Chemistry and Artists' Pigments Ian S. Butler Department of Chemistry, McGill University, Montreal, Quebec, Canada Richard J. Furbacher Department of Fine Arts, Concordia University, Sir George Williams Campus
S i n a the advent of rummercial color houses during the 17th and 18th centuriei ( I j, artists have rarely needed to prepare their own niments. and todav most artists are ienorant of the chemist&underlyi'ng the pigments which theyise routinely. As nart of the final vear's reauirements for a master's deeree e ( 2 ) ,itwas decided to synthesize four common in ~ i n Arts artists' vipments in the laboratow in order to obtain first-hand experiehce with some of the synthetic procedures involved in ~ i p m e nmanufacture. t We now report details of the svntheses &;he two organic and two inorianir pigments selected together wit h a hriefdi~cussionuithrchemistry involved in each case. The syntheses are all easy to perform and we feel that they will be of interest to others as possible undergraduate chemistry laboratory experiments. Once the pigments are mixed with conventional artists' acrylic, they can be used as paints if the students are interested in doing this. For a more comprehensive description of the interrelationship of chemistry and art, the reader is referred to the two excellent series of articles that appeared in the JOURNAL in 1980 and 1981 (3). Organlc Pigments There are two main types of organic pigments commonly used by artists: (a) lakes-pigments formed by the precipitation of a soluble dye onto an inorganic substrate which then becomes an integral part of the final product and (b) toners-full-strength colors which are already in a suitable pigment form. One example of each type will he prepared, viz., alizarin red and phthalocyanine blue, respectively. Alizarin is the principle color constituent of the madder root (Rubia tinetorum) and was first isolated in 1828by Colin and Robiquet (4). Subsequent research has established that it is in fact 1,2-dihydroxyanthraquinone(I). Natural alizarin was used in ancient Egypt, Persia, and India, and an aluminum lake of madder has been identified on some of the artifacts from the Refuge in Corinth, which are
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Journal of Chemical Education
believed to predate the destruction of that city by the Romans in 146 B.C. (5).Artificial alizarin was first synthesized in Germany in 1868 by Graebe and Lieherman from dibromonanthraauinone ( 6 ) .This event is of historical imoortance in organic &mist& &we alizarin was the first natu;al dyrstuff to he made svntheticallv. This discoverv caused. or at least contributed to, the rapid decline of the ~ r e n i hmaddergrowing industry, although small quantities of natural alizarin were still being used as an artists' color up to 1930 (7). The synthesis of alizarin red pigment is a two-step process involving first the preparation of structure I and then its incorporation into a calcium/aluminum lake. The synthetic procedure reported by Fierz-David and Blangey (8)will be followed with minor modifications. The chemical reactions taking place during the synthesis are shown below (9).
I1 I The important processes for the conversion of alizarin into
a lake were kept a closely guarded secret until the German processes were released by the Allied intelligence agents as part of their investigation of all German industries following the Second World War. Factors such as source of water and chemicals affect the product shade so much that even today alizarin red pigments-differmarkedly from one manufacturer to another (101. Alizarin lake contains both cakiilm and aluminum and the structure proposed for the product is shown below (11) (11).
w
+
c=O 'N H.
