Ceramic colorants

Ceramic Colorants Richard A Eppler, Eppler Associates, Cheshire, Connecticut, United States Ullmann's Encyclopedia of Industrial Chemistry Copyright © 2002 by Wiley-VCH Verlag GmbH & Co KGaA All rights reserved DOI: 10.1002/14356007.a05_545 Article Online Posting Date: June 15, 2000 The article contains sections titled: Introduction Classification Pigment Structures Colorant Systems Industrial Production of Ceramic Pigments Use of Ceramic Pigments in Glazes Ceramic Glazes Application Media Quality Control Introduction Color is an important characteristic of many ceramic products In fact, any product for which aesthetics is a consideration for purchase will be enhanced by proper use of color Thus, ceramic products which use color include dinnerware, tile, porcelain enamel, sanitary ware, and some glasses and structural clay products Ceramic material can be colored in three general ways [1] First, the ceramic material itself may contain transition metal ions that are colored This method is rarely used, except for colored glass, because adequate tinting strength and purity of color cannot be obtained this way A second method to obtain color in ceramics is to induce precipitation of a suitable crystalline phase during processing For example, certain oxides, such as zirconium oxide and titanium dioxide, dissolve in vitreous material at high temperature When the temperature is reduced, the solubility is also reduced and precipitation occurs This method is used for opacification, that is, production of an opaque white color For oxide colors other than white, this method lacks the control necessary for reproducible results It is, however, often used for production of nonoxide colorants, such as gold, copper, and cadmium–selenium crystals, dispersed in a vitreous matrix The third and most common method of obtaining color in a ceramic material is to disperse within that material a colored crystalline phase that is insoluble in the matrix This crystalline phase, commonly called a pigment, imparts its color to the matrix The materials chosen for this purpose must possess certain properties beyond the tinctorial strength required of any pigment [2] To be used in a ceramic material, pigments must be resistant to the high temperature and corrosive environment encountered in the firing process Their rates of dissolution or reaction in the ceramic or molten glaze at high temperature must be low This problem is aggravated by the fine particle sizes (1 – 10 µm) that are required for uniform dispersion of the pigment in the ceramic matrix Classification Most of the materials used in ceramics as pigments are oxides because of their greater stability in oxygen-containing ceramic systems [1] The only important exceptions are the cadmium sulfoselenide pigments These nonoxide-containing materials are used because a bright red color cannot be obtained any other way However, these pigments are very difficult to use in ceramics Great care is required to prevent their being oxidized during firing of the ceramic body The various oxide pigments are classified according to their crystallographic structure (see Table 1) [3] These structures vary significantly in the range of concentration of constituent oxides allowed A wide range of cation substitution may be permitted if the ionic sizes of the substituted ions are similar Different formulations are used to vary the color obtained Thus, some of the crystal classes have individual pigments with various primary colors This effect is particularly evident in the spinel class, wherein are found blue, green, pink, brown, and black pigments Table Classification of mixed-metal oxide inorganic pigments * CAS Crystal class, name registry (i.e., category) number Baddeleyite – Zirconium vanadium yellow baddeleyite [6818701-9] Borate – Cobalt [68608magnesium red-blue 93-5] borate Corundum-hematite – Chrome alumina [681872 Basic chemical formula DCMA number (Zr, V)O 1–01–4 (Co, Mg)2 B2 O 2–02–1 (Al, Cr)2 O3 3–03–5 pink corundum – Manganese alumina pink corundum – Chromium greenblack hematite – Iron brown hematite (Al, Mn)2 O3 3–04–5 [6890979-5] [6818735-9] Cr2 O3 3–05–3 Fe2 O3 3–06–7 Garnet – Victoria green garnet [6855301-5] CaO · Cr2 O · SiO2 4–07–3 Olivine – Cobalt silicate blue olivine – Nickel silicate green olivine [6818740-6] [6851584-4] Co2 SiO4 5–08–2 Ni2 SiO4 5–45–3 Periclase – Cobalt nickel gray [68186periclase 89-0] (Co, Ni)O 6–09–8 Phenacite – Cobalt zinc silicate blue phenacite [6841274-8] (Co, Zn)2 SiO4 7–10–2 [1345536-2] [6861013-9] Co3 (PO )2 8–11–1 CoLiPO 8–12–1 [6861024-2] NiO · BaO · 17 TiO2 9–13–4 Phosphate – Cobalt violet phosphate – Cobalt lithium violet phosphate Priderite – Nickel barium titanium primrose priderite Pyrochlore 27-9] [6818699-2] – Lead antimonate yellow pyrochlore [6818720-2] Pb2 Sb2 O7 10–14–4 [7107718-4] (Ti, Ni, Sb)O 11–15–4 [6861143-8] (Ti, Ni, Nb)O 11–16–4 [6818690-3] [6861142-7] [6818692-5] [6841238-4] (Ti, Cr, Sb)O 11–17–6 (Ti, Cr, Nb)O 11–18–6 (Ti, Cr, W)O 11–19–6 (Ti, Mn, Sb)O 11–20–6 [6818700-8] (Ti, V, Sb)O 11–21–8 [6818693-6] [6818753-1] [6818754-2] [6999168-0] (Sn, V)O 11–22–4 (Sn, Cr)O 11–23–5 (Sn, Sb)O 11–24–8 (Ti, Mn, Cr, Sb)O 11–46–7 [7024809-8] (Ti, Mn, Nb)O 11–47–7 Sphene – Chrome tin pink sphene [6818712-2] CaO · SnO · SiO · Cr2 O3 12–25–5 Spinel – Cobalt aluminate blue spinel [6818686-7] CoAl2 O4 Rutile-cassiterite – Nickel antimony titanium yellow rutile – Nickel niobium titanium yellow rutile – Chrome antimony titanium buff rutile – Chrome niobium titanium buff rutile – Chrome tungsten titanium buff rutile – Manganese antimony titanium buff rutile – Titanium vanadium antimony gray rutile – Tin vanadium yellow cassiterite – Chrome tin orchid cassiterite – Tin antimony gray cassiterite – Manganese chrome antimony titanium brown – Manganese niobium titanium brown rutile 13–26–2 – Cobalt tin bluegray spinel – Cobalt zinc aluminate blue spinel – Cobalt chromite blue-green spinel – Cobalt chromite green spinel – Cobalt titanate green spinel – Chrome alumina pink spinel – Iron chromite brown spinel – Iron titanium brown spinel – Nickel ferrite brown spinel – Zinc ferrite brown spinel – Zinc iron chromite brown spinel – Copper