Chemistry and Artists' Colors
Mary Virginia Orna, O.S.U. College of New Rochelle New Rochelle, NY 10801
Part 111. Preparation and properties of artists' pigments
Artists' pigments may he prepared in many ways depending upon t h e nature of t h e starting material and t h e desired oroduct. Naturallv occurrine minerals such a s cinnabar or azurite are prepared by simply grinding t h e material until the desired particle size is reached. Others, such as chrome yellow, are prepared by precipitation of t h e insoluble pigments by interaction of aqueous solutions of soluble salts. Still others, like lead white a n d verdigris, are formed by t h e corrosion of lead and coDDer, res~ectivelv. v fumes of acetic acid. Ultra.. . h. marine blue and chromium oxide green are examples of pigments which are pyrogenetic i n origin, i.e., formed by t h e calcination of t h e starting materials in a furnace. Other pyrogenetic pigments of historical interest are Egyptian hlue and smalt. Synthetic vermillion, zinc white and lampblack a r e examples of pigments prepared by forming fumes, or by combustion or sublimation. Organic pigments a r e prepared by t h e normal routes of organic synthesis, and purified hy washing or recrvstallization, when possihle. T h e following syntheses may b i carrwd out easily in a modestly equipped laboratory or demonsrration tahle:
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1) Pwparntion of Chrome Yellou. Chnme yellow IPbCrO,) may
he prepnrrd rasily by precipitating equimolar quanuties of I'b2' and (Mi2mns from aqueous wlutims of their d u h l e salts. A typical ~.
equation is:
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Pb(N0a)daq) + NazCrOdaq) PbCrOh4 + 2NaNOs(aq) Practical working quantities are 3.0 g of Pb(NO& and 1.5 g of NazCrOd, each dissolved in 25 ml of water. The precipitated product may be filtered with suction, washed with water, and sir-dried. 2) Preparation of Prusian Blue. Prussian Blue is just one of the many names by which this mixd-valence compound of iron is imown. Other names for these so-called "iron blues" are potash blue, Chinese blue, French hlue, and Milori hlue; they are, however, mast accurately described as the "ferriferrocyanide pigments" (26). A typical formula for Prussian blue is Fe(III)NHfle(II)(CN)6.It can be seen from this notation that the iron in the compound is in two different oxidation states, hence the term "mixed-valence" compound. A typical commercial process for the manufacture of the compound is the following:
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The energy necessary to promote this charge transfer is of the order of the energies of red-yellow light. Consequently, Prussian Blue absorbs yellow light very strongly and reflects an intense blue color. A simple laboratory preparation of Prussian Blue (as the potassium, rather than the ammonium, salt) is accomplished by mixing 0.70 g of FeSOc7HzO with 0.82 g of KzFe(CN)s, both dissolved in 25 ml of water. A dramatic blue, finely divided precipitate of FeKFe(CN)s forms, and this may be collected by suction filtration through highretention filter paper such as Whatman No. 5. 3) Preparation of "Thala"B1ue. "Thalo" or phthalocyanine blue was first introduced on the market in 1937 and has been a mainstay of the pigment industry ever since. It is easily made by heating a mixture of phthalic anhydride (or phthalonitrile) with urea and a copper(Il) salt to about 200T in the presence of a molybdate catalyst (27) according to the following equation: 0
I/
0
Phthalie anhydride
NH2
Urea
Phthalowanine blue
The structure of phthalocyanine blue is given in Table 3. A simple laboratory preparation of the pigment may he carried out by intimately mixing 5.0 g of phthalic anhydride, 2.1 g of urea, 2.1 g of CuS01.5H20, and a small pinch of ammonium molybdate catalyst in a small evaporating dish and heating with a gentle Bunsen burner flame with constant stirring. A deep blue product in good yield is formed. When the effervescence due to the emission of Conceases, the product may be purified by adding about 50 ml of 6M HCI and heating the mixture to boiling. This leaches out most of the organic reagents. After filtration, the leaching process may be repeated with 6M NaOH. Violet Grean Orange lnfrasd Ultraviolat Blue Yellow Red
