Waste-Based Pigments for Application in Ceramic Glazes and Stoneware Bodies

The use of wastes, some of them hazards, as raw materials of ceramic pigments has been a way to diminish their environmental impact, to economically valorize them, and to face the depletion of virgin raw materials. In this work were prepared pigments having in their composition only industrial wastes: Cr/Ni electroplating (ES), and sludges from the cutting of natural stones—marble (MS) and granite (GS). The prepared mixtures were calcined at three temperatures (1100, 1200, and 1300 °C) and the obtained powders were characterized by XRD and UV-vis. Their coloring strength and thermal stability were assessed by adding them to different ceramic substrates: glazes (transparent bright and opaque matte) and a stoneware paste. The CIEL*a*b* coordinates of the fired materials were measured. The developed pigments are thermally stable and exhibit good tinting power, originating nicely colored and defect-free ceramic materials.


Introduction
The consuming model of the modern society produces more and more wastes that are no longer useful. In particular, the industrial sector generates huge amounts of waste, reaching levels that are not sustainable. Thus, the growing level of environmental awareness has put waste management as one of the key challenges of our society. The concept of waste valorization, i.e., transforming wastes into raw materials or products, has, therefore, gained relevance. This idea fits with the circular economy concept that is an economic system aimed at eliminating waste and the continual use of virgin raw materials. Circular systems employ recycling, reuse, remanufacturing, and refurbishment to create a closed system, minimizing the use of resource input and the creation of waste [1].
Inorganic pigments-high added-value products-are materials currently used in several sectors, namely in the ceramic and glass industry. These materials must fulfil three fundamental requirements: to have a high coloring power, to be thermally stable at high temperatures, and to be chemically stable when fired with the glazes or the ceramic bodies [2].
In this work, wastes from different industrial sectors: a sludge from electroplating process (ES-rich in Cr and Ni), a sludge from the marble sawing (MS-rich in CaCO 3 ) and a sludge from the granite sawing (GS-rich in SiO 2 ), were used to produce inorganic pigments. Their combination, in

Coloring Potential and Stability
An amount of 3 wt.% of each pigment was added to three types of ceramic supports: a transparent and bright lead-free glaze (TB), an opaque matt glaze (OM) and a stoneware ceramic body (SB). The influence of the pigment content was tested in the OM support to which two other distinct amounts were added (5 and 10 wt.%). The homogenization process of the pigment with the ceramic substrate was conducted by wet ball mixing. The mixtures were dried at 80 • C, disaggregated, and sieved (at 63 µm). The obtained powders were pressed into Ø 2.5 cm pellets and fired in an electric furnace in air. For the glazes (TB and OM) the heating and cooling rates were 10 • C/min and the soaking time at the maximum temperature (1100 • C) was 30 min. For the mixtures of pigments with SB the maximum firing temperature was 1200 • C and the soaking time was also 30 min. The heating rate was 5 • C/min but two dwells of 30 min were performed at 575 • C and 770 • C. The cooling rate was Materials 2019, 12, 3396 3 of 13 10 • C/min. To assess the color stability of the prepared pigments with the temperature the mixtures with TB were also fired at 1085 • C and 1115 • C (1100 ± 15 • C).

Sample Characterization
The chemical composition of the wastes (RM, ES and GS) was obtained by X-ray fluorescence (X'Pert PRO MPD spectrometer, Malvern PANalytical, Almelo, The Netherlands). The loss on ignition (LOI) at 1000 • C was also determined. The particle size distribution was determined by laser diffraction on a Coulter LS particle size analyzer (LS230FM, Beckman Coulter, Brea, CA, USA). Their mineralogical composition was assessed by X-ray diffraction (XRD). XRD was also used to qualitatively determine the crystalline phases present in the prepared specimens. XRD for semi-quantitative phase analysis (QPA) was performed on selected specimens. All of the XRD patterns were recorded at room temperature on a θ/θ diffractometer (X'Pert Pro, Malvern PANalytical, Almelo, The Netherlands), equipped with a fast RTMS detector (PIXcel 1D, Malvern PANalytical, Almelo, The Netherlands), with Cu K α radiation (45 kV and 40 mA, 15 • -80 • 2θ range, with a virtual step scan of 0.02 • 2θ, and virtual time per step of 200 s). QPA was assessed on selected specimens, by way of the Rietveld method [14] on the XRD data; Rietveld analysis was performed via the GSAS-EXPGUI software package [15].
