Less Familiar Elements in Ceramic Pigments C. J. HARBERT The Harshaw Chemical Company, Cleveland, Ohio
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I H E d e f i n i t i o n of ceramic pigments can be given as inorganic compounds, mixed crystals, solid solutions, colloidal suspensions, and inert oxides, which are stable a t elevated temperatures. Other outstanding physical characteristics are permanence to light and, in most cases, acid and alkali resistance. T h e p a l e t t e of c e r a m i c colors is rather limited compared with the vast number of organic colors, but research has contributed many new colors, especially in the field of the less familiar elements. This paper will deal with ten of these elements, divided into two groups according to their oxides :
reflectance of 73 measured on a Hunter reflectometer with magnesium oxide as an arbitrary figure of 100. A s a ceramic color, rutile has been u s e d f o r c o l o r i n g pottery glazes and bodier:. An especially clean ivory to dark tan color can be obtained with a calcined and q u i c k - g u e n c h e d native rutile. A similar ceramic color, a light yellow, is produced by mixing and calcining equal parts of rutile and zinc oxide. The most irrtportant titania color, however, is a recently discovered p i g m e n t c o n s i s t i n g of chromium, antimony, and titania. This strong and RECORDING PHOTOELECTRIC SPECTROPHOTOMETER inert pigment is rich yelloiv in color, and 2 to 3 Der cent are sufficient to obtain a strong yellow colored pottery body. Analysis of this solid soluGroup 2 . Coloring Oxides Group 1. White Oxides tion of chromium antimonate and titania shows approximate1,y Vanadium Neodymium Titanium Cerium 2 to 5 per cent chromic oxide, 10 to 20 per cent antimon,y Molybdenum Selenium Praseodymium Zirconium Tin Uranium pentoxide, and the remainder titania. The additions of elec- ... tropositive element will strengthen the color and a t the same time will change it to a brownish yellow. This new yellow White Oxides color can stand temperatures in body and glazes up to 1300O Cl. TITAKIUM. Formerly considered a rare element, titanium ZIRCONIUM.I n the form of zirconium oxide, zirconium is is the ninth most prevalent element on earth (1). Despite its commercially produced from the c o r n o n minerals zircon and abundance only three minerals are of economic imPortanCebaddeleyite. When transparent and colored, zircon is sold as ilmenite in the form of ferrous titanate, and rutile and brookite a semiprecious and for the past few years artificisll as titanium dioxide. Industrial application of titania of zircon has been produced successfu~~y. ~~~~~~~~i~~of noteworthy importance started only a few years ago with the zirconium as zirconium silicate and oxide has assumed noteproduction of high-grade material on a commercial scale. worthy importance in the enamel industry, zirconium oxide is a stable white powder with a specific gravity of ~ , 7 ~ Since the price and grade of titania made it acceptable for general use in ceramics, the consumption has increased from a refractive index of 2.2, zirconiumis used successfu~~y in year to year. Its physical characteristics (white color, specific the form of zircon and sodium zirconium silicate in enamel gravity 3.9, refractive index 2.5-2.9) and its acidity a t elevated and glaze frits, to produce opacity. zirconiumoxide it is made new compounds and used as a smelt in the frit and more recently as a mill addition possible. Titania in combination with antimony will increase opacifier; opacificationwith zirconium oxide as a mill the acid resistance of enamels when smelted in the frit. While tion is improved by the olloming factors: titania as a paint pigment is the most opaque white material known, it is not considered a true opacifier for enamels on 1. A zinc oxide containing frit should be used. 2. The particle size of zirconia should not be smaller than account of its solubility. a compound of zinc antimony 0.5 micron. titanate, however, it can be rated equal with the best opacifiers 3. zirconiashould have the proper form. on the market. This new opacifier is a solid solution of zinc Zirconium oxide is either monoclinic or tetragonal or a mixantimonate and zinc titanate, in which zinc can be replaced ture of both, depending on the process of manufacture. A with other electropositive elements such as calcium or barium. good zirconium opacifier will have a t least 75 per cent tetragoWhen added at 4 per cent to a commercial enamel, it gives a