FeSOn. 7H20 + NaaFe(CN)s. 10Hz0 (NH&SOI F ~ ( N H ~ F ~ ( C+ N2NazSOa )B 17H~0 The iron-containing product of this reaction is known as "Berlin White" and can be transformed into Prussian Blue by digestion a t around 90-100T with concentrated HzSOn and oxidation with NazCrz07 or NaC103:
+
The presence of iron in two valence states is necessary for the compound to be colored. This is because the iron atoms in the compound undergo mutual oxidation and reduction by the following process:
500 600 700 BOO ... Wavelength (nm) Figure 7. Comparison of the optical spectra of red ruby and "green ruby." 300
400
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Each of the syntheses described above may he carried out i---n -s verv short neriod of time withverv dramatic results. Filtrationdand dr;ing of the products may take place outside of class time if these preparations are used as demonstrations. With sufficient preparation, all three can he carried out by students within a 75-min. laboratory period. An excellent summary of pigments based upon their hue, origin, and chemical composition is given in the review article hy H. Kiihn (28), and an'overview of methods of synthesis is given by Gettens and Stout (29). Information on the preparation of additional pigments, both organic and inorganic, along with copies of "hard-to-get" references, are available upon reProperties of Artists' Pigments What Makes a Pigment? All materials used for coloring other objects may he classified as either pigments or dyes, depending upon the method of application. Typically, dyes are dissolved in a medium into which the object to he dyed is then immersed. A physical or chemical interaction between the dye and a substrate is necessary in order for the dye to become "anchored" in place. Pigments, on the other hand, do not react with a substrate, and therefore must he applied hy first being mixed with a medium, called the vehicle, which allows the pigment to adhere to the surface (9, 30). The more general term for hoth dyes and pigments is "colorant." Pigments and Crystal Structure The ideal pigment is chemically inert, stable to heat and light, and insoluble in the medium into which it is to he dispersed. If a pigment is even slightly soluble in a given medium, its particles tend to migrate, giving rise to a "bleed effect which is highly undesirable unless it can he controlled. The hue characeristim of a pigment may he determined from the intensitv. "..noaition. and shaw of the spectral reflectance curve, and are dependent not oniy upon the nature of the chemical snecies. but also UDOn the crvstal structure. Titanium white, TiOz, for example, exists in the two crystalline forms, rutile and anatase. Although the rutile form is the more highly reflecting of the two, it also has a strong ultraviolet absorption hand which spills over into the visible region, thus giving a cream undertone to the pigment. Phthalocyanine hlue also exists in two crystal forms, ci and 8, the latter form being greener and weaker in color (31). Other common pigments which exhibit different hues, because of different crystal strurt~~res. are the chrome yelluws and the quinacridones. The ndor of a crystal can also he affected by distorting or rhnnging the dimensions ol'the crystal lattice. It is a wellknown tact that when a diamond is irrad~atedin a nuclear reactor. it turns ereen. This is due to a disturtion of the dia" mond's crystal lattice, thus causing a shift in its absorption spectrum. A dramatic example of the change in crystal dimensions involves the expansion of the crystal lattice in ruhy. Ruby is essentially crystalline alumina, AIzO3, containing small amounts of C F in solid solution. The familiar red-blue color of ruhv is due to Cr3+d-d transitions. If the concentration ol'CrJ' in ruby is increased, which can be done by preparing mechanical mixtures of alumina and chromin. CrlO?, in controlled amounts, the alumina lattice expands because C+' is larger than Al"+. This expansion affecLq the ma&pitude of the d-d energy level difference in Cr'' and cause;; a shift in ruby's reflertance spectrum toward longer wavelengths. As a result. t h ~hlue . reflectance band of red rubv is shifted into region and becomes the dominant band perceived the by the eye, thus causing the "ruby" to appear green (32).This shift in reflectance is illustrated in Figure 7. Although there is no such entity as "green ruhy," the appropriate mechanical mixture of A1203 and Cr203which appears green to the human eye is merely ruhy to which a hit more chromia has been added. 268 / Journal of Chemical Education