The color stability was calculated using the value of ∆E: ∆E = (∆L) 2 + (∆a) 2 + (∆b) 2 . Values of ∆E > 2 mean that the color change can be noticed by the naked eye [17].
The particle size distribution of the starting materials is a characteristic that strongly affect their reactivity. Moreover, the granulometric distribution can also influence the mixture homogeneity. The ES sludge outcomes from the flocculation process that occurs during the wastewater treatment in the Cr/Ni electroplating plant. As reported in previous works of the authors [7] the non-calcined ES has an average particle size of ≈112 µm and a wide size range (from sub-micron sizes to ≈500 µm) due to the presence of large agglomerates. The other two sludges exhibit a much narrow distribution and a smaller medium particle size (22.3 µm and 4.6 µm, respectively, for GS and MS).
The mineralogical composition of the wastes has been revealed by XRD, Figure 1a-c. GS (Figure 1a) is mainly (qualitatively) composed of low-quartz, a mica group mineral, a feldspar, and a zeolite group mineral as well-this latter likely coming from atmospheric weathering of the granite. As suggested before, MS is composed by calcium carbonate, as all of the reflections are assignable to CaCO 3 (Figure 1b), whilst GS showed itself to have a very low crystalline nature, the only crystalline phases detected being low-quartz, and eskolaite (Cr 2 O 3 ), as shown in Figure 1c [7]. The particle size distribution of the starting materials is a characteristic that strongly affect their reactivity. Moreover, the granulometric distribution can also influence the mixture homogeneity. The ES sludge outcomes from the flocculation process that occurs during the wastewater treatment in the Cr/Ni electroplating plant. As reported in previous works of the authors [7] the non-calcined ES has an average particle size of ≈112 µm and a wide size range (from sub-micron sizes to ≈500 µm) due to the presence of large agglomerates. The other two sludges exhibit a much narrow distribution and a smaller medium particle size (22.3 µm and 4.6 µm, respectively, for GS and MS).
The mineralogical composition of the wastes has been revealed by XRD, Figure 1a-c. GS ( Figure  1a) is mainly (qualitatively) composed of low-quartz, a mica group mineral, a feldspar, and a zeolite group mineral as well-this latter likely coming from atmospheric weathering of the granite. As suggested before, MS is composed by calcium carbonate, as all of the reflections are assignable to CaCO3 (Figure 1b), whilst GS showed itself to have a very low crystalline nature, the only crystalline phases detected being low-quartz, and eskolaite (Cr2O3), as shown in Figure 1c

Pigments Characterization
The temperatures at which the prepared mixtures were fired (1100, 1200 and 1300 °C) were selected considering results of previous works [7,12]. XRD patterns of the formulations 50GS:ES and 30GS:20MS:ES after thermal treatment at such temperatures are shown in Figure 2a,b. When GS and ES are mixed in equal wt.% amounts, and thermally treated at high temperatures, both the color and the mineralogy changes, Figure 2a. At 1100 °C, the main crystalline phases detected are: (i) a mineral of the spinel group; (ii) nickel oxide (bunsenite); (iii) a mineral of the olivine group (tentatively: Ni2SiO4); (iv), and low-quartz. Increasing the thermal treatment temperature to 1200 °C, the residual low-quartz disappeared, probably enriching the glassy phase. At this isotherm, the mineralogical phases are: (i) the spinel; (ii) the mineral of the olivine group; and (iii) NiO. At the highest temperature, 1300 °C, spinel (68.7 wt.%) and NiO (31.3 wt.%) are the only two crystalline phases, as listed in Table 3.

