARDO, Vol. 44, No. 2, pp. 108–115, 2011 109 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/LEON_a_00114 by guest on 25 September 2021 ech T temporary painters still operate within the confines of subtractive color theory, ce, Fig. 1. achieving silver-white. N derived from practical experience with e (a) chaotic layers, each tuned to I c a color of the spectrum. absorption pigments. Here the primaries S , (b) Three stacks, each tuned to T are red, yellow and blue. A mixture of a primary color. (© a. Parker) these colors becomes darker with each o: Ar primary added until black results, in an N complete reversal to what occurs in “sil- ver” fish. Natural iridescence adheres to the rules of additive/light mixture, based on the primaries red, green and blue, and so does iridescent “pigment.” Con- sequently, for iridescence ever to become a fixture on the painter’s palette, addi- tive and subtractive methods have to be mixed—a concept alien to artists. fades over time. Centuries of practical ex- [10]. In the centuries that followed, his In nature, on the other hand, chemical perience with absorption pigments have technique of grinding up the scales of pigments and structural colors are often led to firm rules for colorant mixing. silver fish such as bleak and herring re- combined. The composition of cuttlefish With the recent arrival of an altogether mained the only way of obtaining pearl skin is a prime example, its individually different type of “pigment” that imitates luster effects. adjustable color cells being arranged in natural thin-film structures, it is instruc- layers (Article Frontispiece). A bottom tive to re-examine certain doctrines. subtractive versus additive layer of silver “mirror” cells reflects colors Fueled by rapid advancements in na- color Theory from the surroundings. The cells above, noscience and manufacturing, the evolu- How can silver coloration in fish be ex- containing chemical pigments, switch tion of iridescent “pigment” technology plained? While the color appears identi- on (expand) and switch off (contract) is gaining momentum. While offering cal to that of the precious metal, no silver accordingly, thus (in combination with both non-toxic and fade-resistant prop- traces are found in fish scales/skin. The the “mirror” cells) assuming any color erties, the ever-expanding flake choices metallic color results from transparent desired via optical mixing. Studying such are most notable for their outstanding multi-layered architectures, known as color mechanisms can be instructive to purity, brilliance and innovative optics. broadband reflectors [11]. In a variation the painter. These qualities have enticed those in the of the thin-film formula, a number of lay- auto, cosmetics and plastics industries. ers of varying thickness are stacked (Fig. a First Generation Unfortunately the technology seems to 1a). Each layer reflects a different wave- of Pearlescent Flakes have bypassed fine art painting. The art length/color of the spectrum, depend- From the 1920s onward, sustained field’s lack of awareness of the medium’s ing on its particular optical thickness. attempts were made by industry to potentials is due, in part, to difficulties One layer reflects red; another slightly replicate natural pearl essence with a syn- in sourcing the product. Although paints thinner layer reflects orange; an even thetic alternative, because natural pearl based on first-generation flakes can now thinner one, yellow; then green, then essence is costly and limited in supply. be bought from specialist art suppliers, blue, etc., until all the colors of the rain- This eventually led to the conception of recent industrial color-variable flakes bow are reflected and bright white light “pearlescent pigments,” based on “a trick remain prohibitively expensive [8] and is observed. This process not only dem- copied from nature” [13], namely light as yet unavailable in paints for artists. onstrates what happens when differently interference. Ever since, (semi-)transpar- The greater stumbling block, however, colored reflections/lights are combined ent flakes generating “metal-like effects is the challenge the creative application but also illustrates the laws of additive .