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ceric salts are used for calcination. Cerous salts will yield nal crystals. The average zirconium opacifier added a t 4 yellow ceric oxide. Another important factor, which applies per cent as a mill addition to a commercial frit will give the to all opacifiers, is that the particle size should not be below enamel a reflectance of 71 to 72 on a Hunter reflectometer. 0.5 micron. This can be controlled by a certain method of Ceramic colors made with zirconium oxide are very few, if precipitating ceric hydroxide. Small amounts of impurities any, and are not commercially important a t present. The left in the precipitate during calcination will further improve fact that Zirconium oxide is very inert and near neutral a t the opacifying power of ceric oxide. A good ceric oxide added elevated temperatures may be an asset for opacifiers but is a t 4 per cent to the mill of a commercial frit will give a reobviously a handicap for the development of ceramic colors. flectance of 75, the highest TIN. The element tin is of all known opacifiers. Ceric widely distributed in the form oxide plays a minor role in of tin oxide, but the only ore ceramic colors; the only incommercially i m p o r t a n t is dustrially applied color is ceric c a s s i t e r i t e (SnOz), which titanate, producing a goldoccurs in a few localities such yellow color in glass. Other as Queensland, Bolivia, and cerium colors, which can be the Straits Settlements. The used for porcelain decorating, bulk of cassiterite is reduced are a blue ceric molybdate to tin metal and used for and a bluish green ceric tungtinning sheet iron and prostate. Ceric oxide as a chemiducing various alloys. A relacal glass decolorizer or an tively small amount is used as oxidizing agent for ferrous tin oxide in ceramics Of the ions in glass is used successdifferent grades, the French fully if the presence of arsenic process tin oxide (oxidized is avoided (7). tin powder by means of a M O L Y B D E N U MChief . very hot flame) is considered minerals of this element are one of the best. This type molybdenite, MoS2,and wulof tin oxide is a fine white fenite; PbMo04. The largest powder of tetragonal crystal user of molybdenum is the form with a specific gravity steel industry. Molybdenum of 6.9 and a refractive index and its salts are important of 2.04. It has been used as reagents. Molybdenum an opacifier for many years oxide, Mooa,is grayish white, and has been recognized as has a specific gravity of 4.5, one of the most reliable opaciand is rhombic. The oxide fiers on the market. Added itself is not an opacifier, but a t 4 per cent to the mill of a its lead and barium salts are commercial frit, it produces a used successfully as opacifiers reflectance of 73 to 73.5. in low-fired glass enamels, Several, ceramic colors are such as lead borate and lead P I L O T - P L A N T P R O D U C T I O N O F C O L O R S IN GLOBAR FURNACE madewith tin oxide as its inain borosilicate. Five to ten per constituent, besides its use as cent of lead molybdate are a so-called stabilizer in leid antimonate yellows. Tin oxide mixed and fired with magsufficient to give a good white opaque glass enamel. Other opacifiers, such as tin or zirconium oxides, are not successfully nesium and cobalt oxides produces a sky blue color, the soused in this low-melting glass because 10 to 20 per cent would called Cerulean Blue. This cobalt magnesium stannate is be required to equal the opacity of lead molybdate and a t the permanent to light, is acid and alkali resistant, and is a highly same time the eutectic would be raised too high to render a esteemod artist’s pigment. A well-known ceramic color is glossy surface. Lead molybdate’s one weakness is its sensitivity chrome tin pink, a very stable rose-red to bluish-red glaze to light, which will darken the glass considerably. Lead stain. The composition is about 1 per cent chromic oxide, molybdate fired a t higher temperatures will turn quite yellow 3 per cent silica, and the remainder calcium stannate. The and can be used as a yellow glaze stain. Lead phosphomolybred color is attributed to colloidally dispersed chromic oxide date (6) has been claimed to be a good opacifier, and a few blue (4) and will change to a bluish red with the addition of and green glaze stains (2) have been mentioned, but indusborax. The purest red is obtained with a high calcium contrially only lead molybdate is of any importance a t present. tent. This bears some resemblance to colloidal gold solutions, where the color changes from red to bluish red with the change of p H and is attributed to different sizes of colloidal particles. Coloring Oxides A similar pigment, although yellow, is vanadium tin yellow VANADIUM. Once regarded as a scarce element, vanadium which will be mentioned later under vanadium. CERIUM.This prominent member of the rare earths group is now known to be abundant on the earth’s crust. Main is now commercially produced from monazite sand. In the sources for commercial production are ilmenite and magnetite, form of ceric oxide it is used to a certain extent in ceramics. uranium ores, and vanadium minerals (vanadinite and Its physical properties are: color, yellow or white; crystal chileite). Vanadium is used mainly in the steel industry. In the form of its oxides (VZOsand V20s) and its salts (espeform, cubic; specific gravity, 7.3; and refractive index, 2.19. cially metavanadates) vanadium is gaining more and more The use of ceric oxide as an opacifier has not made much ground in ceramics. Vanadium oxides are used to produce a headway in this country as compared with the Continent. yellow glass. A more recent discovery is a yellow pigment of The price is still prohibitive, and the production of a good white ceric oxide with high opacifying qualities is not easily exceptional stability-vanadium tin yellow. This inert pigment, consisting of 2 to 3 per cent vanadium pentoxide and accomplished. I n order to obtain the white modification, only
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tin oxide, is somewhat similar to chrome tin pink, inasmuch as both depend on the colloidal distribution of small amounts of coloring oxides on a tin oxide lake. It is obvious that the quality and strength of the color will depend much on the best colloidal distribution of vanadium pentoxide and also on the manufacturing process. Two different stains are used; one is a clean opaque yellow, and the other a strong greenish yellow which is more or less transparent and decidedly crystalline. Both yellows can be used as glaze, underglaze, overglaze, and body stains a t temperatures up to 1400” C. SELENIUM.This element is widely distributed on the earth but only in small quantities. There are a number of rare selenium ores which are of little importance commercially. One of the main sources is selenium in one of its allotropic forms as a by-product in the sulfuric acid process. Elementary selenium, selenium oxide, and its salts are of utmost importance in ceramics. At present selenium produces the only pure red color in vitreous enamels, glass enamels, and lowfired glazes. In the form of selenates and selenites, especially as sodium and barium salts, it is used in small percentages for decolorizing glass ( 8 ) ,an optical process based on the absorption of red and blue light. At higher percentages selenium, in combination with cadmium sulfide, produces a rich ruby colored glass when reheated or. annealed. This ruby glasstis made in one firing by the addition of elementary cadmium (6). With smaller amounts of selenium and cadmium sulfide an amber colored glass can be produced. It is obvious that, for the development df a strong selenium red in glass, the presence of cadmium sulfide is necessary. The red color can be made individually by calcining a mixture of cadmium sulfide, cadmium oxide, and elementary selenium at a dull red heat. The resulting product, in which part of the sulfur is replaced by selenium, contains from 5 to 15 per cent selenium, according to the shade of red desired. This cadmium sulfoselenide is also produced by precipitating a soluble cadmium salt, sodium sulfide, and sodium selenide in the proper proportion. It is used as a paint pigment, especially in the form of a lithopone, which is prepared by carrying out the reaction in the presence of barium sulfide and zinc sulfate; coprecipitated cadmium sulfoselenide, barium sulfate, and zinc sulfide are formed. If cadmium sulfoselenide is used in glass enamels such as lead borosilicate, it is necessary to add about 5 per cent cadmium oxide or carbonate to the batch to prevent black spots for which lead sulfide is responsible. NEODYMIUM AND PRASEODYMIUM. These two elements were discovered in 1885 by Auer von Welsbach. They occur in all cerium minerals in a ratio of a t least 2 parts neodymium to 1 part praseodymium. Pure neodymium salts can be obtained by fractional crystallization of the double ammonium nitrates. Neodymium oxide or oxalate is used in glass to produce a very delicate violet, which shows up bluish in the thinner parts of the glass and reddish in the heavier parts and thus exhibits a unique color effect ( 3 ) . Praseodymium used the same way will give the glass a fine greenish yellow tint. If and when a better and cheaper way of producing or separating these two rare earth elements is found, the ceramic industry will undoubtedly take advantage of it. The price for praseodymium, which is very hard to separate completely from neodymium, is too high to be considered. Neodymium, of a greater staining power than praseodymium, does not have t o be completely pure and is used for that reason to a certain extent in the glass industry. Recently neodymium has been tried and successfully applied as a physical decolorizer for glass. This is not surprising, since it is almost an exact complementary color of the blue-green iron color in glass, and is so bright and pure that it cuts down the addition of a gray in the glass to a minimum.