Pigments Characterization
The temperatures at which the prepared mixtures were fired (1100, 1200 and 1300 • C) were selected considering results of previous works [7,12]. XRD patterns of the formulations 50GS:ES and 30GS:20MS:ES after thermal treatment at such temperatures are shown in Figure 2a,b. When GS and ES are mixed in equal wt.% amounts, and thermally treated at high temperatures, both the color and the mineralogy changes, Figure 2a. At 1100 • C, the main crystalline phases detected are: (i) a mineral of the spinel group; (ii) nickel oxide (bunsenite); (iii) a mineral of the olivine group (tentatively: Ni 2 SiO 4 ); (iv), and low-quartz. Increasing the thermal treatment temperature to 1200 • C, the residual low-quartz disappeared, probably enriching the glassy phase. At this isotherm, the mineralogical phases are: (i) the spinel; (ii) the mineral of the olivine group; and (iii) NiO. At the highest temperature, 1300 • C, spinel (68.7 wt.%) and NiO (31.3 wt.%) are the only two crystalline phases, as listed in Table 3.

Pigments Characterization
The temperatures at which the prepared mixtures were fired (1100, 1200 and 1300 °C) were selected considering results of previous works [7,12]. XRD patterns of the formulations 50GS:ES and 30GS:20MS:ES after thermal treatment at such temperatures are shown in Figure 2a,b. When GS and ES are mixed in equal wt.% amounts, and thermally treated at high temperatures, both the color and the mineralogy changes, Figure 2a. At 1100 °C, the main crystalline phases detected are: (i) a mineral of the spinel group; (ii) nickel oxide (bunsenite); (iii) a mineral of the olivine group (tentatively: Ni2SiO4); (iv), and low-quartz. Increasing the thermal treatment temperature to 1200 °C, the residual low-quartz disappeared, probably enriching the glassy phase. At this isotherm, the mineralogical phases are: (i) the spinel; (ii) the mineral of the olivine group; and (iii) NiO. At the highest temperature, 1300 °C, spinel (68.7 wt.%) and NiO (31.3 wt.%) are the only two crystalline phases, as listed in Table 3.  Additions of MS into the system lead to differences on the color and mineralogy (Figures 2b and  3). At the lowest firing temperature, specimen 30MS:20GS:ES is composed of spinel, NiO, wollastonite and alite. Those two latter mineralogical phases result from the addition of calcium to the system. At the intermediate firing temperature of 1200 °C, we assist at the disappearance of wollastonite at the expenses of an increase in the alite content, together with the crystallization of pseudowollastonite. At 1300 °C, the pigment is (semi-quantitatively) composed of: 35.3 wt.% spinel, 18.0 wt.% NiO, and 46.7 wt.% alite-cf. Table 3, and as graphically reported in Figure 3.  Additions of MS into the system lead to differences on the color and mineralogy (Figures 2b and 3). At the lowest firing temperature, specimen 30MS:20GS:ES is composed of spinel, NiO, wollastonite and alite. Those two latter mineralogical phases result from the addition of calcium to the system. At the intermediate firing temperature of 1200 • C, we assist at the disappearance of wollastonite at the expenses of an increase in the alite content, together with the crystallization of pseudowollastonite. At 1300 • C, the pigment is (semi-quantitatively) composed of: 35.3 wt.% spinel, 18.0 wt.% NiO, and 46.7 wt.% alite-cf. Table 3, and as graphically reported in Figure 3.
