The mechanism for the colour change of iron chromium black pigments in glazes through transmission electron microscopy techniques

Black iron-chromium (Fe-Cr) bearing oxide pigments are generally utilised as effective colourants in a wide variety of applications. However, in the case of their use within ZnO-containing glazes, they yield an undesirable brown colour instead of expected black colour. In order to understand the colour change in this system, we report the use of focused ion beam (FIB) sample preparation technique followed by the use of analytical transmission electron microscopy (TEM) characterisation techniques. According to the results, the formation of a reaction layer between the pigment and glaze was identified with an average composition of Zn_0.48Fe_0.79Cr_1.32O₄. Additionally, the valance of Fe was determined as 3+ in the pigment grain, whereas 2+ in the reaction layer and the glaze, respectively. Therefore, it was concluded that the colour change is occurring as result of the valence variation of Fe, the formation of Zn_0.48Fe_0.79Cr_1.32O₄ compound and the outward diffusion of Fe into the glaze.     

Synthesis of Y(Al,Cr)O3 red pigments by co-precipitation method and their interactions with glazes

New red pigment based on the system YAl_1−yCr_yO₃ (y = 0.01–0.1) was synthesized by co-precipitation method. The precipitant was attained by mixing solutions of yttrium, aluminum and chromium nitrates, respectively, and addition of ammonia as the precipitator. The effects of chromium as dopant and glaze composition on the color shade of resulting pigments were studied. EDX analysis of the prepared pigment particles, which was embedded in glaze, showed the occurrence of reactions between some glaze constituents and pigment particles. Accordingly, a glaze which was enriched in Al₂O₃ and poor in ZnO was more suitable in point of achieving a reddish shade. The resulting pigments were characterized by using X-ray diffraction (XRD), SEM and UV–vis spectrophotometer.     

Firing process and colouring mechanism of black glaze and brown glaze porcelains from the Yuan and Ming dynasties from the Qingliang Temple kiln in Baofeng,Henan, China

Black glaze and brown glaze porcelains were an important part of ancient Chinese iron-based high temperature glazes. The excavation of black glaze and brown glaze porcelains from the Yuan and Ming dynasties at the Qingliang Temple kiln site in Baofeng, Henan, China, in 2014, enriched the firing history of this kiln site and history of Chinese ceramics. In this study, black glaze and brown glaze porcelain samples from the Qingliang Temple kiln site from the Yuan and Ming dynasties were selected and analysed via optical microscopy, laser Raman spectroscopy, focused ion beam scanning electron microscopy combined with EDS and energy dispersive X-ray fluorescence to determine their microscopic morphology, microzone composition, microstructure and chemical composition. Moreover, the main wavelength range of the brown glaze porcelain samples were measured by UV–Vis–NIR spectrophotometer systems. The main conclusions of this study are as follows. The brown glaze porcelain from the Yuan and Ming dynasties at the Qingliang Temple kiln site has two different colour layers, with the surface is brown and the bottom is black. The presence of a glass phase and α-Fe₂O₃ phase in the black glaze porcelain samples, and a rare ε-Fe₂O₃ phase in the brown glaze porcelain samples. The brown colour was a result of ε-Fe₂O₃ precipitation, whilst the black base layer also enhanced the brown-colouring effect. Different glaze formulations were used for brown glazed porcelain, some of which were similar to those used for black glaze porcelain and derived from the transformation of black glaze porcelain through different firing atmospheres and cooling rates. Although the formula of the brown glaze porcelain samples exhibited differences, the main wavelength difference was not large, was within the 645–682 nm range and belonged to the visible red region.     

