Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Pigment, Ceramic

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_181

Synonyms

 Colorant;  Dye;  Paint;  Stain;  Tint

Definition

A ceramic pigment is usually a metal transition complex oxide obtained by a calcination process which shows three main characteristics: (a) thermal stability, maintaining its identity when temperature increases; (b) chemical stability, maintaining its identity when fired with glazes or ceramic matrices; and (c) high tinting strength when dispersed and fired with glazes or ceramic matrices. Other characteristics as high dispensability in vehicles, high refractive index (in order to avoid transparency and to increase its tinting strength), acid and alkali resistance, and low abrasive strength are also suitable.

The main coloring methods of ceramics are based on dyes or pigments. Strictly a dye or soluble colorant is a colored substance that interacts with the matrix to which it is applied and usually is soluble in the media of application called matrix or substrate. Conversely, ideally a pigment or stain is a colored substance insoluble in the pigmented matrix, is stable in the matrix (it does not interact with the matrix), and, in the case of ceramic pigments used in stoneware or glazes, is stable when temperature increases (thermal stability). Obviously a pigment must have a high tinting strength relative to the materials it colors. As a general rule, ceramic stains and ceramic pigments look pretty much the same before and after firing, but not dyes, based on raw oxides as well as salts such as carbonates or nitrates, because they decompose in firing and dissolve in the glazes or stoneware (Fig. 1a) [1a].
Pigment, Ceramic, Fig. 1

(a) Ideal behavior of interaction of pigments (dispersion) and dyes (dissolution) with the matrix. (b) Color evolution with the firing temperature of screen printing (90 threads/cm) deposition of a Co-Ti ink (1 w.% in the organic polyol media) under a glazed CaO-ZnO-SiO2 monoporosa tile (previously fired at 1,080 °C)

Industrial Ceramic Pigments

In the industry there are several steps for the production of industrial ceramic pigments:
  1. 1.

    Grinding. The raw materials (usually oxides or carbonates) are carefully mixed in mills such as ball mills often with mineralizer agents (inorganic salts as NaCl, NaF, and KNO3) in order to facilitate the pigment crystallization.

     
  2. 2.

    Calcination. The mixture is then calcined in either batch kilns or continuous calciners.

     
  3. 3.

    Washing. After calcination, the powders are washed to eliminate residual salts from mineralizers and other leaching components in order to avoid future firing defects and environmental affections.

     
  4. 4.

    Micronization. The mixture is then ground to the necessary fineness in mills. Micronizers and jet mills are used to break agglomerates.

     
  5. 5.

    Tinting strength control. The final production step involves careful control of the color tone.

     

Because these pigments are formed at high temperatures, they generally offer superb thermal stability and are relatively inert. This results in excellent weathering and light fastness properties. Most of these pigments have superior acid and alkali resistance. They are nonmigrating and nonbleeding in nature and do not interact with substrates. The principal disadvantage of ceramic pigments is their low tinting strength. In addition, some are relatively high in cost. This is particularly true with cobalt-containing pigments. Some of these pigments are difficult to disperse. However, the recent development of easily dispersed ceramic pigments should eliminate this problem. A final concern is the inherent hardness of these pigments. Their hardness can lead to processing system damage through abrasion. When using ceramic pigments, processing system components designed for use with abrasive materials should be considered [1b].

Overview

Stable fired pigments solve some of the problems found in using dyes such as: (a) the use of hazardous raw materials, because many of the unstable oxides or salts used as direct dyes are soluble or toxic, its calcination, by combining these elements along with clays, silica, alumina, etc., makes them stable giving a safety manipulation, is the case of vanadium pentoxide a toxic substance that is safety used in the zirconium vanadium blue zircon ceramic pigment, (b) volatilization in the kiln which is colored by the fumes such as it occurs using vanadium oxide or chromium oxide, and (c) the own stability of the color avoiding chemical reactions of the dye with the components of the glaze, for example, eskolaite Cr2O3, used as natural green dye, reacts with tin oxide and silica of the glazes producing malayaite Cr-CaSnSiO5 pink precipitates, and then a pink undesired coloration is given, or reacts with zinc oxide of the glaze to crystallize ZnCr2O4 spinel, imparting undesired blue-brown color; the use of the cobalt-zinc-alumina-chromite blue-green pigment gives a trustworthy stable range of color from green to blue.

