Mary Virginia Orna, O.S.U. College of New Rochelle New Rochelle. NY 10801
Chemistry and Artists' Colors Part 1. Light and color
Introduction The chemistry of artists' materials has its roots in ancient and medieval technology and medicine. Among the earliest "manufactured chemicals" were the synthetic pigments Egyptian hlue, which was produced in the third milennium B.C. ( 1 , 2 ) , and vermillion (HgS), a one-time staple of the artist's palette (3).The technical literature of the Middle Ages abounds with recipes for the synthesis of artists' pigments. A d a c e of honor was accorded to synthetic hlue colorant because of the scarcity and prohibitive cost of natural blues (4). According to Eastlake (5).chemistry remained the professed auxiliary of painting well intu the seventeenth century. Althuugh, in later centuries, chemistry and art have gone their srparate ways and rach discipline has develuped its own specinlind vocal~ularyand methodology, there are still many are:).; 01 hoth fields which areof mutual interest. Oneof these areas of overlap is color. I t is a matter of history that Europe's chemical industrv erew out of dve and ~ i e m e nmanufacture t (6).In 1978, the e"scmated glohz production of synthetic dyes and pigments was close to the two billion dollar mark. Although a very small proportion of this output is actually used bv practicing artists, the very magnitude of the husiness ass&& ongoing research which is constantly producing new H colorants with properties more desirable than many of the 0 traditional artists' pigments. For this reason, the artists' palette has undergone a series of transformations over the past 0 century and a half. One notable example was the introduction of titanium white, TiOz, in the early 1920's. Far superior to its older counterparts, lead white and zinc white, in opacity, and adaptability to various media, Ti02 has almost c stability, completely replaced them in artists' usage and has found use in many new products as well. E The historical and ongoing links between the chemical inM dustry and artistic endeavor which are centered in their mutual interest in colored materials, lead to a series of questions which will form the basis of the topics covered in this feature. S (1) What is the nature of light and color? (2) How is light T modified by colored objects to produce the sensation of color? (3) What features of molecular or crystal structure must he present for a compound to he colored? (4) What physical Y properties of colorants make them desirable as artists' pigments? (5) How are artists' colors synthesized? (6) How are artists' colors classified?
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The Nature of Llght and Color Theories regarding the nature of light and the origin of color go hack to the ancient Greeks. Aristotle himself is credited with making the first important contribution to what is now the modern theory of selective absorption (7). I t was Seneca, a Roman philosopher of the first century, who first noted that a prism reproduces the colors of the rainbow, hut it remained for Isaac Newton in the seventeenth century to formulate modern color theory on the basis of experiment. Light and Color
Newton allowed a narrow beam of sunlight to pass through a prism in a darkened room, and he observed that the light emerging from the other side w3s no longer white light, hut exhibited a series of colors ranging from red, through orange, yellow, green and blue to violet (shown in Fig. 1and in color 256 1 Journal of Chemical Education
Figure 1. Dispersing prism
on the front cover). Newton drew two conclusions from his observations: 1) Sunlight must consist of a mixture of all the colors observed in 2)
the prismatic spectrum. The prism is capable of dispersing the white light into its constituent colors. The various colors travel at various velocities in the prism material, and therefore have different angles of refraction ( R i
The observed variation of angle of refraction with color is due directly to the wave nature of the incident lieht. Lieht is energy of a special form known as electromagnetic rad~aiiun. (The name results from the association of mcillntine elwtric and magnetic fields with the radiation.) A characteristic ~ r o ~ e rof t vall electromaenetic radiation is the freauencv of . ihe'field oscillation, u, wkch remains invariant as the wave travels through anv medium. The freauencv is related to the by theequation velocity of th; wave, c, and the wawle&h, uA = r . It fullows from thi5 relationshio thar hoth Xand r must vary as a wave of a given frequency tiavels through different media. The frequency can also be related to the energy of the wave through the Einstein-Planck relationship, E = hv,where h is the Planck constant with units of energy times time. A convenient value for h is 4.136 X 10-15 eV-sec. An electron volt ( ~ VisI defined as the energy an electron gains when moved through a votential of one volt. If. for examole. each electron "storeb" an ordinary 12-V automobile Lattery has a potential energy of 12 eV, then this amount of energy is expended by each electron as the hattery discharges in use. The energies of electromagnetic radiation vary from more than 3 X lo6 eV to less than eV. The visible portion of this spectrum, i.e., the energy range of the human eve. .. response . &upies only the very small region betwien about 1.7 and i l eV. An analysir of this visihle region relaiinc.uther variables to color is given in Table 1. Color, although arising from the presence of light, is fundamentally a subjective phenomenon. It is the result of a stimulus received by the eye and interpreted by the brain. A complete descri~tionof the color phenomenon must include three factors: the light source, the ohject it illuminates, and the eye-brain physiological-psychological mechanism which receives and perceives the culur. For the purpuses oi thig prtwntation, a short summary of these tigpics will suffice. A more detailed rliscussion 01these pl~unomenacan he found in the accurnpanying paper hy Thomas Rrill entitled ',Why Objects Appear As They Do."