Morphological and structural study of crystals in black-to-brown glazes of Yaozhou ware (Song dynasty) using imaging and spectroscopic techniques

The Yaozhou kiln complex (Shaanxi, China) is a famous black-to-brown ceramic production center due to its significant volume of production, long manufacturing history (from Tang Dynasty to Yuan Dynasty) and variety of decorations. In this work, four representative types of black-to-brown ware from the Yaozhou kilns were selected to study the morphology, structure and distribution of the precipitated crystals in the glazes using a series of imaging and analytical techniques (optical microscopy, XRF, SEM-EDS, TEM-EDS, μRS and UV–vis-IR). The results show that the rare metastable ε-Fe₂O₃ phase, already observed in typical sauce glaze, was also detected in other types of brown glaze decorations, such as light brown stripes on a darker background or irregular brown spots on a black background. Cross-section analyses also showed a three well-separated layered crystalline distribution in the glazes of lighter brown colors as it has been noted in glazes from Qilizhen kilns. Raman analyses revealed that the crystals contained in third layer are of varying nature: ε-Fe₂O₃ in the sauce glaze and magnetite in the light brown stripes. The peak shifts and line broadenings observed in the Raman spectra of ε-Fe₂O₃ crystals were also investigated. They are the result of Al, Ti and Mg substitutions, which were identified using TEM-EDS. Such ionic substitutions would stabilize the metastable ε-Fe₂O₃ crystals by increasing the cationic disorder. In addition, Mg-, Ca- and Fe-rich spinel crystals were observed for the first time in a black glaze of Yaozhou wares.     

Influence of crystallinity on chromatic parameters of enamels coloured with malayaite pink pigments

Chromium-doped malayaite ceramic pigments, CaSnSiO₅:Cr were obtained by solid state reaction using CaCO₃, SnO_2, SiO₂ and PbCrO₄ as precursors. The synthesis process was studied by means of thermal analysis and mass spectroscopy, the results evidencing the volatilization of carbon dioxide and lead in a large temperature range. The major aim of the study was to evaluate the dependence of the pigment colour and crystallinity from the soaking time at a maximum synthesis temperature fixed at 1400 °C. X-ray diffraction data indicate a nearly linear tendency of crystallinity increase with increasing soaking time. The colour of glazes prepared with the pigments, which was monitored by measuring their chromatic coordinates, presents a clear tendency of improvement with increasing crystallinity. This was interpreted in terms of decreasing solubility of the pigments as their crystallite size is increased. SEM and EDX studies confirm that hypothesis, evidencing very small dissolution of the highly crystallized pigments in the glaze.     

Colour in ceramic glazes: Efficiency of the Kubelka–Munk model in glazes with a black pigment and opacifier

In this study the efficiency of the Kubelka–Munk model (already known and consolidated in other industrial sectors) was evaluated in the prediction of the colour of an opaque ceramic glaze obtained by a mixture of black pigment (spinel Ni–Fe–Cr) and zircon opacifier (ZrSiO₄). Glazes with different percentages of black pigment and opacifier were prepared to determine the absorption and scattering optical constants from the reflectance curves measured with a spectrophotometer. After the physical and chemical characterization of the glaze components (frit, pigment and opacifier), suggestions for the adaptation of the Kubelka–Munk model were made to facilitate the experimental procedure of analysis. The result obtained with the adapted Kubelka–Munk model was in good agreement with the experimental reflectance curves. The reproduction of the desired colour was possible with a reduced number of experiments and the model made it possible to correlate the colour with the added pigments concentration facilitating the formulation step.     

Production of brown pigments for porcelain insulator applications

In this study, production of brown pigment for the ceramic insulator applications by using inexpensive natural raw materials or waste materials was undertaken. Different pigment compositions were designed, synthesised and examined. As a source of chromium, chromite, Cr₂O₃ and ferrochrome were used. Limonite, grinding waste, flotation waste and iron oxide scale were used as an iron source whereas manganese oxide and ferromanganese were used as a manganese source. The colour of glazed insulator bodies change from dark brown to light brown depending on the pigment composition. The pigments prepared with ferrochrome, manganese oxide, flotation waste (C6) or iron oxide scale (D4) and calcined at 1300 °C have a darker brown colour and possess suitable L*a*b* values as 30.1, 2.7, 1.6 for C6 and 30.9, 2.1, 0.3 for D4, which are closer to the L*a*b* values (30.1, 2.9, 0.1) of commercial MnFeCr pigments. The results indicate that waste materials containing iron can be used to produce brown pigments to be used in the insulator application in ceramic industry and as a result of this waste material can be converted into a value-added product. Also, ferrochrome can be used successfully as chromium source and help to reduce the cost of the pigment.     