How to give trustworthy and safety color to ceramics is the main proposal of pigment production because adding color to our ceramics is needed to produce ceramic art and it cannot be a tricky proposition. Working with paints, the color you put on your piece is the color of the final paint, but in ceramics the fire also builds the color. Pigment research is addressed to obtain new ceramic pigments in order to reach three main purposes:
  1. (a)

    High thermal stability pigments, addressed to new materials as porcelainized stoneware and correlated use of ink-jet printing techniques, that require obtaining adequate particle size of pigments or precursor solutions that produce the pigment “in situ,” during thermal processing of substrate.

     
  2. (b)

    High chemical resistance pigments, adapted to new glazes and ceramic matrices avoiding particle pigment solubilization or degradation when it reacts in ceramic processing.

     
  3. (c)

    Eco-friendly pigments, with low environmental affection evaluated on its complete life cycle: raw material preparation, transport, processing, product distribution, and waste management.

     
  4. (d)

    Pigments for industrial digital printing (ink-jet). Ink-jet decoration is a relatively recent methodology. Elmquist designed the first practice application of CIJ (continuous ink-jet) in 1951 for tape recorder. Winston developed the first teletype printer by CIJ, and in 1968 the first commercial printer 9600 from AB Dick appeared. A continuous flow of electrical conductive ink drops is thrown to substrate and selectively dispersed by an electric field in the CIJ ancient technology. The DOD (drop on demand) ink-jet was developed later in order to produce only the demanded drops. Firstly, in TIJ (thermal ink-jet) technique, the drops were produced from pressure of bubbles generated into the nozzles by a local heating mechanism. Now, in the PIJ (piezoelectric ink-jet) technique, the drops are produced by pressure due to the deformation when an electric field is applied to a piezoelectric piece disposed in the nozzles.

     
Digital decoration technology is widely developed in other materials (textile, paper, etc.), basically using TIJ technique. It has been claimed for surface ceramic tile decoration due to several advantages based on the fact of absence of contact between the applicator and the decorated surface:
  1. (i)

    High resolution of images. The ink-jet technique allows to throw 2,000–5,000 drops/cm in a strictly controlled way. Using serigraphy, chalcography, or flexography methods, the resolution is very limited. Therefore photographic quality images could be carefully reproduced by ink-jet digital monitored application.

     
  2. (ii)

    Low raw strength of tile and short enameling line required. Due to the absence of contact applicator surface, the raw strength is limited only to the adequate manipulation of raw tile, and then the thickness of piece can be reduced. The number of decoration operations is reduced sometimes at one because it can be applied four colors in a selective way at the same time; likewise the reduced weight of ink deposited and relatively low water concentration reduce the drying time between successive applications, and therefore the length of industrial enameling line is drastically reduced.

     
  3. (iii)

    The topography of the tile surface is not limited and relief surfaces can be easily decorated.

     
  4. (iv)

    Reduction of necessary ink (1 g/m2), fully recycling and washing operations eliminated.

     
  5. (v)

    Simplicity of the decoration process because it can be limited to only one decoration step using the simultaneous four-color application CMYK (cyan, magenta, yellow, black).

     
The main difficulty for ink-jet application in ceramic tile industry is the ink that must show specific properties, basically:
  1. (i)

    High stability, because the precipitation, agglomeration, or viscosity changes can obstruct the nozzles.

     
  2. (ii)

    High color strength is necessary due to reduced weight of ink deposited by dropping; on contrary, diffused and weak colors are obtained and poor decoration is produced.

     
  3. (iii)

    Neutral pH is required in order to prevent corrosive effects on the nozzles, but conductivity is not required in both PIJ or TIJ technologies; contrarily in ancient CIJ technique, the electric charge capacity of drops was necessary for their selective dispersion by an electric field.

