Effect Of Firing Temperature Research Articles

Arsenic leaching from ceramic water filters: Effects of firing temperature and influent conditions

Ceramic water filters (CWFs), fabricated using local sourced clay, have been widely acknowledged as sustainable and effective point-of-use (POU) drinking water treatment approach. However, arsenic leaching from CWFs poses a potential risk of arsenic exposure, thereby limiting their safe application. In this study, CWFs were made from red clay to investigate the critical roles of firing temperatures (900–––1050 °C), solution chemistry conditions (i.e., pH, Na+, Ca2+), as well as operational conditions on arsenic leaching. Overall, arsenic leaching from CWFs followed a flushing-out pattern, with up to 61 % of total leachable arsenic released during initial first 100-pore-volume filtration. Specially, higher firing temperature led to lower initial arsenic leaching but higher cumulative leaching from resulting CWFs over long-term filtration. Feed solutions at circumneutral pH, with low Na+ or high Ca2+ resulted in less cumulative arsenic leaching, while POU operational conditions (intermittent operation and fluctuating flow rate) had negligible effects. Mechanism investigation found that firing process enhanced arsenic mobility, i.e., the exchangeable arsenic fraction was only 5.34 % in unfired clay but 26.7 % − 37.3 % in CWFs. The enhanced arsenic mobility was mainly caused by the thermal transformation of source clay, especially the thermal alteration in the hematite lattice. Our findings provide not only new insights into the process controlling arsenic leaching from CWFs but also shed light on ways to mitigate leaching, thereby improving the safety of CWFs application.

Experimental and numerical study on extended endplate beam-column joints after high temperature to resist progressive collapse

This paper presents experimental and numerical investigations on the collapse-resisting behavior of extended endplate beam-column joints after high temperatures. The failure patterns and structural responses of extended endplate (EP) joints after high temperature to resist progressive collapse were first described, and then the effects of fire temperature and temperature duration on their collapse resistance mechanisms were also analyzed in detail. The test results demonstrate that all specimens withdrew from work owing to the serious deformation of endplate bolt holes or the brittle fracture of high-strength bolts, resulting in the catenary mechanisms of EP joints not being adequately developed. Compared with previous studies, it is shown that the fire temperature exerts the most significant effects on the carrying capacities and deformability of EP joints, and the collapse resistance of corresponding assemblies is most sensitive to the fire temperature. However, the temperature durations have an insignificant influence on the development of flexural mechanism resistance, especially for those after 600 °C and 800 °C. Moreover, the corresponding refined FE models in consideration of the damage and fracture of ductile metals have been established to reproduce the structural response of specimens after high temperature. To improve the post-fire collapse-resisting performance of assemblies, both the lateral plate reinforcement and welding reinforcement methods were proposed and compared with the assistance of validated FE models. From a practical viewpoint, the reinforcement with lateral plates is determined as a superior technique for post-fire EP joints against progressive collapse.

Effect of final temperature and heating rate on the mechanical and optical properties of a zirconia veneering ceramic

PurposeTo evaluate the effect of firing temperature and heating rate on the volumetric shrinkage, translucency, flexural strength, hardness, and fracture toughness of a zirconia veneering ceramic. Material and methodsZirconia veneering ceramic specimens (N = 45) with varying final temperatures (730 °C, 750 °C, and 770 °C) and heating rates (70 °C/min, 55 °C/min, and 40 °C/min) were fabricated (n = 5). Each specimen's shrinkage, translucency, flexural strength, hardness, and fracture toughness were determined. Two-way analysis of variance, Scheffé test, and Pearson's correlation analysis were used to evaluate data (α = 0.05). ResultsThe shrinkage (44.9 ± 3.1–47.5 ± 1.6 vol%) and flexural strength (74.1 ± 17.4–107.0 ± 27.1 MPa) were not affected by tested parameters (P ≥ 0.288). The interaction between the main factors affected the translucency, hardness, and fracture toughness of the specimens (P ≤ 0.007). Specimens with 770 °C final temperature and 70 °C/min heating rate had the lowest (21.8 ± 3.2 %) translucency (P ≤ 0.039). The hardness ranged between 4.98 ± 0.51 GPa (730 °C; 70 °C/min) and 5.60 ± 0.37 GPa (770 °C; 70 °C/min). Fracture toughness ranged between 0.54 ± 0.04 MPa√m and 0.67 ± 0.08 MPa√m with the highest values for specimens fired at 730 °C with 70 °C/min (P ≤ 0.001). There was a positive correlation between translucency and hardness (r = 0.335, P = 0.012), and a negative correlation between fracture toughness and all parameters other than shrinkage (translucency: r = −0.693/P < 0.001, flexural strength: r = −0.258/P = 0.046, hardness: r = −0.457/P < 0.001). ConclusionsHeating rate and final temperature should be considered while fabricating veneered zirconia restorations with tested ceramic as they affected the translucency, hardness, and fracture toughness.