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Table 1. The Vislble Spectrum Color Red
Orange Yellow Green
Blue Violet
Wavelength
Bandwidth
Frequency
(nm)
(nm)
(cm-')
Energy (eV1
647.0-700.0 585.0-647.0 575.0-585.0 491.2-575.0 420.0-491.2 400.0-420.0
53.0 62.0 10.0 83.8 71.2 20.0
15447-14277 17083-15447 17083-17380 17380-20343 20343-23810 23810-24983
1.77-1.92 1.92-2.12 2.12-2.16 2.16-2.52 2.52-2.95 2.95-3.10
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3.1 400
Energy (eV) Wavelength (nm)
1.7 700
Figure 3. Absorption spectrum of a red object.
3.1 400
Energy (eV) Wavelength (nm)
1.7 700
Figure 2. Spectral energy distribution curve of typical daylighl
The Light Source Every source of illumination emits a range of energies, the intensities of which vary across the energy spectrumto yield a spectral enerm distribution curve. A light source which emits enkrgy with roughly constant intensity over the limited response range of the eye, 1.7 to 3.1 eV, or, in terms of wavelength, 700-400 nanometers (1 nanometer, nm = 10-9 m), is perceived by the eye as "white." Dispersion of this light by a nrism or eratine vields the snectral colors raneine from red at around-1.7 e c violet at around 3.1 eV. on;! way of illustratine the enerev ,.. outnut . of a lieht source is shown in Fieure 2.l Thk source represented here is typical daylighr, and'the relative intensitv of the liaht at each wavrlenath (or enermj is plotted with respect to wavelength (and energy). The Object If the light described by the curve in Figure 2 were allowed to fall on an object which absorbed some of the light, as shown in Figure 3, the light reflected to our eyes would no longer consist of significant intensities of all the wavelengths of visible light. The light in the shaded area, which is largely green and blue light, has been absorbed to a great extent. Our eyes then can be stimulated only by the unabsorbed light a t the red end of the spectrum, and so the ohject which yields this reflectance curve is perceived by the eyes as "red." The shaded area in the diagram is called an absorption band, and the unshaded area is the resulting reflectance curve of this "red" ohject. The color characteristics of most colored objects can be described partially by reference to the shape, width, intensitv and vosition of their resoective ahsorvtion hands. The superimposition of the spectral energy distribution curve of Figure 2 on the reflectance curve of Figure 3 yields a composite curve called the "stimulus for color" curve, which stimulates the eve-brain mechanism (9.10). Color, however, is a very complex phenomenon. Objects can modiiv. lieht not onlv. by. reflectance and selertive absorntion. . . but also by transmission, scattering, dispersion, interference, and diffraction. It is the combination of all these uossihle in-
Figure 4. 1931 CIE standard observer
teradions which ultimately determines the appearance of an ohject. The Eye-Brain Detector-Interpreter After modification, the light must strike a detector in order to he evaluated. The most imwrtant detector when discussine color is the human eye, because perceived color is no more than the suhiective versonal evaluation of the lieht reflected or transmitted to thk eye. A complete description of the color perception process must then involve the stimulus for color curve superimposed on the proper response curve for the human eye, which is slightly different for each human being. In order to obviate this latter difficulty, in 1931 the Commission Internationale de 1'Eclairage (CIE) defined the response curve for a "standard observer." This curve, which is illustrated in Fiaure 4 is actually three curves, one for each response regionbf the spectrum, and it is based upon the Young-Helmholtz theory. This theory postulates that since the retina resnonds to different colors in at least three different ways, tiere must he three different kinds of receptors present in the eye, each of which is sensitive to a particular
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Differences in Solar Spectra: Since our atmosphere creates substantial loss of solar radiation due to absorption and scattering throuehout the soectrum. a solar soeetral curve will deoend won the pnth lrngth id runlieht thruugh the atmusphere and tnrrelore, u p m the position d the sun i n IIWsky. Thus, the varying p z ~ t ~ of o nthe sun from irszen~thgive3 rke r u n fnm:lv of solnr spectral run,rs. Volume 57, Number 4. April 1980 / 257