     

There are three main families of digital inks: (a) soluble salts such as cobalt nitrate (CYAN), mixture of chromium nitrate and iron nitrate (MAGENTA), mixture of antimony nitrate. nicke and stabilized titania (YELLOW), and mixtures of cobalt and iron nitrates (BLACK) are used for producing “in situ,” when decorated ceramics are fired, classical ceramic pigments such as Co2SiO4 olivine blue pigment, Fe(Fe,Cr)2O4 iron chromite brown spinel, (Ti,Ni,Sb)O2 chromium-antimony yellow rutile, and CoFe2O4 black spinel, respectively; (b) colloidal or ultramicronized classical pigments particles dispersed using dispersant agents; and (c) nanoparticles of classical ceramic pigments, produced by “soft chemistry” such as polyol or sol-gel routes, or metals absorbed on stable sol particles of oxohydroxy compounds of Al, Ti, Sn or Zr (purple of Cassius and its analogous with Cu).

Since the nineteenth century, needs on color stability, glaze resistance, and sure management lead to stabilized fired pigments. On these pigmenting systems, chromophore agents (usually first raw transition heavy metals and p metals) are inertized in high-thermal and low-solubility ceramic matrices.

The main mechanisms of origin of color based on the stabilization-inertization of chromophores are the following:
  1. (a)

    Structural mechanism. In this case the chromophore cations (usually transition metals that absorb, as it is discussed below, selected visible wavelengths by d-d electron transitions) are forming the ceramic network such as Co(II) in Co2SiO4 olivine. In other structural cases, a solid solution is obtained: pigmenting cations substitute structural cations in ceramic matrix as V4+ substitutes Zr4+ in ZrSiO4 zirconium vanadium blue zircon (a cyan pigment also named Turkish blue zircon or turquoise zircon) formulated by CPMA as (V,Zr)SiO4. The above-discussed classical pigments Fe(Fe,Cr)2O4 (iron chromite brown spinel with iron in solid solution into chromite network) and (Ti,Ni,Sb)O2 (chromium-antimony yellow rutile with Sb+5 and Cr3+ in solid solution into rutile lattice), are also solid solutions.

     
  2. (b)

    Inclusion mechanisms. Colored nanoparticles protected into high stable host crystals (e.g. Fe2O3 hematite into zircon crystal particles that gives the pink coral of iron in the zircon ceramic pigment) are the origin of color in this case.

     
  3. (c)

    Mordant mechanism. The origin of color is in this case a metal-based dye stabilized by a mordant compound. The final color depends on the mordant used, due to the formation of metal complexes that cause a change in the molecular orbital energies and hence a shift in UV-Vis absorption bands; e.g. Au in Sn(OH)4 mordant substrate on Cassius purple pigment [1a].

     

In agreement with above-described purposes of research on new ceramic pigments, three main limitations can be pointed out: (a) thermal and chemical stability, (b) structure base, and (c) environmental safety. These limitations are described and illustrated with three examples.

Thermal and Chemical Stability: Cobalt Inks

Usually cobalt oxide Co3O4 used as cobalt precursor raw material is a mixed valence oxide of Co2+ and Co3+ with spinel structure CoIICo2IIIO4 which decompose on firing by reducing the unstable Co3+ to Co2+ producing a profuse pinhole in the glazes if the time of firing is not sufficient due to oxygen release:
$$ {\mathrm{Co}}_3{\mathrm{O}}_4\to 3\mathrm{C}\mathrm{o}\mathrm{O}+\mathrm{\frac{1}{2}}\ {\mathrm{O}}_2\left(\mathrm{g}\right) $$
(1)
The Co2+ ion dissolves in ceramic glazes and imparts an intense blue color to ceramic glazes up to 0.05 w.% and could be used in novel dye ink-jet application in ceramics. This color intensifies in the presence of ZnO, due to the crystallization of a solid solution of Co2+ in willemite (Zn,Co)2(SiO4)2, or in the presence of BaO which precipitates cobalt-celsian solid solution (Ba,Co)(SiO4)2. In glazes containing MgO and TiO2, cobalt gives green colorations, due to crystallization of green cobalt-ilmenite CoTiO3 more stable in boron glazes [1c].