Optical spectra of all the specimens fired at 1300 • C are depicted in Figure 4a. They all show similar optical characteristics, with the exception of specimen 30MS:20GS:ES_1300, in which a drop-in absorption in the 25,000-15,000 cm −1 region, due to the non-negligible presence of Ca 3 SiO 5 , is detectable, cf. Table 3. In this work, we will describe in detail only the optical spectrum of 25GS:ES_1300, Figure 4b. The UV-vis spectrum of this specimen is dominated by Cr 3+ and Ni 2+ optical signatures. The bands at low energy, in blue in Figure 4b, can be tentatively assigned to Ni(II) 3 [18]. In addition, the weak bands at~17,500-20,000 cm −1 (bands in green in Figure 4a) belong to Ni(II) 3 T 1g ( 3 P) transitions [18]. The bands at higher energy (those in orange in Figure 4b) are tentatively assigned to Cr(III) 4 A 2g → 4 T 1g electronic transitions [19]. The signal at high energy (~35,000 cm −1 ) is due to charge transfers between metals and the oxygen ions [20]. The UV-vis spectrum of specimen ES fired at 1200 • C is also reported for comparison (Figure 4c). In it, the bands in light blue were tentatively assigned to Ni(II) 3 T 1g ( 3 F) transitions; those in dark blue to 3 A 2g (F)→ 3 T 1g (F) transitions of Ni(II); those in orange to Ni(II) 3 T 1g ( 3 P) transitions, and those in violet to Ni(II) 1 T 2g transitions [7].
Absorbance bands in light grey and green are tentatively assigned to Cr(III) transitions: 4 A 2g → 4 T 2g and 4 A 2g → 4 T 1g ( 4 F), respectively [7]. Optical spectra of all the specimens fired at 1300 °C are depicted in Figure 4a. They all show similar optical characteristics, with the exception of specimen 30MS:20GS:ES_1300, in which a dropin absorption in the 25,000-15,000 cm -1 region, due to the non-negligible presence of Ca3SiO5, is detectable, cf. Table 3. In this work, we will describe in detail only the optical spectrum of 25GS:ES_1300, Figure 4b. The UV-vis spectrum of this specimen is dominated by Cr 3+ and Ni 2+ optical signatures. The bands at low energy, in blue in Figure 4b, can be tentatively assigned to Ni(II) 3 A2g( 3 F) → 3 T1g( 3 F) transitions [18]. In addition, the weak bands at ~17,500-20,000 cm -1 (bands in green in Figure  4a) belong to Ni(II) 3 T1g( 3 P) transitions [18]. The bands at higher energy (those in orange in Figure 4b) are tentatively assigned to Cr(III) 4 A2g→ 4 T1g electronic transitions [19]. The signal at high energy (~35,000 cm -1 ) is due to charge transfers between metals and the oxygen ions [20]. The UV-vis spectrum of specimen ES fired at 1200 °C is also reported for comparison (Figure 4c). In it, the bands in light blue were tentatively assigned to Ni(II) 3 T1g( 3 F) transitions; those in dark blue to 3 A2g(F)→ 3 T1g(F) transitions of Ni(II); those in orange to Ni(II) 3 T1g( 3 P) transitions, and those in violet to Ni(II) 1 T2g transitions [7]. Absorbance bands in light grey and green are tentatively assigned to Cr(III) transitions: 4 A2g→ 4 T2g and 4 A2g→ 4 T1g( 4 F), respectively [7].   Optical spectra of all the specimens fired at 1300 °C are depicted in Figure 4a. They all show similar optical characteristics, with the exception of specimen 30MS:20GS:ES_1300, in which a dropin absorption in the 25,000-15,000 cm -1 region, due to the non-negligible presence of Ca3SiO5, is detectable, cf. Table 3. In this work, we will describe in detail only the optical spectrum of 25GS:ES_1300, Figure 4b. The UV-vis spectrum of this specimen is dominated by Cr 3+ and Ni 2+ optical signatures. The bands at low energy, in blue in Figure 4b, can be tentatively assigned to Ni(II) 3 A2g( 3 F) → 3 T1g( 3 F) transitions [18]. In addition, the weak bands at ~17,500-20,000 cm -1 (bands in green in Figure  4a) belong to Ni(II) 3 T1g( 3 P) transitions [18]. The bands at higher energy (those in orange in Figure 4b) are tentatively assigned to Cr(III) 4 A2g→ 4 T1g electronic transitions [19]. The signal at high energy (~35,000 cm -1 ) is due to charge transfers between metals and the oxygen ions [20]. The UV-vis spectrum of specimen ES fired at 1200 °C is also reported for comparison (Figure 4c). In it, the bands in light blue were tentatively assigned to Ni(II) 3 T1g( 3 F) transitions; those in dark blue to 3 A2g(F)→ 3 T1g(F) transitions of Ni(II); those in orange to Ni(II) 3 T1g( 3 P) transitions, and those in violet to Ni(II) 1 T2g transitions [7]. Absorbance bands in light grey and green are tentatively assigned to Cr(III) transitions: 4 A2g→ 4 T2g and 4 A2g→ 4 T1g( 4 F), respectively [7].