Effect of Firing Temperatures on Phase Formation, Microstructure, Dielectric and Magnetic Properties of Ni0.6Zn0.4Fe2O4 Ceramics Synthesized by the Solid-State Combustion Technique

In this study, the effect of firing temperatures on phase formation, microstructure dielectric and magnetic properties of Ni0.6Zn0.4Fe2O4 (NZFO) ceramics were investigated. The NZFO powder were synthesized by the solid-state combustion technique using glycine as a fuel to reduce the reaction temperature. All samples were calcined in the range of 900 °C–1100 °C for 2 h and sintered in the range of 1175 °C–1275 °C for 2 h. A pure ferrite phase was found in the powders calcined above 1000 °C and the average particle size of NZFO powders was increased from 0.39 µm to 0.59 µm when the calcination temperature was increased. The x-ray diffraction (XRD) analysis results confirmed the formation of pure spinel structure with cubic phase in all the ceramic samples. The average grain size (5.54–2.80 µm) was slightly decreased, and the dielectric constant (239–13) tended to decrease with increasing sintering temperature. As the sintering temperature increased to 1250 °C, it was found that the lattice parameters (8.366–8.387 Å), the density (5.29–5.35 g/cm3), and saturation magnetization (81.51–93.92 emu/g) tended to increase after that decrease.

Firing Temperatures Effect on Phase Formation, Microstructure and Electrical Properties of BNBKL Ceramics Fabricated via the Solid-State Combustion Technique

Lead-free 0.85Bi0.5Na0.475Li0.025TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 (BNBKL) ceramics were prepared by the solid-state combustion technique. The BNBKL powders and ceramics were calcined and sintered between 650 to 850 °C for 1–5 h and 1100 to 1140 °C for 2 h, respectively. The effect of firing temperatures on the phase structure, microstructure, and electrical properties of BNBKL ceramics was investigated. X-ray diffraction patterns of BNBKL powders showed a pure perovskite phase structure at a calcination temperature of 800 °C for 3 h. The BNBKL ceramics sintered at different temperatures possessed coexisting rhombohedral (R) and tetragonal (T) phases. The ceramic grains showed a polyhedral shape with directionless growth. The average grain size of the BNBKL ceramics increased from 0.80 to 1.26 µm when the sintering temperature increased from 1100 to 1140 °C. The density of the BNBKL ceramics increased as the sintering temperature increased up to 1120 °C, after that it decreased. The highest density (6.05 g/cm3) and dielectric constant (5855), were obtained at the sintering temperature of 1120 °C. As the sintering temperature increased, the polarization hysteresis loops (P-E loops) of the BNBKL ceramics exhibited less saturation and were more pinched, indicating more relaxor behavior.

Phase formation and electrical properties of SBNLT ceramics prepared via combustion technique

This report investigates the effect of firing temperatures on the phase formation, microstructure, electrical, and energy storage properties of lead-free Sr0.3(Bi0.7Na0.67Li0.03)0.5TiO3 (SBNLT) ceramics, synthesised by combustion technique. The samples were calcined between 700°C and 900°C for 2 h and sintered between 1100°C and 1200°C for 2 h. The ceramics exhibited coexisting rhombohedral and tetragonal phases, which were confirmed by the Rietveld refinement technique. A morphotropic phase boundary (MPB) of the rhombohedral and tetragonal phases, with a ratio of 49:51, was obtained at the sintering temperature of 1175°C. The highest maximum dielectric constant (ε max = 4667), polarisation (P max = 28.80 µC/cm2) and energy density (W = 0.95 J/cm3), with a breakdown strength of 70 kV/cm, were achieved from the sample sintered at 1175°C.