The color evolution with the firing temperature of screen printing (90 threads/cm) deposition of a Co-Ti ink (1 w.% in the organic polyol media) over a glazed “monoporosa” tile (previously fired a 1,080 °C) is shown in Fig. 1b. The green coloration at low temperatures (820, 925 °C) is associated to cobalt-ilmenite CoTiO3 crystallizations with Co2+ in octahedral coordination (absorption bands at 350 nm in the UV range, 700 nm in the visible red wavelength, and 1,200 nm in the IR range, in the spectral and color rendering of Fig. 1). When temperature increases, the cobalt-ilmenite CoTiO3 decomposes and Co2+ solubilizes in the glaze giving intense blue color associated to Co2+ in tetrahedral coordination (absorption bands at 530 nm green, 590 nm orange, and 650 nm red and high reflectivity at 430 nm blue wavelength). At low temperature the color is associated to a pigment mechanism (particles of ilmenite crystallization on the glaze surface), but at high temperatures, the color is due to a dye mechanism (cobalt dissolution in glaze).

Structure Base: The Classical Pigment Palette

A useful colorant classification of the chemicals used as ceramic pigments was reached in 1977 from the requirements of the Toxic Substances Control Act, 94–469 US law, which includes the whole chemical substances used in the USA whether they are toxic or not. The Dry Colors Manufacturer’s Association (DCMA) commends to their Metal Oxide and Ceramic Color Subcommittee of DCMA Ecology Committee the classification of the commercial ceramic pigments giving a standard terminology. This DCMA committee applied a chemical-structural criteria for the classification of ceramic pigments in 14 structural families: (1) baddeleyite [1 ceramic pigment], (2) borate [1], (3) corundum-hematite [4], (4) garnet [1], (5) olivine [2], (6) periclase [1], (7) phenacite-willemite [1], (8) phosphate [2], (9) priderite [1], (10) pyrochlore [1], (11) rutile-cassiterite [11], (12) sphene [1], (13) spinel [19], and (14) zircon [3 ceramic pigments].

In 2010 the Color Pigments Manufacturers Association (CPMA) actualized the classification and chemical description of the complex inorganic color pigments in the fourth edition [2]. CPMA describes the industrial ceramic pigment by a code such as CPMA 1-01-4. The first number on the code is the corresponding number of the structure above discussed (in this case 1=baddeleyite), the second set is the CPMA category number identifying each pigment within a given crystal class (in this case 1=first ceramic pigment on the CPMA list), and the third set of numbers identifies the color of the pigment as follows: (1) violet and red-blue; (2) blue and blue-green; (3) green; (4) yellow and primrose; (5) pink, orchid, coral, and peach; (6) buff; (7) brown; (8) gray; and (9) black (in the above case 4=yellow). Finally, CPMA also gives a classification based upon the predominant use of these pigments; three use categories were adopted for guidance. Category A deals with pigments suspended in glass matrices which require the highest degree of heat stability and chemical resistance to withstand the attack of molten glass. Category B deals with pigments suspended in plastics and other polymers which require only moderate heat stability. Category C deals with pigments suspended in liquid vehicles which require little or no heat stability.

Among the more successful ceramic pigments which gives a complete CMYK palette of color, the pigments based on zircon structure (number XIV in CPMA classification) shown in Fig. 2a and discovered by C. A. Seabright in 1948 stand out.
Pigment, Ceramic, Fig. 2

(a) ZrSiO4 structure from CPMA classification, S.G. I41/ with D2d symmetry for SiO4 group and dodecahedral D2d for ZrO8, (b) CMYK subtractive color model, (c) RGB additive color model, (d) the CMYK zircon palette (cyan (Zr,V)SiO4 DCMA 14-42-2, pink (Zr,Fe)SiO4 DCMA 14-44-5, yellow (Zr,Pr)SiO4 DCMA 14-43-4) including a Cr,Co-ferrite as black component, all 3 % enameled in monoporosa glaze, (e) particles of black spinel pigment by SEM

  1. (a)

    CYAN based on the vanadium blue zircon V-ZrSiO4 (CPMA number 14-42-2). CPMA describes this pigment as an inorganic pigment and as a reaction product of high-temperature calcination in which zirconium (IV) oxide, silicon (IV) oxide, and vanadium (IV) oxide in varying amounts are homogeneously and ionically interdiffused to form a crystalline matrix of zircon. Basic chemical formula: (Zr,V)SiO4. Its composition may include any one or a combination of the modifiers alkali or alkaline earth halides. Use category A. Exceptionally suitable for coloring ceramic glazes and clay bodies. Not generally used in porcelain enamels.

