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IntroductionThe chemistry of artists' materials has its roots in ancientand medieval technology and medicine. Among the earliestmanufactured chemicals were the synthetic pigmentsEgyptian hlue, which was produced in the third milenniumB.C. 1 , 2 , and vermillion (HgS), a one-time staple of theartist's palette 3).The technical literature of the Middle Agesabounds with recipes for the synthesis of artists' pigments. Adace of honor was accorded to synthetic hlue colorant be-

Mary Virginia Orna, O S UCollege of New RochelleNew Rochelle. NY 10801

cause of the scarcity and prohibitive cost of natural blues 4 ) .According to Eastlake 5). hemistry remained the professed

Chemistry and Artists Colorsart 1 Light and color

auxiliary of painting well intu the seventeenth century.Althuugh, in later centuries, chemistry and art have gonetheir srparate ways and rach discipline has develuped its ownspecinlind vocal~ulary nd methodology, there are still manyare:).; 1 hoth fields which areof mutual interest. Oneof theseareas of overlap is color. I t is a matter of history th at Europe'schemical industrv erew out of dve and ~ i e m e n t anufacture6).In 1978, the e scmated glohz production of synthetic dyesand pigments was close to the two billion dollar mark. Al-though a very small proportion of this output is actually usedbv practicing arti sts , the very magnitude of the husiness as-s&& ongoing research which is constantly producing newH colorants with properties more desirable than many of thetraditional artists' pigments. For this reason, the artists'

0 palette has undergone a series of transformations over the pastcentury and a half. One notable example was the introductionof titanium white, TiOz, in the early 1920's. Far superior to itsolder counterparts, lead white and zinc white, in opacity,c stability, and adaptability to various media, Ti02 has almostcompletely replaced them in artists' usage and has found usein many new products as well.The historical and ongoing links between th e chemical in-M dustry and arti stic endeavor which are centered in their mu-tual interest in colored materials, lead to a series of questionswhich will form the basis of the topics covered in this feature.S (1) What is the nature of light and color? (2) How is lightT modified by colored objects to produce the sensation of color?(3) What features of molecular or crystal structure must hepresent for a compound to he colored? (4) What physicalY properties of colorants make them desirable as artists' pig-ments? 5)How are artists' colors synthesized? 6) How arearti sts' colors classified?

The Nature of Llght and ColorTheories regarding the nature of light and the origin of color

go hack to the ancient Greeks. Aristotle himself is creditedwith making the fir st important contribution to what is nowthe modern theory of selective absorption (7). It was Seneca,a Roman philosopher of the first century, who first noted thata prism reproduces the colors of the rainbow, hut it remainedfor Isaac Newton in the seventeenth century to formulatemodern color theory on the basis of experiment .Light and Color

Newton allowed a narrow beam of sunlight to pass througha prism in a darkened room, and he observed that th e lightemerging from the other side w3s no longer white light, hutexhibited a series of colors ranging from red, through orange,yellow, green and blue to violet (shown in Fig. 1and in color256 1 Journal of Chemical Education

Figure 1. Dispersing prism

on th e front cover). Newton drew two conclusions from hisobservations:

1) Sunlight must consist of a mixture of all the colors observed inthe prismatic spectrum.2) The prism iscapable of dispersing the white light into its con-stituent colors. The various colors travel at v rious velocitiesin the prism material, and therefore have different angles ofrefraction Ri

The observed variation of angle of refraction with color isdue directly to th e wave nature of the incident lieht. Lieht isenergy of a special form known as electromagnetic rad~a iiun.(The name results from the association of mcillntine elwt ricand magnetic fields with the radiation.) A characteristic~ r o ~ e r t vf all electromaenetic radiation is the freauencv ofihe'field oscillation, u, wk ch remains invariant as the wavetravels through anv medium. The freauencv is related to thevelocity of th; wave, c, and the wawle&h, X y theequationuA = r . It fullows from thi5 relationshio thar hoth Xand mustvary as a wave of a given frequency tiavels through differentmedia. The frequency can also be related to the energy of thewave through the Einstein-Planck relationship, E hv whereh is the Planck constant with units of energy times time. Aconvenient value for h is 4.136 X 10-15 eV-sec.An electron volt( ~ V Is defined as the energy an electron gains when movedthrough a votential of one volt. If. for examole. each electronstoreb an ordinary 12-V automobile Lattery has a po-tential energy of 12 eV, then this amount of energy is ex-pended by each electron as the hattery discharges in use. Theenergies of electromagnetic radiation vary from more than 3X lo6 eV to less than eV. The visible portion of thisspectrum, i.e., the energy response range of the human eve.&upies only the very small region betwien about 1 7 and i leV. An analysir of this visihle region relaiinc uther variablesto color is given in Table 1.Color, although arising from the presence of light, is fun-damentally a subjective phenomenon. It is the result of astimulus received by the eye and interpreted by the brain. Acomplete descri ~tio nf the color phenomenon must includethree factors: the light source, the ohject it illuminates, andthe eye-brain physiological-psychological mechanism whichreceives and perceives the culur. For the purpuses oi thigprtwntation, a short summary of these tigpicswill suffice.Amore detailed rliscussion 1these pl~unomena an he foundin the accurnpanying paper hy Thomas Rrill entitled ',WhyObjects Appear As They Do.