Preparation of foam ceramics from solid wastes: a study on the relationship between firing regime and properties by grey system theory

Foam ceramics were prepared by using 80 wt.% sand shale, 8 wt.% coal fly ash, 8 wt.% steel slag and 4 wt.% magnesite as raw materials and 0.3 wt.% SiC as foam agent. Effects of firing temperature and holding time on performance and phases of foam ceramics were investigated. Grey system theory was used to explore the correlation between pore structure parameters and thermal conductivity. The foam ceramics sintered at 1160 °C for 50 min exhibit the optimal comprehensive properties: bulk density of 0.651 g/cm3, flexural strength of 4.75 MPa, total porosity of 73.58%, closed porosity of 54.16%, and thermal conductivity of 0.081 W/(m·K). The Micro-CT results show that the pores with the average pore volume of 1.083 mm3 are relatively independent and uniform; and the pore wall thickness presents a normal distribution, which is conducive to improving the properties. Among the pore structure parameters, the fractal dimension has the greatest correlation with the thermal conductivity of foam ceramics.

Revealing the Individual Effects of Firing Temperature and Chemical Composition on Raman Parameters of Celadon Glaze

The Raman polymerization index (or IP value) is often used as a positive indicator of the firing temperature of the glazes of ancient ceramics. Previous studies have also reported that the IP value was negatively correlated with the chemical composition of the glaze. However, these findings were derived from data on the potential integrative effects of temperature and composition. To explore their individual effects, we prepared celadon glaze samples with controlled composition and firing temperatures. Particularly, according to the typical content of K2O and CaO in celadon glaze, four categories, or, in total, fifteen compositional formulations, were designed, and each formulation was fired at multiple temperatures (from 1180 to 1250 °C by 10 °C). The chemical compositions and the glassy matrix of samples were analyzed by an energy-dispersive X-ray fluorescence spectrometer and Raman spectroscopy, respectively. A positive correlation between firing temperature and IP value (correlation coefficient = 0.56) was detected only in the samples with low contents of K2O and CaO. However, no significant correlation was found when combining the samples with broad variation in chemical composition. Additionally, both the K2O content and CaO content were negatively related to the IP value, with regression coefficients of −9.645 and −5.332, respectively. Our results help to clarify the technology of ancient ceramic production and to improve its preservation.

The effects of firing temperature and Ti/Si ratio on the H2 permeation characteristics of microporous TiO2-SiO2-organic chelating-ligand composite membranes

Titania-silica composite materials were synthesized and successfully employed in the preparation of microporous gas separation membranes via sol–gel processing. Acetylacetone (ACA), an organic-chelating ligand (OCL), was introduced as a reaction modulator to control the hydrolysis/condensation reactions of titan alkoxide, which resulted in formation of well combined titania and silica structures. The membrane material had a molar ratio of Ti/ACA = 1/1 and was coated onto an intermediate layer with an average pore diameter of about 1 nm. The effect of firing temperature on gas permeability was examined at 300 and 500 °C under a nitrogen atmosphere. As a result, for the case of Ti/Si/ACA = 1/1/1, the membrane fired at 300 °C under nitrogen (300 N) showed a molecular sieving permeation mechanism and a permeance that depended on molecular diameter. This membrane achieved H2 permeance of 0.63 × 10-6 mol/(m2 s Pa), H2/CH4 selectivity of 72.8, and H2/SF6 selectivity of 534.7. The membrane fired at 500 °C under nitrogen (500 N) showed a similar molecular sieving performance, albeit with smaller values for H2 permeance and H2/SF6 selectivity. For this study, the Ti/Si ratios were also altered and studied. Membranes with ratios of Ti/Si/ACA = 1/2/1 and 2/1/2 were prepared by firing under nitrogen at the optimal temperature of 300 °C, and the Ti/Si molar ratio effect on the gas permeance was examined. H2/SF6 selectivity was increased with an increase in the silica content (Ti/Si/ACA = 1/2/1 membrane: H2 permeance 1.26 × 10-6 mol/(m2 s Pa), H2/SF6 = 1350.7). On the other hand, H2/SF6 selectivity became smaller as the titania composite content was increased (Ti/Si/ACA = 2/1/2 membrane: H2 permeance 1.16 × 10-6 mol/(m2 s Pa, H2/SF6 = 53.0). Therefore, by optimizing the titania content, it is expected to achieve the H2 gas separation performance of the TiO2-SiO2-ACA membrane according to the application of the membrane.