chromite black spinel – Iron cobalt black spinel – Iron cobalt chromite black spinel – Manganese ferrite black spinel – Chrome iron manganese brown spinel – Cobalt tin alumina blue spinel – Chromium iron nickel black spinel – Chromium manganese zinc [6818705-3] [6818687-8] Co2 SnO 13–27–2 (Co, Zn) Al2 O 13–28–2 [6818711-1] [6818749-5] [6818685-6] [6820165-0] [6818709-7] [6818702-0] [6818710-0] [6818751-9] [6818688-9] Co(Al, Cr)O 13–29–2 CoCr2 O4 13–30–3 Co2 TiO4 13–31–3 Zn(Al, Cr)2 O 13–32–5 Fe(Fe, Cr)2 O4 13–33–7 Fe2 TiO4 13–34–7 NiFe2 O4 13–35–7 (Zn, Fe)Fe2 O 13–36–7 (Zn, Fe)(Fe, Cr)2 O 13–37–7 [6818691-4] [6818750-8] [6818697-0] CuCr2 O4 13–38–9 (Fe, Co)Fe2 O 13–39–9 (Co, Fe)(Fe, Cr)2 O 13–40–9 [6818694-7] [6855506-6] (Fe, Mn)(Fe, Mn)2 O4 13–41–9 (Fe, Mn)(Fe, Cr, Mn)2 O4 13–48–7 [7175083-9] [7163115-7] [7175083-9] CoAl2 O4 /Co2 SnO 13–49–2 (Ni, Fe)(Cr, Fe)2 O 13–50–7 (Zn, Mn)Cr2 O 13–51–7 brown spinel Zircon – Zirconium vanadium blue zircon – Zirconium praseodymium yellow zircon – Zirconium iron pink zircon * [6818695-8] (Zr, V)SiO 14–42–2 [6818715-5] (Zr, Pr)SiO 14–43–4 [6818713-3] (Zr, Fe)SiO 14–44–5 Reprinted with permission [3] Color variations within a given structural class are of practical importance Normally, pigments within a given class have excellent chemical and physical compatibility Thus, it is possible to mix them to obtain intermediate shades Pigment Structures The structures of inorganic solid-state materials are governed by several principles [4] In the first place, the nearest-neighbor cation–anion distances almost totally determine the lattice energy of an ionic solid material at room temperature Moreover, the preferred coordination polyhedra of the anions surrounding each cation in ionic solid phases are determined almost solely by the ratio of the ionic size of the cation to that of the anion Furthermore, the structures that can be built from any combination of cations and anions are subject to the rules of electrostatic neutrality Thus, in a stable ionic structure, the valence of each anion, with changed sign, is exactly or nearly exactly equal to the sum of the strength of the electrostatic bonds to it from the adjacent cations Lastly, the coordination of a cation is larger when its charge is less, and is smaller as its field strength increases The result of these principles is that for any given stoichiometry and ionic size of the materials to be used, only one or two structures exist that will accommodate them For example, the spinel class, which contains 19 pigments, is restricted to those materials of A2 BX4 stoichiometry with: 0.06 nm < rA < 0.100 nm 0.055 nm < rB < 0.100 nm where rA and rB are the ionic radii of the respective cations, and X is oxygen Similar restrictions apply to the other structures found in Table Colorant Systems The pigments given in Table are the alternatives available for coloring ceramics Their specific applications are described in this chapter Opacifiers Whiteness or opacity is introduced into transparent ceramic materials, such as glazes, by the addition of substances that will disperse in those materials as discrete particles These particles scatter and reflect some of the incident light To this, the dispersed substance must have a refractive index that differs appreciably from that of the clear ceramic material The refractive index ( ) of most ceramic materials is 1.5 – 1.6 and, therefore, the refractive indexes of opacifiers must be either greater or less than this In practice, opacifiers of higher refractive index are used Some examples of opacifiers are tin(II) oxide ( ), zirconia ( ), zircon ( ), and titania ( for anatase or 2.7 for rutile) In glazes and other ceramic coatings fired at temperatures in excess of 1000 °C, zircon [1490-68-2 ] is the opacifier of choice [5] Its solubility in many ceramic glazes is about % at high temperature and – % at room temperature A customary mill addition would be – 10 % zircon Consequently, most opacified glazes contain both zircon that was placed in the mill and went through the firing process unchanged and zircon which dissolved in the molten glaze during firing, but which recrystallized on cooling The effectiveness of a zircon opacifier is a function of particle size The finer the particle size, the greater is the opacity of the pigment Because this greater fineness is achieved by milling, the finer zircons are also the most expensive On the other hand, zircon for smelting into frit is best when of an intermediate size Therefore, the effectiveness of zircon opacifiers can be improved in partially or fully fritted glazes by smelting some of the zircon into the frit Zirconia is rarely used as an opacifier because in the vast majority of ceramic glazes it reacts with the silica in the glaze to produce zircon [6] Therefore, because zircon is less expensive than zirconia, zircon is preferred Tin oxide is a more effective opacifying agent than any of the other possibilities because its solubility is lower However, the price of tin oxide is so high that it is no longer an economic solution Its use is restricted to those special cases, such as chrome – tin pinks, where it also enhances the effectiveness of the coloring pigment In porcelain enamels and in glazes where the firing temperature is less than 1000 °C, titania [13463-67-7] in the anatase crystal phase is the opacifying agent normally used [7], [8] It is the most effective opacifying agent because it has the highest index of refraction However, at ca 850 °C, anatase inverts to rutile in ceramic systems Once inverted to rutile, titania crystals are able to grow rapidly to sizes that are no longer effective for opacification Moreover, as the rutile particles grow, their absorption band extends into visible wavelengths, leading to a pronounced cream color Thus, while titania is an effective opacifier at lower temperatures, it cannot be used above 1000 °C The solubility of titania in molten silicates is ca – 10 % in most cases, and at room temperature is reduced to ca % Thus, titanium dioxide is customarily used at concentrations of ca 15 % When a pastel color is required, an opacifier plus a pigment is added to the coating If this is done, the overall