Synthesis of black pigments containing chromium from leather sludge

The black ceramic pigments with spinel structure have been prepared by using Cr-rich leather sludge in this paper. The washed Cr-rich leather sludge calcined at 1100 °C for 1 h as chromium oxide precursor (named as CA) was mixed with an appropriate proportion of other industrial metallic oxides, followed synthesizing black ceramic pigment by sintering. Both non-washed and washed sludge fired at 1100 °C were characterized by X-ray fluorescence (XRF) in order to determine their chemical compositions and X-ray diffraction (XRD) analysis to confirm that CA mainly contains Cr₂O₃ crystal phase. The results show that CA could be used as a source of chromium to prepare black pigment. The crystalline phases of obtained pigments were characterized by XRD. Furthermore, the morphology as well as the composition of pigments was investigated by scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). The color coordinates of pigments were examined and compared with the commercial pigments based on CIE-L* a* b* values measured using UV–vis spectroscopy. The obtained pigments sintered at 1200 °C with 35–55 wt% content of CA possess the excellent black spinel structure and color effect. Under optimized conditions, the pigment has low average spectral reflectance (7%).     

The effects of Al and Ba on the colour performance of chromic oxide green pigment

On the basis of the novel hydrogen reduction of chromite ore, an activated sintering method was devised to prepare chromic oxide green pigment. Using XRD, SEM, EDX, ICP-AES, UV and reflectance colorimetry it was found that the colour performance of the synthesized pigment was markedly improved by adding Al and Ba, which resulted in the formation of an Al₂O₃–Cr₂O₃ solid solution and a secondary BaCr₂O₄ phase, respectively, within the pigment. The colour performance of the chromic oxide green pigment doped with 0.1% Al and 0.55% Ba conformed to commercial pigment standards.     

Effect of the milling process on the properties of CoFe2O4 pigment

In this study, CoFe₂O₄ pigments were synthesised using both co-precipitation and conventional ceramic methods. Pigment particles prepared using the conventional ceramic method were subsequently milled to submicron size. The effects of the solvent, dispersant and milling type in the milling process were investigated. This study showed that planetary milling in a diethylene glycol (DEG) medium with sodium tripolyphosphate (STPP) was an effective method for producing submicron-sized pigment powders from pigments synthesised using the conventional method. With this method, submicron-sized pigment particles (approximately 190 nm) were obtained after milling for 4 h. Planetary milling was more efficient in reducing particle size compared to attrition milling. Co-precipitated pigment had a more intense black colour, due to the nanoscale particle size (<100 nm). However, conventional ceramic pigments also had an adequately intense black colour that increased after milling compared to unmilled conventional pigments. When considering production of industrial scale submicron-sized pigments, the milling of these pigments to submicron size can be a good alternative method for the production of ink colourants.     

Influence of firing temperature on the color developed by a (Zr,V)SiO4 pigmented opaque ceramic glaze

The analysis of the physical interactions between pigments, opacifiers and glazes is fundamental to understand the optical behavior of ceramic glazes. Furthermore is important to verify if the glaze devitrifies because the crystallized phases can contribute to the optical properties of the system. The size and the quantity of the formed crystals can change significantly the glaze color. The mean goal of this study was to evaluate the influence of firing temperature on the color stability of an opaque ceramic glaze colored by a blue vanadium–zircon pigment taking into account all the optical components. Quantitative X-ray diffraction analysis was conducted in order to evaluate the pigment dissolution at the three studied temperature and the quantity of the in situ formed zircon crystals from the used frit. The reported study demonstrated the importance to consider all the components in a multicomponent optical system as a ceramic glaze.     