nortion of the snectrum (11). Modern research has shown that there are indeek three different types of cones in the retina. Each contains one of three lieht-sensitive . oiements which enable the eye to respond to each of the primary colors-red, green, and blue (12,13). Liaht Modlfication to Produce Color Chrmistry has traditionally been defined as the study of matter and the changes that take place in matter. Therefore, the chemistry of color could hp bn)adly defined as the study of the chanres induced in matter as a result of its interaction with visib~e~electroma~netic radiation. If impinging electromagnetic radiation of the visible region induces no color in the chemical species under observation, then the object will simply interact physically with the visible radiation. It could also, of course, selectively absorb ultraviolet or infrared radiation. On the other hand, if the species under ohservation is capable of ahsorhine .. lieht .. between about 1.7 and 3.1 eV. then some \.isible rlectromagnetic radintion u,ill he absorbed and most of the rest of it will be reflected or transn~itted.The iundamental chemical questions therefore become:
Table 2. Colors of Absorbed LlgM and Corresponding Complementary Colors
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1) What is the process which allows some materials to absorb
visible radiation? 2) Why can some materials undergo this process while others
cannot?
3) Why is this process different for different materials?
The answers to these questions are not simple, and they involve a reiteration of several important theories of atomic and molecular structure. The conceots of molecular structure develooed from the theory of atomic structure provide us with a picture of vibrating nuclei linked bv electrons located in uermitted orbitals of different energies. The consolidation of the quantum theory by the successful interpretation of the energy distribution of blackbody radiation, the line spectra of atoms, and the band spectra of molecules, leads us to helieve in the existence of permitted energy levels. Absorption of energy by these atoms, molecules, and crystals serves to move the body from place to place (translational energy), to cause electrons in permitted energy levels of the system to enter higher energy levels (electronic energy), to allow the atoms in the system to change their distances with respect to one another (vibrational enerev). and to allow naseous molecular systems to rotate (rota&al energy). he energies of visihlelight are too great to he absorbed as rotational or vibrational energy, hut in many instances, the energies of visihle light are sufficient to promote electrons from the mound state to excited states. For example, the energies necessary to promote hydrogen'ssingle electron from thrgrounrl sti~te(n = 1 1 to~~xciredrr~rpsareall~reatcr than the energies amesponding to visihle light (Lyman Sericsj. Howe\w, the transitions invol\.ing the promorion of an elerrnm from n = 2 to higher states fall within the visihle repion (Ralmer Series). On the hasis of this single observation, it is now oossible to begin to answer our fuudamentd chemical In the firstplace, the process which allows some species to absorb visible radiation is a n electronic process dherehy radiation is absorbed to promote the speciesinto the excited states. In the second place, not all species can undergo this process because their permitted energy leuel differences, which are determined by the structure of each species, lie outside the visible region. Thirdly, different mnterials exhibit different colors, because the energy leuel spacings in their molecules. atoms. and crvstals are different. However. all .. atoms, molecules, and crystals, whether colored or colorless, exhibit absorption of electromagnetic radiation. Since there is no difference in principle between electronic transitions resulting from absorption from the visible or ultraviolet regions, one can say that color is not connected with any one special feature of molecular structure (14,15).