coating–pigment–opacifier system must be considered in selecting materials The opacifier and the pigment must be compatible For example, zircon opacifiers should be used with zircon or zirconia pigments Chrome–tin pinks are stronger if some tin oxide opacifier is used Titanium-based pigments are used in enamels when titania is the opacifier Black Pigments Black ceramic pigments are formed by calcination of several oxides to develop the spinel structure [9], [10] The formulation of black pigments is an excellent illustration of the wide flexibility of this structure for incorporating various chemical elements Table lists five different black spinel pigments: copper chromite black spinel, iron cobalt black spinel, iron cobalt chromite black spinel, manganese ferrite black spinel, and chromium iron nickel black spinel Selection of a particular black pigment depends somewhat on the specific materia l with which the pigment is to be used If care is not taken, the pigment may show a green, blue, or brown tint after firing A particularly important problem is the tendency of some ceramics to attack the pigment and release any cobalt that may be present Thus, in some cases, using a cobalt-free pigment is desirable The relatively high price of cobalt oxide also encourages use of cobalt-free pigments One black pigment that is not a spinel is chromium black hematite This pigment is limited to zinc-free systems, as it reacts with zinc oxide to yield a brown spinel The basic black pigment is iron cobalt chromite black spinel In some systems, however, it has a slightly greenish tint In zinc-containing bases, therefore, iron cobalt black spinel is recommended For a black with a slightly bluish tint, iron cobalt chromite black spinel with some manganese and a higher concentration of cobalt is used, and for a black with a brownish tint, manganese ferrite black spinel is the pigment of choice In those instances where a cobalt-free system is desirable, copper chromite black spinel can be considered for use in systems fired at < 1000 °C, such as glass colors or porcelain enamels Chromium black hematite can be used in zinc-free systems This is the least expensive black on the market for use in materials fired at > 1000 °C However, the presence of zinc oxide in the material results in a chemical reaction that alters the color For systems containing zinc and fired at > 1000 °C, chrome iron nickel black spinel can be considered This pigment can be used with most glaze systems and at all firing temperatures from cone 06 up to sanitary ware firing temperatures of cone 11 or 12 (for cone temperature equivalents, see Table 2) Table Cone temperature equivalents Cone number 010 09 08 07 06 05 04 03 02 01 10 11 12 * ** Orton standard pyrometric cones * , °C Seger cones, °C Large cones, Small cones, 150 °C** 300 °C** (used in Europe) 894 923 955 984 999 046 060 101 120 137 154 162 168 186 196 222 240 263 280 305 315 326 900 920 940 960 980 000 020 040 060 080 100 120 140 160 180 200 230 250 280 300 320 350 919 955 983 008 023 062 098 131 148 178 179 179 196 209 221 255 12 64 300 317 330 336 335 From the Edward Orton, Jr., Ceramic Foundation, Columbus, Ohio Temperature rise per hour Gray Pigments The simplest way to obtain a gray pigment is to dilute a black pigment with a white opacifier This dilution must be done with great care to provide an even color, without specking Therefore, in most cases, use of a compound that has been formulated to give a gray color is preferred [10] More uniform results are obtained when a calcined pigment, such as cobalt nickel gray periclase, is used For certain special effects in underglaze decorations, a beautiful deep gray can be prepared by using tin antimony gray cassiterite The limitation on the use of this material is the high cost of the tin oxide base material An important point to note with gray is the many subtle shade variations that are possible With appropriate blending of three or four carefully chosen pigments, many different shades are possible On the other hand, uniformity of color in this area requires careful quality control Blue Pigments The traditional way to obtain blue in a ceramic material is with cobalt, which has been used as a solution color since antiquity [11] Today, cobalt is reacted with aluminum oxide to produce the spinel CoAl2 O4 or with silica to produce the olivine Co2 SiO Some formulations are mixtures of these two materials Cobalt silicate involves the use of a higher percentage of cobalt oxide than does aluminate spinel However, the color is only modestly more intense In the spinel system, the shade can be adjusted toward turquoise or green by additions of chromium oxide replacing alumina and zinc oxide replacing cobalt (see section on Green Pigments) At the lower temperatures encountered in porcelain enamels and glass colors, pigments based on cobalt continue to be fully satisfactory both for stability and for tinting strength, which is quite high At the higher temperatures encountered with ceramic glazes, however, difficulties arise from partial dissolution of the pigment The cobalt oxide diffuses into the glaze, giving a defect commonly called cobalt bleeding Thus, in glazes, cobalt pigments have been largely replaced by pigments based on vanadium-doped zircon [2], [12], [13] These pigments are less intense than cobalt pigments and tend toward turquoise Therefore, they are not applicable in all cases However, when they are applicable, they give vastly improved stability The zircon–vanadium blue pigment is made by calcining a mixture (in the stoichiometry of zircon) of zirconia, silica, and ammonium metavanadate in the presence of a mineralizer [2] The latter materials, which are selected from various halides and silicohalides, facilitate transport of silica during the reaction forming the pigment Although there is extensive literature on this subject making many claims with respect to composition, the fact is that for development of a strong blue color, the stoichiometry of zircon must be retained and such mineralizers used as will simultaneously optimize the various transport processes and incorporate the optimum amount of vanadium into the zircon structure when it is