-
C-N-
C-NH,
0
In 1928, workers nt Scottish Dyes Ltd. noticed a blue substanceon the wallsofa vat that had been used for the preparation of phthalimide from phthalic anhydride and ammonia (12). This impurity, which was later shown to be iron phthalocyanine with the iron coming from the vat walls, was retained for investigation into its possihle use as a colorant. While this was not the first svnthrsis o l a metal nhthalocva n b e complex, it was the firsttime that the pigment potentik was recoenized. Research into metal ~hthalocvaninecomplexes WHS quickly undertaken and, in i9:15.co&erphthalocvanine (Monastral Hlue) appeared on the market (13). - T h e structure of copper yhthalocyanine is based i n the 16-membered tetrabenzoporphyrazine ring (111). The structure is extremely thermodynamically stable owing to extensive delocalization throughout the ring. Of the six known crystalline forms of copper phthalocyanine, the a-form is preferred as a pigment because its crystals fall within the optimum particle size range for color, covering, and tinting power. In the presence of organic solvents, crystals of the a-form are slowly converted into the larger ones of the B-form. This leads t o the paint in a can gradually becoming
greener as the particles increase in size, and a t the same time the paint loses as much as 90% of its color strength (14). Despite this defect, a-copper phthalocyanine is one of themost widely used blue pigments because it has the unique property of being able to reflect almost pure blue light and is one of the most permanent blues known. Copper phthalocyanine is synthesized by two different general methods and there are literally hundreds of local variations and modifications of these processes. However, those based on phthalonitrile are more expensive than those mine ohthalic anhvdride and urea. An example of the ohthalic anh&de/urea process has been selected for'our purphse (15). From the various color chanees that take place durine the synthesis, it will he evident ihat several intermediates are involved. The steps that occur in the reaction have been the subject of considerable research; those shown below are the currently accepted ones.
D-Copper phthalocyanine is obtained initially from the synthesis, and it has a hard texture and large particle size. Therefore, as a final procedure in the pigment preparation, acid-pasting will be performed. This procedure involves dissolving the hot product in hot concentrated H2S04 and pouring the solution into a large volume of water, thereby precipitating the pigment in the a-rather than the 0-form. Inorganic Pigments
The two inorganic pigments selected are cadmium sulfide yellow and chromium oxide green. Cadmium pigments are among the most important inorganic pigments used by artists. Cadmium sulfide occurs naturally in small amounts as the rare mineral, Greenockite. While this natural pigment was certainly used over 2000 years ago, it is seldom used today (16). The first synthetic cadmium yellow was prepared by GayLussac in 1818 (17).but it only became commerciallv available some 30 years later. hloreovrr, lt was apparently tirst shown ~ u b l ~ rat l v the 1851 Great Exhibition in the Crvstal Palace in ~ n g l a n d(18). One of the chief properties of cadmium sulfide vellow which make it especiallygodd as an artists' color is its resistance to attack by hydrogen sulfide. Unfortunately, many other pigments are not so resistant and this continues to be a serious problem for art galleries because hydrogen sulfide is a common air pollutant in large cities. The major process for the production of cadmium sulfide yellow involves the reaction of cadmium chloride with H2S gas (eqn. (I)), and this is the procedure we will follow. CdClz + H2S
-
CdS + 2HCI
(1)
Historically, this reaction was performed in porous, earthenware bottles, e.g., Woulff bottles (19). The product must be finished before it can be used as a pigment and this is accomplished by calcination, i.e., heating to "65OoC. Calcination alters the structure of the cadmium sulfide and yields a pigment of improved brightness and fastness (16).Industrially, this is a crucial step because overcalcination produces flat green shades, while undercalcination gives reddish shades, and uneven calcination results in color variations even within the same batch of pigment. Chromium oxide green is the most stable green pigment ever developed. Its use as a ceramic glaze was suggested in Volume 62 Number 4 Aoril 1985