Microstructure and coloring mechanism of iron spots on bluish white porcelain from Jingdezhen of the Song Dynasty

Jingdezhen is famous for its bluish white (Qingbai) porcelains of the Song Dynasty, and those decorated with iron spots are distinctive among them. Herein, iron spots on a bluish white porcelain were investigated using a series of microscopic and spectroscopic characterizations. We found the decreasing iron content from more than 8 wt% to about 2 wt% during the glaze color transition from rusty to brown and finally into green, which built a connection on the coloring mechanism of iron-rich crystallized glaze and celadon glaze. We identified the rare ε-Fe₂O₃, a promising magnetic material, in both the dark brown crystals and the triangular crystals in the rusty area, which is its first discovery among bluish white porcelains. Based on these findings, we discussed the coloring mechanism of iron-spot decoration along with the physical form of the iron oxide crystals, indicating the partially reducing atmosphere during firing process.     

Ceramic Pigments for Producing Black Glazes

A new black-color ceramic pigment has been developed on the basis of the CoO - Fe₂O₃ system under a decreased temperature of synthesis (1000 – 1200°C). The optimum temperature range of the spinel-forming reaction in the specified oxide system is determined. When the obtained pigment is introduced into raw glaze for sanitary ceramics in milling, the resulting glaze coatings have intense and stable black color within a sufficiently wide interval of firing temperatures.     

The influence of Ca2+ and Zn2+ doping on the development of sustainable pigments based on GdFeO3 perovskite: From a reddish colour towards a pure black

Black pigments are very commonly used and arouse widespread interest in the ceramic industry. Nevertheless, these pigments contain toxic elements that are detrimental to human health. In view of this, the present work is focused on the development of sustainable black pigments prepared by a coprecipitation method at 1200 °C. Samples with the nominal formula (Gd_1–xCa_x)(Fe_0.95Zn_0.05)O_2.975–x/2 (x = 0.00, 0.05, 0.10) showed single-phase orthorhombic perovskite. The presence of dopants played an important role in the reduction of Fe³⁺ to Fe²⁺ and caused different local distortions in the structure which explained the final black colouration of these pigments in comparison with the red GdFeO₃ sample. The loss of symmetry and the increase in the number of d-d transitions of iron may explain the aforementioned changes. Co-doped pigments reached low C* values, improving on the purest black colour obtained in a commercial black ceramic pigment, which contains toxic elements. In addition to presenting good NIR solar reflectance values of up to 8%, the final colours of these pigments were also stable after their application in a commercial transparent glaze at 1080 °C that could be used for tiles.     

Environmental-friendly yellow pigment based on Tb and M (M = Ca or Ba) co-doped Y2O3

A yellow inorganic ceramic pigment with general formula Y_1.86?xM_xTb_0.14O_3?x/2 (M = Ca and/or Zn) with x = 0.06, 0.32 and 0.64 were synthesized by a modified Pechini method. XRD, SEM and HRTEM/EDX analysis showed the formation of solid solution at 1300 °C when x = 0.06 and 0.32. The best b* yellow coordinates were obtained for Ca and Zn co-doped Y_1.86Tb_0.14O₃ samples. The intensity of the yellow colour in the samples is related to the presence of Tb⁴⁺ ions. Samples with higher concentration of Tb⁴⁺ ions lead to a better yellow colour. The chemical stability of these pigments was determinate in an industrial glaze. The glazing tests indicated that the powder samples with x = 0.06 and 0.32 fired at 1300 °C were stable in the glaze. These results make it a potential candidate for environmental friendly yellow ceramic pigment to be used in applications such as pigment for glazes or inkjet printers.     

Effect of iron oxide content on the crystallisation of a diopside glass–ceramic glaze

The effect of iron oxide content on the crystallisation of a diopside glass–ceramic glaze was investigated using a glass–ceramic frit in the K₂O–ZnO–MgO–CaO–Al₂O₃–SiO₂ system and a granite waste glass. Measurements by X-ray diffraction (XRD) combined with scanning electron microscopy (SEM) and EDX microanalysis showed that the distribution of Fe³⁺ ions among different crystalline phases such as franklinite (ZnFe₂O₄) and hematite Fe₂O₃ depends on the iron content in the original diopside mixture. Thus, the original glaze crystallises to franklinite or hematatite when iron content is greater than 2 and 15%, respectively.     