formed With these pigments, use of zircon for opacification is generally desirable In addition, at least some zirconium oxide in the glaze is preferred to stabilize the pigment Green Pigments Five of the more important methods to obtain green pigmentation in a ceramic material are discussed in this section [14] 10 Historically, the basis of most green pigmentation was the chromium ion Although chromium oxide itself may be used to produce a green color, this procedure has a number of limitations First, pure chromium oxide has some tendency to fume or volatilize during the firing of the ceramic coating, which leads to absorption into the refractory of the furnace used Second, if tin-containing white pigments or pastel colors containing tin are also in the furnace, the chrome will react with the tin to form a pink coloration Finally, the ceramic material into which chromium oxide is placed must meet particular requirements It must not contain zinc oxide, which produces an undesirable dirty brown color As already mentioned, no tin oxide may be used as opacifier or as a constituent of the glaze More satisfactory results are obtained if chromium oxide is used as a constituent in a calcined ceramic pigment One such system is cobalt chromite blue-green spinel In these spinels, varying amounts of cobalt and zinc appear in tetrahedral sites and varying amounts of alumina and chromium oxide appear in octahedral sites Greener pigments are obtained by using a higher concentration of chromium oxide and a lower concentration of cobalt oxide Conversely, shades from blue-green to blue result from lowering the amount of chromium oxide and raising the amount of cobalt oxide These pigments should not be used in low concentration because they give an undesirable dirty gray color The final type of chromium oxide containing green is Victoria green garnet This material is prepared by calcining silica and a dichromate (sodium or potassium) with calcium carbonate to form the garnet CaO · Cr2 O3 · SiO This pigment gives a beautiful bright green color but is transparent When the color is applied thinly, it has a tendency to blacken Victoria green garnet is not satisfactory for opaque glazes or pastel shades because the tone always has a gray cast and lacks brilliance It can be used only in zinc-free coatings with high calcium content In the presence of zinc, the stability of the garnet structure is inadequate In addition, because this is a difficult pigment to manufacture correctly, the price is high, reflecting the care required Because of all of the difficulties mentioned in the use of chromium-containing pigments, and also because there is a definite limitation on the brilliance of green pigments made with chromium, most ceramic glazes use pigments in the zircon system [2] Originally, pigments in the zirconia–vanadia–silica system were recommended However, because these pigments are, in fact, in-place mixtures of a zircon–vanadium blue pigment and a zirconia–vanadium yellow pigment, superior products can be obtained by preparing the pigments separately, using the optimum preparative conditions for each one Moreover, because the zircon–praseodymium yellows are the strongest in their color family, their use as the yellow constituent of a green blend gives even better results Therefore, the cleanest, brightest, most stable greens are obtained by blending a zircon–vanadium blue and a zircon–praseodymium yellow The bright green shades are obtained from a mixture of ca two parts of the yellow pigment to each part of the blue pigment Finally, copper compounds are used in certain low-temperature firing applications [14] The use of copper is of little interest to the majority of industrial manufacturers, but the 11 colors obtained from it are of great interest to art potters because of the many subtle shades that can be obtained This variety arises because the pH of the glaze affects the color obtained from copper If the glaze is alkaline, a turquoise blue color results; if the glaze is acidic, a beautiful green color develops Copper oxide dissolves in the glaze composition, and is, therefore, a transparent color Because copper oxide volatilizes quite readily, it should not be used above 1000 °C Another limitation on the use of copper colors is the fact that copper oxide renders many lead-containing glazes unsafe for contact with food or drink Therefore, copper pigments should never be used on such articles Yellow Pigments Although a number of systems exist for preparing yellow ceramic colors, there are technical and economic reasons for the use of a particular yellow pigment The pigments of greatest tinting strength, the lead antimonate yellows and the chrome–titania maples, not have adequate resistance to molten ceramic coatings Therefore, other systems must be used if the firing temperature exceeds ca 1000 °C Three of these higher temperature systems are considered in this section Zirconia–vanadium yellows are prepared by calcining zirconium oxide with small amounts of ammonium metavanadate [13], [15] Titanium dioxide or iron oxide may be used to alter the shade In the absence of these latter materials, lemon-yellow is obtained; in their presence, orange-yellow results In ceramic coatings, zirconia–vanadium yellows are usually weaker than tin–vanadium yellows and muddier than praseodymium–zircon yellows However, they are economical stains for use with either zinc-containing or zincfree coatings They are stronger and brighter in low-lead, low-boron glazes Zirconium silicate is the preferred opacifier Tin–vanadium yellows are prepared by introducing small amounts of a vanadium oxide into the cassiterite structure of tin oxide [16] The shade may be varied by addition of titanium dioxide or iron oxide In the absence of these materials, a lemon-yellow shade is obtained A stronger yellow may be made by adding titanium dioxide to the color batch, and the increased strength of this modified yellow is accompanied by an increase in the apparent redness of the pigment when iron oxide is added Tin–vanadium yellow pigments develop a yellow color in all ceramic materials, although the actual shade may be influenced by the nature of the substrate