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1809 by Vanquelin (20), the discoverer of chromium, h u t it did 1860 (21). Cheminot appear as an artists' coloruntil tally, chromium oxide green is almost pure chromic oxide (CmOs); it is an extremely hard material that scratches quartz, topaz, and zircon. When mixed into paints, t h e pigment has p w r opacity, low tinting power and a relatively dull shade. It also reflects infrared light in a similar manner t o chlorophyll, t h e ereen o i w e n t in nlants. and for this reason i t is used exten&ely in the formuiation of military camouflage paint (22). T h e most common industrial method for t h e production of chromium oxide green pigment is t h e high-temperature reduction of sodium dichromate (23). In practice, t h e dichromate salt is mixed mechanically with excess elemental sulfur and the mixture is placed in a brick chamber fitted with a flue t o allow escape of the sulfur dioxide gas which is formed during t h e reaction. On heating to llOODC,the mixture hecomes hrown h u t turns green a s i t cools. T h e solid mass is broken u p and the unwanted alkali metal salts (eqn. (2)) are leached out with water. T h e pigment is then filtered off, dried, and finally powdered in a hall mill. We will essentially mimick this industrial process in our preparation of chromium oxide green. 20NazCrz07 + 388
-
2OCrzOa
+ 12NanSOr+ 8Naz0 + 12S02
(2)
All the chemicals necessary for the pigment syntheses can be readilv obtained from anv of the maior suppliers and can he used without further purification. Preparation of Alizarin Red L)issolve a mixture of sodium anthraquinone-2-sulfonate (LOO g), NaOH (2W gj, andsodium rhlorate l28g1 in water (670 ~ L ITransfer . this solution to a stainlesu-steel bomb and heat at 1 8 5 T with continuous stirrine for 67 h. (Caution: Durine this time the oressure in t h hOmh ~ will r k c h nlmmt 12 -~ ~ ~-atm ~)Allow , . . &e reaction mixture to cool tu room temperature and open the bomb carefully. Pour the reactron mixture intoa large round-bottomed 11-k containing water r? 1.1 and stir for several minutes. The resulting suspension should be boiled for ahout 30 min. Aiter nroling to room temperature, carefully add sufficient 501 H2S04to give pH 2 when the alirarin product will be orwioitattd from the solurwn. Filler off the finelv divided precipitate at 50% and wash the precipitate repeatedly with wate; until the washings become clear. (Important: Do not attempt to dry the alizarin completely as this will make the subsequent production of the lake extremely difficult ( 2 4 ) . )Next, slowly add 342 mL of a 10%solution of soda ash to 770 mL of a boiling 10%solution of aluminum sulfate, and stir the reaction mixture continuously for 1h. Filter the reaction mixture after cooling to room temperature and dilute the filtrate to 1600 mL. To this solution, add the following materials in sequence: sodium phosphate (10% solution, 164 mL), calcium chloride (10% solution, 290 mL), Turkey Red oil (10%solution, 260 mL), and 448 mL alizarin solution (made from 100 g of the suspension in 448 mL water). Bring this mixture slowly to the boil over a 2-3-h period. Following this, continue boiling for 5 hand then allow the reaction mixture to cool to room temperature. Subsequent filtration, washing with water, and drying to evaporation will yield the desired alizarin red lake (11, -170 g). ~~~~~
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Preparation of Cadmium Sulfide Yellow D i l v e cadmium chloride (4 g) in about 1L water and then bubble HzS gas from a cylinderthrough the gas for about 1h while continually stirrine vieorouslv. (Caution: Owine to the toxicitv and obnoxious smell i f hh;droged s&fide, it is impe&ive that thisreaction he performad . in fumehood.) Filter off~ the resultins vellow .. an efficient ..-~.~.~~. ~ ~~, ~ ~ ~ ". precipitate and dry the product in vacuo. Finish the pigment by calcination at 650DCin a muffle furnace (yield, 1.5-2.0 g). ~
~
~
Preparation of Chromium Oxide Green By grinding them together in a mortar, intimately mix sodium dichromate (29.6 g) and elemental sulfur (4.4 g). Place the mixture in a oorcelain crucible and heat with a hot Bunsen burner flame for a&ut 20 min. The mixture will rapidly turn brown and froth due to the evolution of SOz gas. On cooling to room temperature, a hard, mottled green-brown cake will he formed. Grind up this residue in a mortar and wash repeatedly with water until a uniform green product is ohtained (yield -10 g).
Acknowledgment W e especially thank J. Finkehine and C. Coyle (McGill University) for their technical assistance in completing this project. One of us (I.S.B.) thanks the C.E.G.B. Berkeley Nuclear Laboratories, England, and the C.N.R.S. Laboratoire d e Chimie de Coordination, Toulouse, France, for their hospitality while h e was on sabbatical leave, during which time this manuscript was prepared.