URANIUM. The last and heaviest element in the periodic table is found in the form of its oxides, U03,U02,in the mineral pitchblende and as K ~ 0 ~ 2 U 2 O 3 ~ V 2 0 ~in~ 2carnotite. H20 It is more or less a by-product in the manufacture of radium. Despite its relatively high price, uranium in the form of sodium uranate, uranium oxide (U308), and uranium hydroxide is used quite extensively in ceramics. A prepared yellow stain of sodium uranate, alumina, and silica is employed in high-fire ivory and yellow glazes, the color depending on the amount of yellow used. If sodium uranate is added a t 15-20 per cent to a low-fired lead glaze, a brilliant tomatored glaze is produced. Uranium also plays an important part as a so-called luster. “Lusters” as used here mean compounds or soaps of metal oxides and resin acids, which, when applied to glazed pottery, impart an iridescent color effect to the surface. Uranium luster is of a greenish yellow- iridescence. Glass stained with uranium oxide has a unique color display; it shows a wine yellow color in transmitted light and a greenish yellow fluorescence in reflected light.
Conclusion Most of the colors and opacifiers mentioned have been found and commercially applied within the last ten years. With the progress of commercial production of other less familiar elements-to mention just a few, columbium and tantalum, gallium and indium, and some of the rare earth group-the research chemist will have an opportunity to apply his knowledge and experience with the aid of modern tools for the benefit of a more complete color palette in the ceramic industry and consequently for our everyday life.
Literature Cited (1) Clarke, F. W., and Washington, H. S., U. S. Geol. Survey, Pro-
fessional Paper 127 (1924). (2) Demyr, Leon, Argile, No. 173, 7-11 (1937).
(3) Loeffler, J., GlashiLette, 66, 63-5 (1936). (4) Mellor, J. Wr., Trans. Cemm. Soc., 36, 16-27 (1937). (5) Shively, R. R., U. S. Patent 2,079,339 (May 4, 1937). (6) Silverman, A, Ibid., 1,983,151 (Dee. 4,1934). (7) Stransky, H., Keram. Rundschau, 45, 515-16 (Nov. 10, 1937). (8) Weyl, W., Glass Ind., 18, 73-8, 93, 117-20, 167-71 (1937). RECBIVEDJanuary 21, 1938. Presented as part of the Symposium on the Less Familiar Elements, the Second Annual Symposium of the Division of Physical and Inorganic Chemistry, American Chemical Society, held in Cleveland, Ohio, December 27 t o 29, 1937.
Humid Aging of Fly Ash Brick (Correction) We wish to correct certain errors in the data reported in the arENG.CHEM.,29, 427, 428 (1937)l. ticle under the above title [IND. In Table I the last four samples, instead of being aged 4 weeks ( 2 dry 2 humid box) were aged 2 weeks (2 days in atmosphere before molding and 12 days in humid box after molding). They were placed in a drying oven for 3.73 hours before molding to remove excess moisture. This correction should apply to the notation on Figure l on which these four yield points should be replotted correctly according t o the data in Table I. Also the unit of molding pressure on the abscissa should read lo2 lb./sq. in. instead of 108 1b.jsq. in. The values in the last column of Table I1 should be changed to read one-half as large as reported. Consequently the ordinate in Figure 2 should be divided by two, so as t o read from 0 t o 3 instead of 0 to 6. J. MASON PILCHEB
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FRANK c. VILBRANDI’