Color Development in Ceramic Bodies
As already said, the coloring hue and tinting power of the prepared waste based-inorganic pigments was evaluated by adding them to a transparent and bright lead-free glaze  Figure 5 presents the images of the TB glaze colored samples; the measured CIEL*a*b* color coordinates are patent in Table 4. The processing temperature of the pigments has a notorious influence in the developed color for both tested formulations: 50GS:ES and 30GS:20MS:ES. This was expected from changes on mineralogy (Figure 2). In general, there is a darken tendency when processing temperature rises, as normally happens in all ceramic systems. With 50GS:ES formulation, the temperature rises from 1100 to 1300 °C promotes a decrease of the L* value from 47.2 to 36.8 (≈22%), a* from 3.7 to 2.3 (≈38%) and b* from 14.3 to 5.5 (≈61%). The pigment containing the three studied wastes-30GS:20MS:ES-exhibits a similar behavior; however, the influence of the firing temperature is more pronounced. The decrease of L* is ≈18% (from 45.7 to 37.3), a* ≈ 58% (from 1.2 to 0.5) and b* ≈ 71% (from 11.1 to 3.2). In it, the black continuous line represents the recorded data, the dash-dotted red line is the cumulative peak fit. (c) Optical spectrum of specimen ES fired at 1200 • C (modified from Reference [7]).

Color Development in Ceramic Bodies
As already said, the coloring hue and tinting power of the prepared waste based-inorganic pigments was evaluated by adding them to a transparent and bright lead-free glaze [TB: SiO 2 (51-53 wt.%); Al 2 O 3 (41-43 wt.%); ZrO 2 (7-9 wt.%); P 2 Figure 5 presents the images of the TB glaze colored samples; the measured CIEL*a*b* color coordinates are patent in Table 4. The processing temperature of the pigments has a notorious influence in the developed color for both tested formulations: 50GS:ES and 30GS:20MS:ES. This was expected from changes on mineralogy (Figure 2). In general, there is a darken tendency when processing temperature rises, as normally happens in all ceramic systems. With 50GS:ES formulation, the temperature rises from 1100 to 1300 • C promotes a decrease of the L* value from 47.2 to 36.8 (≈22%), a* from 3.7 to 2.3 (≈38%) and b* from 14.3 to 5.5 (≈61%). The pigment containing the three studied wastes-30GS:20MS:ES-exhibits a similar behavior; however, the influence of the firing temperature is more pronounced. The decrease of L* is ≈18% (from 45.7 to 37.3), a* ≈ 58% (from 1.2 to 0.5) and b* ≈ 71% (from 11.1 to 3.2).   At 1300 • C, the growth of ES percentage (from 25 wt.% to 75 wt.%)-formulations 75GS:ES, 50GS:ES and 25GS:ES-intensified the hue of the pigment. TB mixtures exhibits a decrease of L* (from 45 to 36.8), a* (3.9 to 1.2) and b* (12.7 to 3.4) values, which is in accordance with the increase of the spinel phase amount (see Table 3).