Pigments and Light-Scattering Another important property of a pigment is its light-scattering power. White pigment particles interact with light by reflection and scattering, black pigments interact mainly by ahsor~tiou.and colored pigments interact by a combination of all bf these processes.~lfapigment in a paint is unable to effectivelv scatter the light striking it, a good deal of the light and the paint surface is will simpiy travel through the said to be transparent. On the other hand, a high degree of light-scattering by a pigment allows only little transmittance of light. . The higher the light-scattering power, the more opaque the pigment. The light-scattering power, and therefore the hiding power, of a pigment depends on two factors, namely, the pigment pzticle size, and the refractive index of the pigment relative to the medium in which it is dispersed. I 1 a r f , r /Sire ~ A pane of glass is nurmally transparent hecause must of the light striking the glass is transmitted, and very little is reflected, scattered, or absorbed. l f n series of random scrutches are made on the glasj, the pane becomes less transparent hecau.;e sume light is scattered at the scratch interfaces. If the pane is broken, still more l i ~ h is t scattered. and finally, if the broken pieces of glass are ground up, so much light is scattered by the glass that a once transparent h j e c t is nuw opaque. A gradual derrease in particle size has increased the scattering power, and therefore the hiding nower. of the elass. The effect of changing the particle size can also hk observed in materials whichHhsorb light. If a green niement. like malachite. is crushed and then graded for size in& the ;ages 40-50 mi'crometers (pm), 20-25pm and 10-12 pm, as the particle size decreases, the amount of light scattered increases relative to the amount of light absorbed, so the smaller particles appear lighter in color (31). In fact, some pigments can only be ground very coarsely, because too much grinding causes their colors to fade. In general, the scattering ;over of pigment particles reaches a maximum when the particle size approaches half the wavelength of the incident light hut falls off precipitately on either side of the maximum. Refractiue Index. The light-bending power of a medium is measured by its refractiveuindexwhi& according to Snell's Law. is the sine of the angle of incidence divided by the sine of the angle of refraction. When light enters paint films of greater density than air, this ratio increases with decreasing angle of refraction, i.e., with increasing bending of the light wave. If pigment particles are suspended in the medium and have very near the same refractive index as the medium, then the pigment will bend the light to the same degree as the medium and very little light will he scattered. Therefore, the pigment-medium system will be transparent. If, on the other hand. the niement particles have refractive indices very much greater oriess than that of the medium, the light will he bent a t verv different angles hv hoth pigment and medium, and a great ieal of light will he scatterkd. Such apigment-medium svstem will he auite onaaue. Thus, the light-scattering power wiil depend"pon the difference between its own o?a refractive index, and that of the medium. For example, chalk, or whiting, has a refractive index of 1.6, which is very close to that of linseed oil, 1.5. Therefore, chalk scatters very little light when suspended in linseed oil and is so transparent that it is not even classified as a pigment. On the other hand, when chalk is susnended in a water-glue medium and spread out to dry, it is very ettrctive as a pigment, hecause after the water dries. it is the chalk-air interface (with a refractive indexdif~~ference of 0.6) which determines the light-scattering power. Thus, "whitewash," when properly used, can he very opaque. For the same reason, titanium dioxide, Ti02, with arefractive index of 2.6. is a very effective light-scatterer in any medium, and is q u i t e ~ p n ~ ulittle e . ~dema,nstration ~ of this property can In made with the \.arious '.liquid paper" ccrrrectiun tluids on the market today. These products are nothing more than a suspension of Ti02 in a volatile solvent. When applied to a ~