Solution combustion synthesis of FeCr2O4 powders for pigment applications: Effect of fuel type

Solution combustion synthesis of iron chromite was reported using iron(III) nitrate nonahydrate and chromium(III) nitrate nonahydrate as starting materials, as well as glycine, urea, citric acid, and ethylene glycol as fuels. The influence of fuel type on the structure, molecular, microstructure as well as chromatic properties of samples was investigated. The X-ray diffraction (XRD) patterns showed that unlike themodynamical prediction, glycine fuel led to strongest combustion and consequent highest XRD peak intensities and lower lattice parameters. Moreover, the change of fuel type and mixing of fuels affected XRD data. Fourier transform infrared analysis showed that the band position of Cr–O and Fe–O bonds were shifted to higher frequencies by using of fuels with weaker combustion reactions. In addition, scanning electron micrographs showed that different morphologies of FeCr₂O₄ particles were obtained depending on the fuel type and ratios. Energy-dispersive X-ray spectroscopy analysis of the samples showed that oxygen concentration of samples was less than that of stoichiometric ratio of FeCr₂O₄ due to local reducing atmosphere. Furthermore, chromatic properties of the powders showed that the pigment synthesized with glycine fuel has a better and lighter grayish brown color than the other ones and can be used as a suitable industrial candidate to create a brown color in the ceramic glaze.     

The morphology and structure of crystals in Qing Dynasty purple‐gold glaze excavated from the Forbidden City

Ancient Chinese purple‐gold glaze (zijinyou) is popular for its beautiful figuration, unique allure and fine craftsmanship. To understand the crystalline nature in the purple‐gold glaze, the morphology and structure of crystals precipitated in the glaze layer of purple‐gold glaze porcelain fired during the Qing Dynasty were characterized by a variety of methods combining X‐ray and electron‐based techniques. A large quantity of single‐phase twinning ε‐Fe₂O₃ crystals with lengths of 1‐3 μm, widths of less than 1 μm, and thickness of approximately 150 nm are found dispersed across the glaze surface to a depth of approximately tens of micrometers. These crystals show stratification across the cross‐section of the purple glaze consisting of 4 sublayers according to the crystal size. The formation of ε‐Fe₂O₃ crystals primarily contributed to the reddish‐brown tones of the purple‐gold glaze. The presence of anorthite, a strong reducing atmosphere during the firing process and the vitreous nature of the glaze influenced the growth of ε‐Fe₂O₃ crystals. These results suggest the controllability of single‐phase ε‐Fe₂O₃ crystals by identifying and understanding the underlying chemical processes in ancient Chinese crystalline glaze porcelain, and the findings will provide insights for modern material scientists in preparing ε‐Fe₂O₃ crystals with large sizes and high purities.     

Minimisation of toxicity in nickel ferrite black pigment

Abstract

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.     

Multi-analytical identification of pigments and pigment mixtures used in 17th century Portuguese azulejos

A multi-analytical approach was used to study the pictorial layers of a set of 17th century historic glazed tiles (azulejos) of Portuguese manufacture. The pictorial layer was studied by μ-EDXRF, μ-Raman, SEM-EDS and OM. Although the established palette comprises few pigments, it was found that the tiles were enhanced by the use of pigment mixtures, which were identified by μ-Raman and μ-EDXRF. As expected, the blue colour derives from cobalt oxide, purple from manganese oxide and emerald-green from copper oxide. Regarding the yellow pigment, μ-Raman results show it is consistent with the composition of a ternary oxide, whereas μ-EDXRF shows a high intensity of Zn in this colour, indicating a composition close to a Pb-Sb-Zn ternary oxide. Some compounds from the original ores were also indentified: in two very dark blue samples, cobalt and nickel olivines (α-Co₂SiO₄, α-Ni₂SiO₄), and in a dark brown sample, braunite (a manganese silicate).