The partial substitution of GS by MS, keeping constant the ES amount-compositions 50GS:ES and 30GS:20MS:ES-and the processing temperature (1300 • C), did not promote a significant alteration of the pigments coloring power (similar L* value). However, these two pigments do not have the same crystalline composition (see Table 3), what is reflected in the developed color-a* and b* values exhibit a variation of 58% and 85%, respectively.
The color stability was accessed on formulations processed at 1300 • C by varying 30 • C (1100 ± 15 • C) the firing temperature of the mixture TB loaded with 3 wt.% of pigment. The obtained results (Table 4) show that all of them have a very good thermal stability; ∆E values are always lower than 2, meaning that the color change cannot be noticed by the naked eye [17]. This is a consequence of the formation of relatively stable crystalline phases within that temperature range (see Table 3).
The OM glaze incorporated three distinct amounts (3, 5, and 10 wt.%) of all the pigments processed at 1300 • C. The aspect of the obtained samples and their CIEL*a*b*coordinates are patent in Figure 6 and Table 5, respectively. The increase of the pigment amount enhances the darkness of the samples, but the hue remains almost unchanged: a* and b* color coordinates do not exhibit significant changes; only the L* value exhibits a notorious decrease (between 27% and 29%).
At 1300 °C, the growth of ES percentage (from 25 wt.% to 75 wt.%)-formulations 75GS:ES, 50GS:ES and 25GS:ES-intensified the hue of the pigment. TB mixtures exhibits a decrease of L* (from 45 to 36.8), a* (3.9 to 1.2) and b* (12.7 to 3.4) values, which is in accordance with the increase of the spinel phase amount (see Table 3).
The partial substitution of GS by MS, keeping constant the ES amount-compositions 50GS:ES and 30GS:20MS:ES-and the processing temperature (1300 °C), did not promote a significant alteration of the pigments coloring power (similar L* value). However, these two pigments do not have the same crystalline composition (see Table 3), what is reflected in the developed color-a* and b* values exhibit a variation of 58% and 85%, respectively.
The color stability was accessed on formulations processed at 1300 °C by varying 30 °C (1100 ± 15 °C) the firing temperature of the mixture TB loaded with 3 wt.% of pigment. The obtained results (Table 4) show that all of them have a very good thermal stability; ΔE values are always lower than 2, meaning that the color change cannot be noticed by the naked eye [17]. This is a consequence of the formation of relatively stable crystalline phases within that temperature range (see Table 3).
The OM glaze incorporated three distinct amounts (3, 5, and 10 wt.%) of all the pigments processed at 1300 °C. The aspect of the obtained samples and their CIEL*a*b*coordinates are patent in Figure 6 and Table 5, respectively. The increase of the pigment amount enhances the darkness of the samples, but the hue remains almost unchanged: a* and b* color coordinates do not exhibit significant changes; only the L* value exhibits a notorious decrease (between 27% and 29%).  Since are appearing new markets for products made with colored stoneware pastes, namely household and decorative stoneware pieces, their production has been gradually growing.