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ColorRedOrangeYellowGreenBlueViolet

Table 1 The Vislble SpectrumWavelength Bandwidth Frequency Energy

(nm) nm) (cm- ) (eV1647.0-700.0 5 3.0 15447-14277 1.77-1.92585.0-647.0 62.0 17083-15447 1.92-2.12575.0-585.0 10.0 17083-17380 2.12-2.16491.2-575.0 83.8 17380-20343 2.16-2.52420.0-491.2 71 .2 20343-23810 2.52-2.95400.0-420.0 20.0 23810-24983 2.95-3.10

3 1 Energy eV) 17400 Wavelength nm) 700Figure 2. Spectral energy distribution curve of typical daylighl

The Light SourceEvery source of illumination emits a range of energies, theintensities of which vary across the energy spectrumto yielda spectral enerm distribution curve.A light source which emitsenkrgy with roughly constant intensity over the limited re-sponse range of the eye, 1.7 to 3.1 eV, or, in terms of wave-length, 700-400 nanometers (1nanometer, nm = 10-9 m), isperceived by the eye as "white." Dispersion of this light by anrism or eratine vields the snectral colors raneine from red

at around-1.7 e c violet at around 3.1 eV. on; way of illus-trati ne the enerev outnut of a lieht source is shown in Fieure..2 l T h k source represented here is typical daylighr, and'therelative intensitv of the liaht at each wavrlenath (or en er mjis plotted with respect to wavelength (and energy).The Object

If the light described by the curve in Figure 2 were allowedto fall on an object which absorbed some of the light, asshownin Figure 3, the light reflected to our eyes would no longerconsist of significant intensities of all the wavelengths ofvisible light. The light in the shaded area, which is largelygreen and blue light, has been absorbed to a great extent. Oureyes then can be stimulated only by the unabsorbed light atthe red end of the spectrum, and so the ohject which yields thisreflectance curve is perceived by the eyes as "red." The shadedarea in the diagram is called an absorption band, and theunshaded area is the resulting reflectance curve of this "red"ohject. The color characteristics of most colored objects canbe described partially by reference to the shape, width, in-tensitv and vosition of their resoective ahsorvtion hands. Thesuperimposition of th e spectral energy distribution curve ofFigure 2 on the reflectance curve of Figure 3 yields a compositecurve called the "stimulus for color" curve, which stimulatesthe eve-brain mechanism 9.10).Color, however, is a very complex phenomenon. Objects canmodiiv lieht not onlv by reflectance and selertive absorntion.. .but also by transmission, scattering, dispersion, interference,and diffraction. It is th e combination of all these uossihle in-

1 Energy e V) 1 7400 Wavelength nm) 700Figure 3. Absorption spectrum of a red object.

Figure 4. 1931 CIE standard observer

teradions which ultimately determines the appearance of anohject.The Eye-Brain Detector-Interpreter

After modification, the light must str ike a detector in ordertohe evaluated. The most imwrtant detector when discussinecolor is the human eye, because perceived color is no morethan the suhiective versonal evaluation of the li eht reflectedor transmitted to thk eye. A complete description of t he colorperception process must then involve the stimulus for colorcurve superimposed on the proper response curve for thehuman eye, which is slightly different for each human being.In order to obviate this latter difficulty, in 1931 the Com-mission In terna tiona le de 1'Eclairage (CIE) defined the re-sponse curve for a "standa rd observer." This curve, which isillustrated in Fiaure 4 is actually three curves, one for eachresponse regionbf the spectrum, and it is based upon theYoung-Helmholtz theory. Thi s theory postulates th at sincethe retina resnonds to different colors in a t least three dif-ferent ways, ti er e must he three di fferent kinds of receptorspresent in the eye, each of which is sensitive to a particular

Differences i n Solar Spectra: Since our atmosphere createssubstantial loss of solar radiation due to absorption and scatteringthrouehout the soectrum.a solar soeetral urvewill deoend won thepnth l r n g t h id unlieht t h r u u g h the atmusphere and tnrrelore upmt h e position h e sun i n I IW ky Thus, t h e varying p z ~ t ~ o nf t h esun from irszen~th ive3 rk r u n fnm:lv of solnr spectral run rs.Volume 57, Number 4 pril 198 257

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