material These are opaque pigments, which need minimum amounts of opacifier However, these pigments are sensitive to reducing conditions Moreover, any blends with chrome-bearing pigments should be avoided The reason is that tin oxide and chrome oxide combine easily to form a compound with a color similar to that of chrome–tin pink, which shows up in the ceramic material as a brown discoloration Finally, grinding the tin–vanadium yellow pigment should be minimized because it tends to weaken the pigment The primary deterrent to the use of tin–vanadium yellows, however, is not any technical deficiency Rather, it is the high cost of the tin oxide that is the major component The result of this high cost, together with the quality of the praseodymium–zircon pigments, 12 has been a decline in the use of tin–vanadium yellows Praseodymium–zircon pigments are formed by calcination of ca % praseodymium oxide with a stoichiometric mixture of zirconium oxide and silica in the presence of mineralizers to yield a bright yellow pigment [2], [16], [17] This pigment is quite analogous to zircon–vanadium blue pigments in that the crystal structure is that of zircon Praseodymium–zircon pigments have excellent tinting strength in high-temperature coatings They can be used in almost any ceramic coating, although preferably with zircon opacifiers They blend well with other pigments, particularly with other zircon and zirconia pigments These pigments are being increasingly used for all applications in which the firing temperature exceeds 1000 °C For lower temperature applications, the tinting strength of the lead antimonate pigments is unsurpassed, except by cadmium sulfoselenides [18] Lead antimonate pigments, which have traditionally been called Naples yellow, are exceptionally clean and bright and have good covering power, requiring little or no opacifier The primary limitation is their instability in ceramic coatings above ca 1000 °C, which leads to volatilization of the antimony oxide Substitutions of cerium oxide, alumina, or tin oxide are sometimes made for a portion of the antimony oxide to improve its stability Thus, although these materials have limited usage in ceramic glazes, they are the pigment of choice in porcelain enamel For the brightest, low-temperature applications, cadmium sulfoselenide yellow can be considered [19] The pure cadmium sulfoselenide colors are produced in a range from primrose yellow through yellow to orange and red Cadmium sulfide itself is yellow to orange, depending on details of its manufacture and the ratio of the alpha to beta forms of the crystal The primrose yellow and light yellow shades are made by precipitating small amounts of zinc sulfide along with the cadmium sulfide One final orange-yellow pigment needs to be considered This is the pigment formed when chromium oxide is added with antimony oxide to titanium dioxide to form a doped rutile [18] This material gives an orange-yellow or maple shade, and is useful in lower melting ceramic coatings Like the lead antimonate yellows, it begins to decompose at ca 1000 °C Although it is of limited use in high-firing ceramics, it is one of the largest volume pigments used in porcelain enamel, where it forms the basis for some of the highvolume appliance colors Brown Pigments By far the most important brown pigment used in ceramics is zinc iron chromite brown spinel [20] This family produces a wide palette of tan and brown shades and can be controlled with reasonable care to produce uniformity within the production variables existing in commercial plants Within the spinel structure, the zinc oxide is found on the tetrahedral sites and the chromium oxide on the octahedral sites The iron oxide is distributed in such a way as to fulfill the requirements of the structure Consequently, adjustment of the formula does result in alteration of the shade For example, a substantial increase in chrome and decrease in zinc results in greener shades 13 in zinc-free coatings and yellow to gray shades in zinc-containing coatings Minor addition of manganese to this system results in yellowish and grayish shades, whereas addition of minor amounts of nickel oxide results in a much darker brown Because they are comparatively inexpensive, these pigments are the brown selected for most applications However, two systems closely related to the zinc iron chromite brown spinel have been developed to improve the firing range and stability of brown pigments The first of these is the addition of alumina to the zinc iron chromite brown spinel This creates a pigment that is a hybrid of the zinc iron chromite brown spinel and the chrome alumina pink spinel It produces warm, orange-brown shades with improved firing stability This pigment is used in coatings that are high in zinc and alumina and low in calcium oxide The alumina–to–zinc ratio is kept as high as practical to improve the brightness and cleanliness of the pigment Another related pigment is a tin-containing iron chromite brown spinel, which is sometimes called a tin tan As produced, this material is a mixture of chromium oxide, tin oxide, and iron aluminate It is always used in a zinc-containing coating to obtain optimum brown shades Most likely, this is because the pigment reacts with zinc from the coating during the firing process to produce a zinc iron chromite brown spinel pigment In coatings that are free of zinc, this pigment produces shades of gray to dark mahogany The pigment has excellent stability at low concentration Therefore, it makes an excellent toner for some tan and beige shades in blends with various pink pigments The final brown pigment to be considered is chrome iron manganese brown spinel Manganese is well-known as the colorant in amethyst-stained glass and, with iron oxide, it has been responsible for the deep brown glazes associated with electrical porcelain insulators, artware, and bean pots It is used, therefore, where a deep brown shade is needed However, in producing medium to light shades of brown, the presence of manganese often causes poor surface and unstable color with tendencies to volatilization Therefore, the use of this pigment is rather restricted Pink and Purple Pigments Only a