     
  2. (b)

    MAGENTA based on the pink coral Fe-ZrSiO4 (CPMA number 14-44-5). CPMA describes the zirconium iron pink (peach, coral) zircon, as an inorganic pigment, as a reaction product of high-temperature calcination in which zirconium (IV) oxide, silicon (IV) oxide, and iron (III) oxide in varying amounts are homogeneously and ionically interdiffused to form a crystalline matrix of zircon. Basic chemical formula: (Zr,Fe)SiO4. Its composition may include any one or a combination of the modifiers alkali or alkaline earth halides. Use category A. Exceptionally suitable for coloring ceramic glazes. Not generally used in clay bodies or porcelain enamels.

     
  3. (c)

    YELLOW based on the yellow of praseodymium Pr-ZrSiO4 (CPMA number 14-43-4). CPMA describes the zirconium praseodymium yellow zircon, as an inorganic pigment, as a reaction product of high-temperature calcination in which zirconium (IV) oxide, silicon (IV) oxide, and praseodymium (III, IV) oxide (Pr6O11) in varying amounts are homogeneously and ionically interdiffused to form a crystalline matrix of zircon. Basic chemical formula: (Zr,Pr)SiO4. Its composition may include any one or a combination of the modifiers alkali or alkaline earth halides. Use category A. Exceptionally suitable for coloring ceramic glazes and clay bodies. Not generally used in porcelain enamels.

     
  4. (d)

    BLACK usually obtained from spinel structure in CPMA classification such as the ferrite (Mg0,5Fe0,5)(Co0,5Fe1,4Cr0,1)O4 shown in Fig. 2d. CPMA describes the iron cobalt chromite black spinel as an inorganic pigment and as a reaction product of high-temperature calcination in which iron (II) oxide, cobalt (II) oxide, iron (III) oxide, and chromium (III) oxide in varying amounts are homogeneously and ionically interdiffused to form a crystalline matrix of spinel. Basic chemical formula: (Co,Fe)(Fe,Cr)2O4. Its composition may include any one or a combination of the modifiers Al2O3, B2O3, CuO, MnO, NiO, or SiO2. Use categories A, B, and C. Predominantly used for coloring category A substrates.

     

Adequate mixtures of above-described pigments may produce all range of visible colors, using the subtractive color vision model CMYK (where C=cyan, M=magenta, Y=yellow, and K=key associated to a mixture of C+M+Y which gives the black color) shown in Fig. 2b. in opposition to the additive color vision model RGB (R=red, G=green, B=blue) shown in Fig. 2c. Really the black color is not necessary because it could be built mixing CMY, but in ink and paints, it is used in order to simplify and to cheapen the printing manufacture. A detailed study of the CMY zircon pigments can be found in several doctoral theses [4, 5]. Really the color given by CMY zircon pigments is influenced by temperature and glaze or ceramic matrix; therefore usually a wider color palette must be used on pigmenting ceramics.

A total of 44 chemical substances are listed in the CPMA classification referred exclusively to industrial ceramic pigments. But the world of pigments is wider and, for example, on the industrial classification, the main red colors do not appear: the unstable purple of Cassius discovered by Johann Rudolph Glauber in 1659 and the sulfoselenide dated on 1909 and classified as harmful substance due to cadmium presence. The reddish ceramic pigments have been during centuries the driving force in the ceramic pigment field research [6]. The investigation on new host structures for pigmenting also has received attention, but usually, searching the red shades.

As example, a new color alternative to classical zircon palette can be obtained from the perovskite, a new and versatile lattice for ceramic pigments not considered on CPMA classification of industrial pigments. Neodymium perovskite pigments have been obtained by nonconventional methods [7, 8, 9]. Red, blue, green, and gray ceramic pigments based on FeNdO3 perovskite (unmineralized sample or using BaF2+MgF2 flux agent for red and blue pigments, respectively) and CrNdO3 (unmineralized sample or using BaF2+MgF2 flux agent for green and gray pigments, respectively) can be obtained by ammonia coprecipitation from a solution of NdCl3.6H2O, FeCl3.6H2O, CrCl3.6H2O salts in water media.