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Preparation of Copper Phthalocyanine Blue In a three-necked, 500-mL round-bottomed flask equipped with a thermometer and a water-cooled condenser and an inlet for the addition of other materials, place a mixture of phthalic anhydride (37 g), urea (40 g), ammonium molyhdate (0.2 g), and 1,2,4-trichlorobenzene (260 g). Heat the mixture on a sand hath to 195'C and stir for about 1 h. While maintaining these reaction conditions for afurther 4 h, add euprous chloride (10 g) in small portions. (Note: The condenser will become blocked from time to time with phthslie snhydride which has sublimed from the reaction mixture. This can he
338
cleared by gently melting the accumulated sublimatewith an external flame, with the condenser cooling water turned off, and pushing the molten solid back into the reaction vessel with a glass rod or long spatula.) Allow the reaction mixture to cool to room temperature and paste off on a medium-porwit~, sintered filter the hluish-green glass funnel.Wash this paste successively with the followinghot sol,,is: 2 N H C ~2, N NaOH, methanol, ethanol, and water. Continue the washing procedure until the washings become colorless. The yield of the final, dried royal-blue pigment should he -25 g.
Journal of Chemical Education
Literature Cited (1) hU&e,c.L.,'~M~Ul0dBand M~tenalsdPaiotingofthecrnrt Shhools and Masters," Vol. I. Dover Publications, he., New York, 1960,pp. a 1 2 . (2) Furbeher, R. J.. M.F.A. Theair Conmrdia Univ., Montrsal,Qucbae,Canada, 1978. (3) (a) 0ma.M. V., J. CHEM. EDUC.,57,253(PartII,234 (PsrtII), 267 (Psrtlll).(1980): (b) J. CHEM. EDUC..58. (1981). (4) Gettens, R. J., and Stout, G. L., "Painting Msterisls-A Short Encyelopedi: Dover Publiealions, Ine., New York, 1966,~. 91. (5) Allen, R. L. M., "Color Chemistry: Appleton-Century-CroEt, Nea. Ymk, 1971, P. 241. (6) VeJataraman, K., (Editor), "The Chemistw of Synthetic Dyes: Acsdemie Proas, Now York. 1952.~.318. (7) Harriron,A.W. C.."The ManufaeturedLaknand Precipitated Pigments: Leonard Hill Publ., London. 1930 (reviad by Remminmn,J. S.. and Francis. W., 1957).P. 239. (8) Ficrz-David. H. E.,and Blangcy, L., "Fundamental Proenses GEDye Chemistry," Inteneience Publ.. New Ywk, 1949,pp. 314-315. (9) Ref. (6). P. 819. (10) Ref. 171, p. 245. (11) Fien-David, H. E., and Rutishauser,Hdu. Chim. Acto, 23,1298 (1940). (12) "Color Index: Vol 4, Society of Dyers and Calowi.ts, Bradford, England, 1971. p. A617
(13) ~ ~ 1 G i .137. p. (14) Mcser, F. H..andThomas, A. L.,"Phthal~cy~cynineCompoundr," ReinholdPubl., New York. 1963. p. 166. (15) Ref. (8).pp. 338-339. (16) Curtis,P. J.,snd Wlight,R.B.,J. OilCol. Chem. Assoe.37.26 (1954). (17) Gay-Lumc, J. L., Ann. Chim. Phya., 2.8 (18181. See also ref (6). Vol. 5, 1971, p. ... BY,.
(18) Laurie, A. P., "The Pigmcnta and Mediums of the Old Masters: MacMiUan & Co., Publ., London, 1914. p. 16. (19) Remmington, J. S.. and Francis. W.."Pigmenta,Their Manufacture. Praperti~a,and Uw:'3rded..hnsrd HillPubl.. London. 1954.rr.IL7. (20) Ref. (4).p. 107.' (21) Laurie, A. P., "New Light on Old Masters." The Sheldon Press, London, 1935,p. 44. (22) Rohin8on.D. J..in"Pigment Handbook: (Editor: Patton, T. C.), Wilay-lnterseien~~ Publ., New York, 1973,pp.352.333. (23) "Pigments. Dyestuffs, and Lakes" (Part 6 of "Paint Technolagy Manual Series"), (Editors: Taylor, C. J. A,, and Marks, S.J.Reinhold Publ. (on behalfof the Oiland Color Chemists Assocl. New York. 1966. n. 173. (24) R e t (8).P.317.
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