~
~
~
piece of paper, the solvent quickly evaporates. The resulting white hlotch reflects light as well as the paper when viewed in reflected lieht. hut if' the naoer is held uo to the lieht. no light is transGitted by the bio&h and the area appears black in transmitted lieht. This simole orocedure can demonstrate very effectively the hiding capacity of high refractive index pigments. For colored pigments, a further complication arises out of the fact that in the region of the absorption band, the refractive index of the oiement chanees markedlv with wavelength. As a result, &&in wavelengths are scattered to a greater degree than others. eivine rise to different colors of the when viewed riflected and transmitted light. Other properties of pigments such as effect on vehicle viscosity, bulkingvalue, oil absorption and gloss (33) are certainly criteria with which to judge desirability, hut with respect to artists' pigments, these are not nearly as important as stability, insolubility, and light-scattering power. Classification of Artists' Pigments Dves and oiements mav be classified accordine to either thei; chemichconstitution or their application. ~ f ; e"Colour Index," first published in 1924 and now in its third edition, is the leading work in the field of colorant classification (34). I t provides a twofold system of numbering colorants based upon both usage and chemical structure. In Part I, each colorant is given a usage number, and in Part 11. a five-dieit ~~~~~~~~~~~~~~~~~~~~number. ~ & sections h are appropriately cross-referenred. For example, hydrated chromium sesquioxide green, Cr20(OH)r,hns heen designated C.1. Pigment Green 18, chemical constitution number 77289. Althouch this compound can he called Guignet's Green, Permanent Green, or Viridian, its C.I. usage number unambiguously identifies the colorant as chromium oxide ereen (35). Althoueh onlv one artists' pigments manufacturer presentl; gives the C.I. ;sage number on each tube of its ~aints.universallv institutine such is a practice which would be high]; desirable: ~ r e s e n t l ~ , - m a n ~ artists' pigments have a variety of common names, which are
used to identify chemically different pigments. For example, two different manufacturers of artists' ~ s the same .i"e m e o tuse name, cerulean blue, for two entirely different colorants, namely barium manganate blue (Pigment Blue 33) and copper phthalocyanine with some added zinc oxide (Pigment Blue 15 Pigment White 4), respectively. Such confusion could be avoided by giving the C.I. usage numher on the label, thus alerting the artist to the fact that the colorants are different even though they are marketed under the same name. Further examples of such confusion are given in reference (19). Concluslon We have seen in the three parts of this paper that color is a very broad field which cuts across the disciplines of physics, chemistry, biology, psychology, and reaches over into various applied fields such as art and engineering. In exploring this area. we have moved from the realm of theoretical ohvsics to the very practical syntheses and classification of colorants. Since it is an area which touches unon so manv. asnects . of life and learning, it lends itself very nicely to incorporation into secondary school and colleee curricula as a link between theow and practice.
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1 2 6 S u u n d A . k n i e r r o c , a n ~ d cP.rmcnn: l n l n Rille "cn " ' r h ~P i ~ m ~ H n ltn d b "11 \'"I 1 " ~ E l l l n r1'slktI.T C . \ l . l e y lhlCC.rLlnle h e w \'..r& 19i3.pp 4 I 1 2-, M ~ r Y. r H I:hr*marlnr l'lpmcnu. '.a "rhcl'.;tnml Handbmk \' I I.' !'a l ' a t m T C , \ V . I e s . l ~ k r . . # e n : e h e n \ rL,l973,pp 4 4 . . u \ IZh, Kllhn H..'Artlna'C.~lc.l.ri.' C b o R e . . s l , 1.19il.pp ,.?C 129, Cntma. R .I ~ n Strut. d 1; L Palntlnz \ l o t ~ . , l . \ >h rr Enr\rb (4a: U v
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(30)Hallas, G., '"~olourChemiptry. P a n 11: Charsctcriaia of Dyes and Pigment*,.. School
Science Reuielu. 56.699 (1975). (31) Patteram, D.:'The Colour of Pigment Crysta1a:'in pigm men^: An lntmduction to Their Physiesl Chemistry? (Editor: Patterson. D.1.Ameriean Elsevier, New York, I9fil. on.FnLm
Volume 57, Number 4. April 1980 1 269