Dried powders were successively fired at 1,000, 1,100, and 1,200 °C during 6 h of soaking time, and the obtained powders were 5 % enameled in a conventional glaze (1,050 °C). Characterizations of samples are shown in Figs. 3 and 4:
Pigment, Ceramic, Fig. 3

Samples fired at 1,200 °C: (a) powder optical lens view (×40), (b) DRX, crystalline phases. p(perovskite MNdO3, M = Fe or Cr), o(neodymium oxide Nd2O3), (c) enameled samples, (d) UV-Vis-NIR spectroscopy

Pigment, Ceramic, Fig. 4

SEM micrographs of samples fired at 1,200 °C

  1. (a)

    Powder optical lens view (×40) (Fig. 3a) shows red, blue, green, and gray color of the respective perovskites.

     
  2. (b)

    XRD (X-Ray Diffraction) analysis carried out on a diffractometer, using Cu Kα radiation, 20–70 °2θ range, scan rate 0.05 °2θ/s, 10 s per step, and 40 kV and 20 mA conditions, shows that perovskite is the only crystalline phase detected in CrNdO3 samples, but residual Nd2O3 peaks are always observed on FeNdO3 samples (Fig. 3b).

     
  3. (c)

    5 w.% enameled samples in a conventional glaze (1,050 °C) show the visual red, blue, green, and gray color of the respective perovskite color associated to CIEL*a*b* color measurements (Fig. 3c), which were measured following the CIE (Commission Internationale de l’Eclairage) colorimetric method using a spectrophotometer, with standard lighting D65 (natural daylight) and standard observer of 10°. On this method, L* is a measure of brightness (100=white, 0=black) and a* and b* of chroma (−a*=green, +a*=red, −b*=blue, +b*=yellow).

     
  4. (d)

    UV-Vis-NIR (Ultraviolet-Visible-Near Infrared) spectroscopy of enameled samples, collected using a spectrometer through diffuse reflectance technique, shows bands associated to Fe3+ in octahedral coordination (in FeNdO3) or Cr3+ in octahedral coordination (in CrNdO3), respectively, along with sharp bands associated to Nd3+ f-f transitions [12]. Samples with BaF2+MgF2 flux agent addition show similar bands to unmineralized samples, but the absorbance level increases in the Vis-NIR range producing dark colors (dark blue and black color, respectively, on Fig. 3c).

     

The micrographs of samples of unmineralized and BaF2+MgF2 mineralized samples fired at 1,200 °C, carried out by scanning electron microscopy (SEM), are shown in Fig. 4. Both samples show higher cubic crystalline morphology, but unmineralized sample shows higher size and heterogeneous particles that show low aggregation; in contrast BaF2+MgF2 mineralized sample shows a bimodal distribution of particle size (small 0.5 μm edge cubic particles and big 1.5 μm particles).

The preparation method of ceramic pigment allows obtaining new and best ceramic pigments. There are different new methods of preparing ceramic pigments that produce new microstructures and solid solutions [1, 8].

Chemical methods as alternative to solid oxide reaction can be illustrated with above-discussed CrNdO3 perovskite. A polyol route was used from NdCl3.6H2O and CrCl3.6H2O: salts were dissolved in 200 ml of DEG (diethylene glycol) to prepare 5 g. of final product, then refluxed at 150 °C during 2 h, and then charred at 500 °C/1 h and fired at 1,100 °C with soaking time of 6 h. SEM micrograph of powders charred at 500 °C shows aggregates of incipient cubes (Fig. 5c), well developed in fired sample at 1,100 °C (Fig. 5d). Powder develops, 5 w.% enameled in conventional ceramic glaze (1,080 °C), good black shades (L*a*b* = 50.5/−0.6/1.0) without mineralizer addition (compare Figs. 5b and 3c).
Pigment, Ceramic, Fig. 5

CrNdO3 fired at 1,100 °C: (a) powder optical lens view (×40), (b) enameled sample, (c) SEM micrograph of powder (500 °C), (d) SEM micrograph of powder (1,100 °C)

