Black Ceramic Pigments Research Articles

Ni-free, black ceramic pigments based on Co—Cr—Fe—Mn spinels: A reappraisal of crystal structure, colour and technological behaviour

A reappraisal of crystal structure, optical properties and technological behaviour of black spinels in the Co—Cr—Fe—Mn system was carried out to define the best compositions and disclose the role of crystal chemistry in colouring performance and pigment-glaze interactions. Twenty ternary and quaternary spinel formulations were designed using crystallochemical criteria and prepared by simulating the industrial synthesis. Powder samples were characterized through X-ray diffraction (Rietveld), optical, Raman and Mossbauer spectroscopies, and technological testing in several glazes and glassy coatings. Black spinels fall in a relatively narrow field of unit cell and inversion parameters in between “chromites” and “magnetites”. Their cation distribution is governed by Co2+ occupancy of tetrahedrally-coordinated site T and Cr3+ occupancy of octahedrally-coordinated site M, with Mn3+ hosted at M, while Mn2+ and Fe3+ are distributed over both sites. Nevertheless, Raman and Mossbauer spectra indicate a growing disorder in cation partitioning going towards the iron-rich terms. The pigment technological behaviour depends to a large extent on crystal chemistry of spinels, with no effect by grain or crystallite size. The best colouring performance corresponds to recommended compositions able to withstand corrosion and change in crystal chemistry in contact with melted glaze. Three different pathways can give rise to excellent black pigments: (a) strongly disordered spinels having an inversion parameter i∼0.2; (b) moderately disordered spinels, free of Mn3+, with i<0.1; (c) apparently ordered and normal spinels halfway from cobalt chromite and cobalt ferrite. Poor technological performances are due to different conditions leaving room to colour bleaching (low Cr amount), glaze bloating (excess of Mn3+) and limited resistance to corrosion (oversaturation of site M by Cr+Fe with formation of kenotetrahedral spinels or eskolaite—hematite solid solutions).

Black ceramic pigments based on open-hearth slag

Black ceramic pigments based on open-hearth slag are obtained. The possibility and sequence of physicalchemical processes occurring during kilning of pigment mixes are evaluated and the nature of the color-carrying phases in the products of their heat-treatment is determined. The relation between the crystal-phase composition and the optical indicators of the slag-containing pigments is studied. It is shown that such pigments are promising for coloring various glazes.

Development of a black ceramic pigment from non stoichiometric hydrotalcites

Abstract Non stoichiometric layered double hydroxides (LDHs) with the hydrotalcite-like structure and containing Co 2+ and Cr 3+ cations in the brucite-like layers have been prepared by coprecipitation at room temperature from aqueous solutions of the corresponding chlorides. Five samples have been prepared, with Co/Cr molar ratios ranging from 2/1 to 1/2. The solids have been characterised by element chemical analysis for cations, powder X-ray diffraction, differential thermal analysis, thermogravimetric analysis, temperature-programmed reduction, Fourier transform-infrared and visible–ultraviolet spectroscopies, and particle size and specific surface area determination. The colorimetric parameters have been also measured. Their calcination at high temperature to yield homogeneously mixed metal oxides has been carried out. Both the original and the calcined solids have been dispersed in two enamels and the colorimetric parameters have been determined too. The optimum composition to yield a black pigment (bulk or dispersed in colourless or white enamels) has been determined.

Synthesis of coloured ceramic pigments by using chromite and manganese ores mixtures

The aim of this work is not only the synthesis of black ceramic pigment with spinel structure using local and inexpensive minerals (chromite and manganese ores) but also throw some light on the relations between the structure and the colour of obtained pigment. Ultimate utilization spinel solid solution in ceramic materials is mostly due to their structure characteristics, their thermal and chemical stability. In this study colour pigments were ned by calcinations at 1250 ºC starting from a mixture of chromite and manganese oxide. Different compositions were tested containing 30, 40 and 50 wt.% of manganese oxide (low and high content respectively). The phase composition and microstructure characterization of both raw material and obtained pigments were evaluated by X-ray diffraction, X-ray fluorescence, polarizing microscope and scanning electron microscope. Furthermore, the colour measurements of the obtained pigments and tiles were evaluated. Composition of all pigments reveal the spinel structure with Cr2FeO4; hematite was also recorded in the mixture of low manganese. The degree of lightness is relatively same in the pigments mixtures of both manganese types. However, tiles could be considered lightness compared with its pigments, especially of the high manganese type. The solid state calcinations reactions are indeed able to form intense brown pigments that can be used as a stain for industrial ceramic applications. The production of less expensive black ceramic pigments from low cost and less pure raw materials that can be utilized in Egypt to substitute for the imported pure oxides or salts is proved superior performance for producing painted pigments.