short step in the color spectrum separates brown and pink This is reflected in the chrome alumina pink spinels, which are similar in crystal structure and behavior to the zinc iron chromite brown pigments except for the absence of iron oxide [21] Chrome alumina pinks are combinations of zinc oxide, aluminum oxide, and chromium oxide Depending on the concentration of zinc, the crystal structure may be either spinel (zinc aluminate–chromite) or corundum (solid solution of chromium in aluminum oxide) The latter is analogous to the composition of a ruby In general, a ceramic coating formulated for chrome alumina pink spinels should be free of calcium oxide, with low concentrations of lead oxide and boric oxide, and with a surplus of zinc oxide and alumina Using an improper glaze results in a brown pigment in place of the desired pink Sufficient zinc oxide must be in the coating to prevent the glaze from attacking the pigment and removing zinc from it A surplus of alumina prevents the molten coating from dissolving the pigment 14 A related, but somewhat stronger, pink pigment is manganese alumina pink corundum This pigment is formulated by addition of magnesium oxide and phosphate to aluminum oxide A pure, clean pigment is obtained The use of a proper formulation, however, is important A zinc-free system with a high concentration of alumina is required Unfortunately, the manufacture of this pigment involves serious pollution problems As a result, several companies have stopped manufacturing it and there is question as to its continued availability The most stable pink pigment is the iron–zircon system [2], [22], [23] It is made by calcining a mixture of zirconium oxide, silica, and iron oxide, using a stoichiometry that will produce zircon This pigment is sensitive to minor variations in the production process, so that one manufacturer's pigment may not duplicate another's [24] Shades extend from coral to pink The pigment is stable in all coating formulations, but those without zinc oxide are bluer in shade The final pink system, and the only one to produce purple and maroon shades as well as pinks, is chrome–tin pink These are pigments produced by calcining mixtures of small amounts of chromium oxide with substantial amounts of tin oxide In addition, most such materials have large quantities of silica and calcium oxide in the formulation The chemistry of these materials is complex and only recently has their chemical composition been determined [25] Mixing ca 90 % tin oxide with small amounts of chromium oxide and either calcium oxide or cerium oxide, together with boric oxide as a mineralizer, gives chrome tin orchid cassiterite This material is a solid solution of chromium oxide in tin oxide Although this is not the crystal structure of most chrome–tin pinks, residual amounts are present in almost all cases It is this residual amount of chromium-doped tin oxide that gives most chrome–tin pinks a somewhat gray or purple overtone For most chrome–tin pinks, addition of substantial amounts of calcium oxide and silicon oxide is required to make chrome tin pink sphene Only in the presence of these materials can pink, red, or maroon shades be obtained In this case the crystal structure is tin sphene (CaO · SnO · SiO2 ) in which chromium oxide is dissolved as an impurity The color of this pigment depends to a great extent on the ratio of the concentration of chromium oxide to that of tin oxide Generally speaking, when this ratio is : 5, the resulting color is green; : 15, purple; : 17 – 20, red or maroon; and : 25, pink These pigments are calcined at 1260 – 1320 °C and, under the right conditions, are stable at these temperatures They can be used in coating materials that are low in zinc and high in calcium oxide Either tin oxide or zirconium silicate opacifiers can be used, but tin oxide as a mill addition improves the strength and stability of the pigment Gold purple, commonly called Purple of Cassius, is an old pigment consisting of tin oxide gel colored by finely divided gold This pigment can be used in low-temperature materials, such as porcelain enamels, where it has good coverage and brilliance It is, however, an expensive pigment This is due not only to the high price of gold, but also to the difficult methods of preparing the pigment 15 Red Pigments There are no oxide systems which can be used to produce a true red pigment that is stable in ceramic systems Therefore, orange, red, and dark red pigments are obtained by the use of cadmium sulfoselenide pigments [19], [26] The specific shade results from varying the ratio of the concentration of cadmium sulfide to that of cadmium selenide An orange pigment is obtained at a ratio of ca : 1, a red pigment at 1.7 : 1, and a deeper red at 1.3 : These pigments are prepared by one of several chemical processes involving wet precipitation of suitable raw materials, such as cadmium carbonate and elemental sulfur or selenium, followed by calcination at 500 – 600 °C under an inert atmosphere Cadmium sulfoselenide pigments require the use of a glaze specially designed for this purpose This glaze contains only small amounts of lead oxide because high-lead flux materials react with selenium in a cadmium sulfoselenide pigment to form lead selenide, which is black The glaze is a low-alkaline borosilicate type It contains a few percent of cadmium oxide, which reduces the potential for dissolution of the pigment in the glaze during firing It is free of vigorous oxidizing agents, such as nitrates, which oxidize the pigment, completely destroying the color These pigments are temperature sensitive Therefore, although they can be used in glass colors, in porcelain enamels, and in low-temperature glazes fired up to ca 1000 °C, they cannot be used in higher temperature applications In order to extend the range of these colors, an inclusion pigment system has recently been developed [2], [27] In this system, cadmium sulfoselenide is incorporated in a clear zircon lattice during manufacture In this way, the superior stability of zircon is imparted to the pigment However, these pigments are difficult to make and not all shades have been made successfully The color palette extends from