Therefore above neodymiums could be an example of alternative to cyan, pink, and black pigments to the classical zircon color palette. Finally an alternative to yellow of praseodymium in zircon could be a CaxY2−xVxSn2−xO7 yellow pigment based on pyrochlore crystal structure (Fd-3m) which describes materials of the types A2B2O6 and A2B2O7, where the A and B species are generally rare-earth or transition metal species, e.g., Y2Ti2O7 [9]. A yellow pigment with the pyrochlore structure CaxY2−xVxTi2−xO7 is known since 1993 as a substitute for the decreasing variety of available yellow ceramic pigments due to the severe regulation of toxic lead and cadmium. The solubility limit of vanadium in this pigment was found to be 1.5 w.% as V2O5 or 0.13 as x in the above formula expression. Characterization of vanadium in the vanadium pyrochlore yellow pigment by electron spectroscopy by chemical analysis and electron spin resonance showed that the oxidation state of vanadium was V5+ and its yellow color mostly originated from V5+ substituted for Ti4+ [10, 11].

XRD (X-Ray Diffraction) results (Fig. 6a) indicate the crystallization of Y2Sn2O7 as the only crystalline phase in all samples except in x = 0.32 that shows very weak peaks associated to YVO4 along with perovskite peaks. Spectral and color rendering of samples by UV-Vis-NIR reflectance diffuse spectroscopy (Fig. 6b) shows a sharp band centered at 250 nm associated to the Sn4+−O2− band transfer and an additional band centered at 500 nm responsible of yellow color and associated to V5+−O2− band transfer. The maximum of absorbance is observed for x = 0.16 sample. CIEL*a*b* results summarized in Table 1 indicate that the yellow b* parameter increases with x until x = 0.16 and decreases for x = 0.32 indicating a solubility limit of vanadium in this pigment around x = 0.16. The enameled powders were 5 w%. glazed in a CaO-ZnO-SiO2 (1,080 °C) single firing glaze but do not produce color; in Na2O-CaO-PbO-SiO2 (1,000 °C) double firing glaze, the color of all samples is similar (CIEL*a*b* = 78/5/35) (Fig. 6d).
Pigment, Ceramic, Fig. 6

(a) XRD diffractograms of CaxY2−x VxSn2−xO7.0,14Na2SiF6 powders, CRYSTALLINE PHASES: P = Y2Sn2O7, S = SnO2, Y = Y2O3. (b) UV-Vis-NIR spectra of CaxY2−x VxSn2−xO7.0,14Na2SiF6 powders, (c) SEM micrograph of x = 0.16 sample, (d) 5w% x = 0.16 glazed sample

Pigment, Ceramic, Table 1

CIEL a*b* colorimetric coordinates for the CaxY2−xVxSn2−xO7 yellow pigments

Sample (x)

L*

a*

b*

0.04

84.3

1.2

26.6

0.08

87.2

0.2

30.8

0.16

85.7

1.1

34.5

0.32

85.8

0.8

33.5

In order to increase the reactivity of the system, the ceramic optimized composition x = 0.16 was prepared by ammonia coprecipitation (CO) and Pechini route (CI). In both routes Y(NO3)3.6H2O, Ca(NO3)2.4H2O, SnCl2.2H2O, and VOSO4.8H2O (ALDRICH) were used as precursors. In the CO route the precursors were solved in water (250 ml for 5 g of final product), then concentrated ammonia was dropped at 70 °C in continuous stirring until pH = 8; finally powder was obtained by drying in oven at 110 °C. In CI route, a molar ratio metallic cations/ethylene glycol/citric acid = 1:1:1 was used. Firstly precursors were solved in water (250 ml for 5 g of final product) and citric acid was solved at 70 °C in vigorous stirring, and then ethylene glycol was added and maintained 8 h for esterification. The obtained ester was dried at 110 °C and submitted to charring treatment at 250 °C. Na2SiF6 flux agent was added to CO or CI powders by manual mixture in an agatha mortar using acetone media. XRD results obtained at 1,000 °C/6 h for both CI and CE show peaks associated to pyrochlore along with unreacted SnO2 and Y2O3 peaks; CO sample only shows weak peaks associated to SnO2 along with pyrochlore. At 1,200 °C all samples show pyrochlore as the only crystalline phase detected. CO at 1,200 °C shows the best pigmenting results in double firing glaze (L*a*b* = 80.0/5.0/43.7) than CE (79.3/2.6/35.4) and CI (81.7/0.1/31.8).