258 1 Journal of Chemical Education
(nm)
Energy lev)
400-420 420-450 445-490 490-510 510-530 530-545 545-580 580-630 630-720
3.10-2.95 2.95-2.76 2.76-2.53 2.53-2.43 2.43-2.34 2.34-2.28 2.28-2.14 2.14-1.97 1.97-1.72
Wavelength
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Color of Absorbed Light
Color Seen
Violet Violet-Blue Blue Cyan Green Green-Yellow Yellow Orange
Green-Yellow Yellow Orange
Red
Red
Magenta Violet
Violet-Blus Blue Cyan
Because vibrational and rotational energy levels are su~erimoosedon the electronic levels in a molecule. a numher bf wavelengths on both sides of the principal absorption hand are alsoabsorbed, giving rise to broad bands which are characteristic of ultraviolet and visible absorption spectra. When these broad bands correspond to eachof several different regions of the visible spect;um, they are capable of inducing a mental color response interpreted as a single color. For example, if red, green, and blue lights are mixed in the proper proportions, the mental color response of the human eye is "white!'If hlue light is subtracted from this mixture, and only the red-green combination remains, the human eye interprets this combination as "yellow." Together, hlue and yellow "complete" the visible spectrum; thus they are termed complementary colors. When a chemical compound absorbs the wavelengths of hlue light from a "white2'light source, the remaining waveleneths will he reflected to the eve and interpreted & the color yellow. Newton himself first recognized these relationships and organized the spectral colors into a color circle. when two colo& directly opposite one another in the circle were mixed in equal proportions, the result was white (considered to be a t the center of the circle). This view leads to an infinite numher of complementary colors, and a numher of variations on Newton's original color circle are in use today (16,17). Table 2 is a rough rendition of Newton's color circle in tabular form. Brill's paper (p. 259) contains a color circle diagram. Literature Clted
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8
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~ " l ~ mOhio, b ~1956. ~ , p. 203. Oma. M. V., Low, M . J. D., and Beer N. S., "Synthetic Blue Pigments: IX-XVI Centuries.I: Literature."Studies in Conssruafion. 25, WO (February, 1980). Eastlake, C.L.," M e t h d ~ a n dMaterials of Painting of thecleat Schoolsand Masfem, Val.I."Daver.NawYork,1960.p.11.
O'Sullivan.Dennot A., "Outlmk Clouds for Color Chemicals Businpas." Chemical and Engineering Neus,57.16 Web. 26.1979). Arislotle, YOn Sense and the Sensible." ( ~ a w l o t o cB-. J. E.1 The Clarendan P-, Oxford. 1908. sect. 2 & 3. Newton. I.. "Optickr,"in "Great Books of the Wasfern Wodd. vol. 34." (Editor Hutchin* R. M.), Encyclapedia Britsnniee, New York, 1952, pp. 386412. Billmeyer. F. W.. .lr. and Saltrman, Mar. "Principles of Color Technology."WileyInterscience. New York, 1966, pp. %12. Oma, M. V., "The Chemical Originaaf Color," J. CHEM. EDUC.,55.478 (1978). MacNichal. E. F., Jr., "Three-Pigment Color Vinion," Scientific Amerreon, 221.48 (Deeembr 1964). Wsld. G., "The Rempton of Human Color Vision," Sliance. 145. lW7 (September
19611.
W a s s e m n , G.S., "The Physiology of Color Vision."Color Research and Applirorian.