yellow through orange to red Dark reds are not yet available Precious Metal Compositions Although bright gold, burnished gold, and the corresponding silver and platinum preparations are not ceramic pigments in the strict sense, they still play a considerable role in the decoration of ceramic materials [28] The production of these materials consists essentially of the reaction of pinene with sulfur or hydrogen sulfide to give a pinene thiol, which then reacts with tetrachlorauric acid to yield an auric sulforesinate This gold resinate is dissolved in an organic solvent and then reacted with various additives, in the form of metal organic compounds, to affect the color tone and to achieve the necessary adherence These materials are applied to the outside surface of the ceramic object and fired at 500 – 850 °C Lusters The pigments that are designed as lusters are closely related to the precious metal compositions because they are also usually prepared and applied as organic compounds of metals After firing, they precipitate as a thin, often irridescent layer on the substrate They are not a coherent metallic layer as with precious metal coatings, but rather are an oxide layer Colored lusters result when compounds of transition metals are 16 included in the formulation Iron lusters give light brown-red to golden coatings; cobalt lusters in high dilution, a chocolate brown; copper lusters, a reddish brown; nickel lusters, a light brown; and manganese lusters, a gray brown Industrial Production of Ceramic Pigments Ceramic pigments are customarily prepared by solid-state reactions For that reason, rapid, uniform, and reproducible conversion requires intimate mixing of the raw materials, which must be of optimum particle size for the given reaction In most pigment preparations, additives called mineralizers are included to increase the rate of reaction and make the mixture more uniform For zircon pigments, these mineralizers are usually alkali and alkaline-earth halides [13], [15] Boric acid is often used as a mineralizer for spinel pigments Much of the art of ceramic pigment manufacture is concerned with selection of particular mineralizers for a given reaction After careful mixing, the pigments are calcined in either batch kilns or continuous calciners In this operation, careful attention must be paid to the control of the temperature of the kiln The advantages of batch calcination procedures lie in greater production flexibility and the ability to prepare smaller quantities when required The continuous tunnel kiln provides greater product quality and greater consistency in calcining conditions However, its use requires a minimum production level After calcination, hard clumps of calcined, sintered products are broken in jaw or roll crushers and then ground to the necessary fineness in mills Depending on the particular pigment and the particle size required, either wet or dry ball mills may be used Wet ball milling yields a finer product, but it is considerably more expensive than dry ball milling Some pigments must be washed to remove soluble constituents that would otherwise cause difficulty in the final application Washing is particularly required for pigments containing vanadium oxide The ground, suspended particles are washed in filter presses or decanters The water is removed mechanically and the remaining pigment slip or filter cake dried The final production step involves careful control of color tone by adjustment with toners Toners are formulations at various extremes of the color spectrum covered by a given pigment family and are used to adjust the color of products to specifications Use of Ceramic Pigments in Glazes There are five ways in which ceramic colors can be applied to a glazable ceramic article: as a body stain, as an engobe, as an underglaze color, as a colored glaze, and as an overglaze or glass color The use of a body stain refers to a pigment added to the body formulation itself The technique of using engobes may be described as that of applying a ceramic pigment to a raw body Underglaze decorating is the application of color to a bisque body In colored glazes, the pigment is dispersed in the glaze itself Finally, overglazes or glass colors are applied to the already formed and fired glaze as an overcoat 17 The selection of a technique depends on the requirements of the particular application For example, if an engobe or body stain is used, it must be stable to the bisque as well as to the glost fires An underglaze color or a colored glaze need be stable only to the glost fire On the other hand, an overglaze or glass color need not be stable to either of these firings Moreover, the range of colors and effects which can be obtained is directly related to the stability Some colors, such as bright red, can only be obtained in overglaze decoration or in specially formulated glazes On the other hand, the durability of a decoration or color in service depends largely on the distance between the outer surface of the ware and the pigment Therefore, overglaze decorations are distinctly inferior to other techniques of application with respect to durability in service Ceramic Glazes The ceramic pigment and application method must be compatible with the ceramic glaze that is to be used Glazes having a wide range of firing temperatures are available, from hobby glazes firing at cone 010 to sanitary ware glazes from cone to cone 12 [29-31] Low-firing glazes, which mature at cone 010 to 01, are used primarily by hobbyists and artware potters The reason is that it is only at these temperatures that a full palette of colors can be used in-glaze as well as overglaze In particular, the cadmium sulfoselenide red pigments can be used only in this firing range and only with a specially designed glaze formulation In addition, a wide range of special effects can be used at these firing temperatures These effects include crackle glazes, high-calcium–alumina matte glazes, and crystalline glazes containing materials such as rutile The glazes that fire out at cone 01 to cone consist of most of the dinnerware and tile glazes and a minority of high-temperature artware The color palette at these temperatures is somewhat