Ceramic Pigments and the European REACH Legislation: Black Fe2O3-Cr2O3, a Case Study

A Rietveld study for three ceramic pigments is presented. Two commercial black ceramic pigments, belonging to the (CrxFe1−x)2O3 family, are shown to be single crystalline phase with a negligible amorphous phase content. This conclusion is drawn from the existence of a unique crystalline phase and from the analysis of the values of the unit cell parameters. Furthermore, a much complex laboratory-prepared brown ceramic pigment has also been studied. This sample was synthesized by adding Al2O3 and MgO dopants in small quantities, below 3 wt%, to the previous system. The Rietveld analysis has shown the stabilization of two corundum structures with a compositions close to Cr0.50Fe1.50O3 and Al0.30Cr1.70O3, plus a spinel structure with composition close to MgCr2O4. An aluminum-rich amorphous oxide phase in low percentage is needed to balance the chemical analysis derived from the Rietveld study and that from XRF. Full mineralogical phase analyses of commercial pigments, plus the required solubility tests, may allow narrowing down of the number of toxicity tests required by REACH legislation by appropriate grouping of a set of ceramic pigments in representative compound(s). We anticipate that working in this direction may lead to significant savings in terms of costs and also in the required use of animals.

Synthesis of black ceramic pigments from secondary raw materials

Black ceramic pigments were prepared from industrial wastes as raw materials, namely, Cr/Ni-rich sludge generated from Cr/Ni plating and Fe-rich galvanizing sludge generated during steel wiredrawing. The pigment was based on the chrome–iron–nickel black spinel (Ni,Fe)(Fe,Cr) 2O 4 catalogued as 13-50-9 in the DCMA classification and prepared by the common solid-state reaction method, under optimal formulation and processing conditions. The characterised synthesised pigments were tested in typical ceramic glazes and ceramic bodies. Optimal colour development was achieved when the spinel compound with the three elements Fe, Ni and Cr was the major phase formed. This situation was obtained for mixtures in which the amount of Cr/Ni-rich sludge varied between 50 and 75%. The colouring properties were similar to those imparted by a commercial black pigment.

Synthesis of nickel-iron spinel by non-conventional methods

Nickel-iron black ceramic pigments of (Fe0.8Mg0.2)(Fe0.2Ni0.8Cr)O4 and (Fe0.8Mg0.1Zn0.1)(Fe0.2Ni0.8Cr)O4 compositions have been prepared by non conventional methods such as coprecipitation, microemulsion and polymeric gel routes (alkoxides hydrolysis-condensation), and compared with ceramic samples prepared by the solid state reaction from oxide precursors. The mineralising effect of a 2NaCl:NaF flux agent has been also analysed. Both non conventional methods and mineraliser presence increase the reactivity of the samples. The highest reactivity and the best black colour are obtained in the polymeric gel sample prepared from alkoxides; coprecipitated and microemulsion fired pigments are partially solubilised by glazes due to the small particle size exhibited, leading to brown colours.

Minimisation of toxicity in nickel ferrite black pigment

Black ceramic pigments are usually prepared from a mixture of transition metal oxides, including toxic and hazardous elements such as Ni, Co, Cr, and Mn. Spinel type black ceramic pigments based on Ni(Fe,Cr)2O4 prepared by a ceramic method have been optimised to reduce toxic and hazardous components. Ni (an A1 carcinogen as classified by ACGIH)has been partially substituted by inert elements such as Mg and Zn and the content of Cr (an A1 carcinogen in hexavalent form) has been minimised, to obtain a black ceramic pigment in which the important properties (colour and stability) are maintained. The black pigments obtained have been glazed and compared with commercial pigments. While maintaining adequate colouring properties, the concentration of toxic elements in the composition has been reduced.

Determination of Nickel(II) Oxide in Ceramic Pigments by In Situ X‐ray Diffraction Quantitative Analysis

Nickel(II) oxide (NiO) still is extensively used as a raw material for the synthesis of traditional ceramic pigments, despite the fact it is potentially carcinogenic. One of the reasons is that, unfortunately, no widely accepted methods exist for its determination in ceramics so far (the main reason being experimental difficulties). Quantitative phase analysis via powder X‐ray diffractometry, for example, may fail because of the presence of glass and because NiO diffraction peaks overlap those of spinel in spinel‐based pigments. This communication elucidates how to address these experimental difficulties and determine free NiO in the case of black ceramic pigments.