Consequently, the consumption of ceramic pigments for this type of application has been increasing as well and the actual waste-based pigments were also tested for this purpose. The aspect and the CIEL*a*b* color coordinates of the stoneware paste without and with 3 wt.% of pigments (fired at 1300 • C) is presented in Figure 7. Since are appearing new markets for products made with colored stoneware pastes, namely household and decorative stoneware pieces, their production has been gradually growing. Consequently, the consumption of ceramic pigments for this type of application has been increasing as well and the actual waste-based pigments were also tested for this purpose. The aspect and the CIEL*a*b* color coordinates of the stoneware paste without and with 3 wt.% of pigments (fired at 1300 °C) is presented in Figure 7. The addition of the developed pigments to the stoneware paste did not induce the appearance of defects neither change of the material's technical properties allowing to obtain nice colored stoneware bodies. The pigments made from GS-and ES-originated colors from dark beige (camel) to light brown, tendentiously darker with increasing amounts of ES. The 30GS:20MS:ES_1300 pigment also originates light brown hues (Figure 7), but darker than those obtained with the binary mixtures. This behavior is opposite to that observed on glazes (TB and OM). Finally, we should mention that the addition of the actual pigments did not induce the appearance of defects neither changes on the technical properties allowing to obtain nice colored stoneware bodies.

Conclusions
This work proved that the production of stable inorganic pigments only from wastes is feasible. Binary and ternary combinations were validated: (i) granite sludge (GS) and electroplating sludge (ES); and (ii) GS + ES + marble sludge (MS).
Three processing temperatures (1100, 1200 and 1300 °C) were tested and it was concluded that firing at highest temperature is advisable to ensure higher chemical and thermal stability. After being fired at 1300 °C the crystalline phases present in the mixtures of GS and ES (25GS:ES, 50GS:ES and 75GS:ES) are a spinel and NiO. A small amount of α-Fe2O3 (2.9 wt.%) was detected in the composition with the higher amount of GS (75GS:ES). The partial substitution of GS by MS (30GS:20MS:ES) leads to the formation of another crystalline phase-Ca3SiO5. In general, darker hues are obtained when the ES content increases.
The developed waste-based pigments are suitable for coloring glazes (transparent bright lead free and opaque matte) and stoneware pastes. The developed hues change with the substrate nature. In bright and transparent lead-free glaze, the pigments developed colors that change from light camel The addition of the developed pigments to the stoneware paste did not induce the appearance of defects neither change of the material's technical properties allowing to obtain nice colored stoneware bodies. The pigments made from GS-and ES-originated colors from dark beige (camel) to light brown, tendentiously darker with increasing amounts of ES. The 30GS:20MS:ES_1300 pigment also originates light brown hues (Figure 7), but darker than those obtained with the binary mixtures. This behavior is opposite to that observed on glazes (TB and OM). Finally, we should mention that the addition of the actual pigments did not induce the appearance of defects neither changes on the technical properties allowing to obtain nice colored stoneware bodies.

Conclusions
This work proved that the production of stable inorganic pigments only from wastes is feasible. Binary and ternary combinations were validated: (i) granite sludge (GS) and electroplating sludge (ES); and (ii) GS + ES + marble sludge (MS).
Three processing temperatures (1100, 1200 and 1300 • C) were tested and it was concluded that firing at highest temperature is advisable to ensure higher chemical and thermal stability. After being fired at 1300 • C the crystalline phases present in the mixtures of GS and ES (25GS:ES, 50GS:ES and 75GS:ES) are a spinel and NiO. A small amount of α-Fe 2 O 3 (2.9 wt.%) was detected in the composition with the higher amount of GS (75GS:ES). The partial substitution of GS by MS (30GS:20MS:ES) leads to the formation of another crystalline phase-Ca 3 SiO 5 . In general, darker hues are obtained when the ES content increases.
The developed waste-based pigments are suitable for coloring glazes (transparent bright lead free and opaque matte) and stoneware pastes. The developed hues change with the substrate nature. In bright and transparent lead-free glaze, the pigments developed colors that change from light camel to dark grey. In the case of the opaque matt glaze, hues fluctuate from beige to dark grey. In a stoneware paste, the achieved colors oscillate between camel and light brown. All the prepared colored products are free of defects and show suitable technical properties. This confirms the chemical and thermal stability of the prepared pigments. In transparent lead-free glaze, color changes induced by temperature variations (1100 ± 15 • C) are not detected by the naked eye (∆E < 2).