reduced, but still quite extensive Spinel blacks, blues, bluegreens, and browns are suitable, as are all the pigments based on zirconia and zircon: zircon–vanadium blues, zircon–praseodymium yellows, zirconia–vanadium yellows, zircon–iron pinks, and zirconia grays Tin–vanadium yellows are stable as are chrome–tin pinks Chrome–alumina pigments are stable with proper glaze formulation The colors used in glazes up to cone 12 are much the same as those used at cone 01 to cone Tin–vanadium and zirconia–vanadium yellows are quite stable Chrome–tin pigments are acceptably stable if a suitable formulation is used The same is true of chrome–alumina pinks Chrome–iron–zinc browns are satisfactory with suitable formulations The zircon pigments are all stable to cone 12, although zircon– praseodymium yellow begins to lose strength at higher temperature Colors containing chromium oxide are stable to cone 12 in a zinc-free glaze Application Media For application methods other than in-glaze, the pigment is mixed with an organic medium, which serves only to apply the decorative color to the ceramic substrate This 18 medium must then burn or evaporate completely in the firing of the ceramic The materials that are used are divided into two types: hydrophilic media and lyophilic media Hydrophilic media are always needed if the decorating color is to be applied to a wet substrate, or if aqueous suspensions are to be applied over the decoration in a subsequent operating step A typical example would be underglaze colors applied to bisque-fired pieces that are subsequently to be glazed Hydrophilic preparations can be based on glycerol, ethylene glycol, poly-(ethylene oxide), or polyglycols as binding agents, and water or other alcohols as solvents Lyophilic media are used primarily in overglaze decoration In one application, they are a component in the decalcomania that are extensively used in overglaze decoration In terms of volume, however, the largest amount of lyophilic media is used in silk screening These materials generally consist of a methacrylate-based binding agent, which will depolymerize on firing of the ceramic decoration, and hence, completely evaporate Pine oil or turpentine usually serves as the solvent Quality Control Quality control in the production of ceramic pigments is primarily a matter of controlling the color of the pigment after application [32] The human eye is an extremely sensitive measuring device for color, particularly on a comparative basis [33] Thus, control of color solely by electronic procedures is difficult [34] In many cases, electronic quality control techniques must be backed up with preparation of trial glazes for visual comparison Moreover, it is usually necessary to adjust the color of each lot of pigment to ensure adequate reproducibility from lot to lot In addition, for some applications, the particle size of the pigment is critical If vanadium oxide is used in the formulation, residual free vanadium must be controlled References A Burgyan, R A Eppler, Am Ceram Soc., Bull 62 (1983) 1001 – 1003 Link R A Eppler, Am Ceram Soc., Bull 56 (1977) 213 – 216 Link DCMA Classification and Chemical Description of the Mixed-Metal Oxide Inorganic Colored Pigments, 2nd ed., Dry Color Manufacturers' Assoc., Alexandria, Va 1982 R A Eppler, J Am Ceram Soc 66 (1983) 794 – 801 Link F T Booth, G N Peel, Trans Brit Ceram Soc 58 (1959) 532 – 564 Link C W F Jacobs, J Am Ceram Soc 37 (1954) 216 – 220 Link R D Shannon, A L Friedberg, Univ Ill Eng Exp Sta Bull 456 (1960) – 49 Link R A Eppler, J Am Ceram Soc 52 (1969) 89 – 99 Link R A Eppler, Am Ceram Soc., Bull 60 (1981) 562 – 565 Link 10 W F Votava, Am Ceram Soc., Bull 40 (1961) 17 – 18 Link 11 R K Mason, Am Ceram Soc., Bull 40 (1961) – Link 12 Harshaw Chemical Co., US 441 447, 1948; US 025 178, 1962 (C A Seabright) 13 C A Seabright, H C Draker, Am Ceram Soc., Bull 40 (1961) – Link 19 14 P Henry, Am Ceram Soc., Bull 40 (1961) – 10 Link 15 F T Booth, G N Peel, Trans Brit Ceram Soc 61 (1962) 359 – 400 Link 16 E H Ray, T D Carnahan, R M Sullivan, Am Ceram Soc., Bull 40 (1961) 13 – 16 Link 17 R A Eppler, Ind Eng Chem Prod Res Dev 10 (1971) 352 – 355 Link 18 The Colour Index, 3rd ed., Soc Dyers & Colourists, Bradford-London 1971 19 R A Eppler, D S Carr: Proc 3rd Int Cadmium Conference, International Lead Zinc Research Organization (ILZRO), New York 1982 20 J E Marquis, R E Carpenter, Am Ceram Soc., Bull 40 (1961) 19 – 24 Link 21 R L Hawks, Am Ceram Soc., Bull 40 (1961) – Link 22 Glidden Co., US 189 475, 1965 (J E Marquis, R E Carpenter) 23 Harshaw Chemical Co., US 166 430, 1965 (C A Seabright) 24 R A Eppler, J Am Ceram Soc 53 (1970) 457 – 462 Link 25 R A Eppler, J Am Ceram Soc 59 (1976) 455 Link 26 Glidden Co., US 643 196, 1953; US 777 778, 1957 (B W Allan, F O Rummery) Fabriques de Produits Chimiques de Thann et de Mulhouse, US 528 834, US 528 835 ,1970 (J Gascon) 27 Fabriques de Produits Chimiques de Thann et de Mulhouse, US 445 199, 1969 (B H P Fehr, J Gascon) H D DeAhna, Ceram Eng Sci Proc (1980) 860 – 862 Link 28 Du Pont, US 924 540, 1960 (J B D'Andrea) 29 R A Eppler in D R Uhlmann, N J Kreidl (eds.): Glass Science and Technology, vol 1, Academic Press, New York 1983, pp 301 – 338 30 C W Parmalee, C G Harman: Ceramic Glazes, 3rd ed., Cahners Publ., Boston, Mass 1973 31 F Singer, W L German: Ceramic Glazes, Borax Consolidated, London 1964 32 K Shaw: Ceramic Colors and Pottery Decoration, MacLaren & Sons, London 1962 33 D B Judd, G Wyszecki: Color in Business, Science and Industry, WileyInterscience, New York 1963 34 D A Klimas, A Canonico, Am Ceram Soc., Bull 63 (1984) 445 Link 20 ... of the pigment in the ceramic matrix Classification Most of the materials used in ceramics as pigments are oxides because of their greater stability in oxygen-containing ceramic systems [1] The... the color of products to specifications Use of Ceramic Pigments in Glazes There are five ways in which ceramic colors can be applied to a glazable ceramic article: as a body stain, as an engobe,... of application with respect to durability in service Ceramic Glazes The ceramic pigment and application method must be compatible with the ceramic glaze that is to be used Glazes having a wide