SEM micrographs of powders fired at 1,200 °C indicate the presence of aggregates of submicrometric particles in all samples (Fig. 6c). Size of aggregates in CE and CO sample (2–6 μm) is similar but more compact in CO sample and higher than in citrate powder (1–4 μm). Also size of particles forming aggregates is higher for CE (500 nm) than for CO (300 nm) and higher than CI powder (150 nm). Likewise SEM-EDX (Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy) microstructural analysis of powders fired at 1,200 °C (not shown) indicates that codopants, vanadium, and calcium present a homogeneous distribution on samples. However the global content of calcium is higher than vanadium, probably due to vanadium evaporation during firing. Likewise the content of vanadium is slightly higher in CO and CE sample than in CI sample in agreement with color intensity observed on glazed samples.

Environmental Safety: Ecotoxicity Evaluation

The environmental safety using ceramic pigments is an important limitation that gives the severe regulation of toxic components such as lead or cadmium. It is necessary to check the safety of the pigments. The leaching test is usually used for determining the ecotoxicity of solids. For example, the European leaching test was performed at a ratio of liquid to solid sample L/S = 16 L/kg. A mixture of 10 g solid sample and 160 mL distilled water was combined in borosilicate glass bottles and then agitated for 24 h at 10 rpm with a rotary agitator. The leachate was then collected by filtration through a 0.45 μm membrane filter. The toxicity of the leachates is evaluated using a battery of bioassays including the photobacterium Vibrio fischeri or Photobacter phosphoreum (Microtox test) and the crustaceans Daphnia magna. Microtox test was based on the bioluminescence measurement of the marine bacteria V. fischeri, within exposure time to leachates of 15 min., using the DeltaTox PS1 Analyzer.

The Microtox ecotoxicity test results for the above-discussed pigments are shown in Tables 2 and 3. In the case of the yellow of the pyrochlore Ca0,16Y1,84V0,16Sn1,84O7, both unmineralized ceramic CE and sonocoprecipitated SONO powders fired at 1,200 °C/6 h are compared in Table 2. In the SONO method, the chloride precursors were dissolved in water/diethylene glycol = 1:1 v/v media and were coprecipitated in ultrasonic bath by dropping 12 % ammonia solution until pH = 8. In agreement to European laws, the limit of EC50 (equivalent concentration of leached that decreases the bioluminescence of V. fischeri to 50 %) is 3,000 ppm. Therefore the pigment accomplishes the European regulations.
Pigment, Ceramic, Table 2

EC5O Microtox results for pyrochlore of tin pigments

Sample

EC50 (ppm)

Ca0,16Y1,84V0,16Sn1,84O7 CE 1,200 °C/6 h)

296,400

Ca0,16Y1,84V0,16Sn1,84O7 SONO 1,200 °C/6 h

65,639

Pigment, Ceramic, Table 3

EC5O Microtox results for perovskite of neodymium pigments

Sample

EC50 (ppm)

FeNdO 3

86,664

2 %BaF2+8%MgF2

SONO fired at 1,100 °C/3 h

CrNdO 3

54,398

2 %BaF2+8 %MgF2

SONO fired at 1,100 °C/3 h

(Mg 0,5 Fe 0,5 )(Co 0,5 Fe 1,4 Cr 0,1 )O 4

2,336

SONO fired at 1,000 °C/3 h

In the case of the yellow of the neodymiums above described (violet of FeNdO3 with 2%BaF2+8%MgF2 SONO fired at 1,100 °C/3 h and black of CrNdO3 with 2%BaF2+8%MgF2SONO fired at 1,100 °C/3 h), the Microtox results are compared with the reference black of spinel (Mg0,5Fe0,5)(Co0,5Fe1,4Cr0,1)O4 SONO fired at 1,000 °C/3 h) in Table 3. In agreement with European limit of EC50 the perovskite of neodymium pigments accomplish the European regulations but not the spinel black due to released chromium. The pigments based on rare-earth components usually give eco-friendly pigments [12].

Cross-References

Notes

Acknowledgments

Authors acknowledge the financial support given by FUNDACION CAJA CASTELLÓN-UJI, P1-1B2010-09 project.

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© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Departamento de Química Inorgánica y OrgánicaUniversitat Jaume I, Edifici Científico-TècnicCastellóSpain