Microstructure and strength of microporous MgO refractory aggregates with nano-sized pores

ABSTRACT In this work, microporous MgO refractory aggregates with nano-sized pores were prepared by in situ decomposition method using Mg(OH)2 as raw material. The effects of firing temperatures (1600–1700°C) and compacting pressures (100–200 MPa) on the microstructures and properties of microporous MgO refractory aggregates were thoroughly studied. The results showed that the microporous MgO refractory aggregates contained two types of pore structures, which were intra-particle pores with pore sizes of 180.0–230.0 nm formed by in situ decomposition of Mg(OH)2 and inter-particle pores with pore sizes of 1.5–3.0 μm derived from particle packing between Mg(OH)2 pseudomorph particles, respectively. Besides, the firing temperatures had a great influence on the intra-particle pore size and microcrystallite size. And the compacting pressures not only influenced the intra-particle pore size via packing behaviors but also affected the inner firing behaviors of the pseudomorph particles due to the increase in H2O vapor pressure. Overall, at a compacting pressure of 150 MPa and firing temperature of 1650°C, the sample had the best comprehensive performance with a bulk density of 1.92 g/cm3, a compressive strength of 11.9 MPa, an apparent porosity of 45.0%, a relative aggregate tube strength of 25.2% and a median pore size of 262.3 nm.

Synthesis and Sintering Reaction Mechanism of High-Performance MgO-CaO-Fe2O3 Clinker

High-performance MgO-CaO-Fe2O3 clinker was prepared using magnesite from Xinjiang (with high calcium and low silica), calcium oxide, and ferric oxide as raw materials. Microstructural analysis and thermogravimetric analysis, combined with HSC chemistry 6 software simulations, were used to investigate the synthesis mechanism of MgO-CaO-Fe2O3 clinker and the effect of firing temperature on the properties of MgO-CaO-Fe2O3 clinker. The results show that MgO-CaO-Fe2O3 clinker with a bulk density of 3.42 g·cm-3, water absorption of 0.7%, and excellent physical properties can be formed by firing at 1600 °C for 3 h. In addition, the crushed and reformed specimens can be refired at temperatures of 1300 °C and 1600 °C to achieve compressive strengths of 17.9 MPa and 39.1 MPa, respectively. The main crystalline phase of the MgO-CaO-Fe2O3 clinker is the MgO phase; the 2CaO·Fe2O3 phase generated by the reaction is distributed between the MgO grains to form a cemented structure with a small quantity of 3CaO·SiO2 and 4CaO·Al2O3·Fe2O3 also distributed between the MgO grains. A series of decomposition and resynthesis chemical reactions occurred during the firing of the MgO-CaO-Fe2O3 clinker, and the liquid phase appeared in the system once the firing temperature exceeded 1250 °C. The presence of the liquid phase promoted intergranular mass transfer between the MgO grains, ensuring the continuous growth of the MgO grains and furthering the densification of the MgO-CaO-Fe2O3 clinker.

Use of rice husk as a source of silica for the production of self-flowing refractory castables

The objective of this study was: i) to obtain rice husk ash (RHA) with high purity silica for the production self-flowing refractory castables with different percentages of RHA as a partial substitute for microsilica; ii) analyze the mechanical, physical, thermal, and microstructural properties of the formulations studied in relation to the effect of firing temperatures. The rice husks were washed with acetic acid in autoclave, dried at 105 °C for 24 h and calcined at 600 °C for 4 h. The ashes obtained were analyzed by particle size distribution, X-ray fluorescence spectrometry (FRX), X-ray diffraction (XRD) and scanning electron microscopy with X-ray microanalysis (SEM-EDS). The presence of amorphous silica of high purity was verified with irregular grains and sizes between 40 and 145 μm. Refractory castables were produced by replacing 0%, 5%, 10% and 15% (mass) of the microsilica by rice husk ash. After curing, the specimens were dried at 110 °C for 24 h and a part was sintered at 800 °C for 5 h, while the other was sintered at 1430 °C for 8 h. The samples were submitted to tests of free creep; apparent density; linear variation; resistance to bending, compression and thermal shock. The formulation with the best mechanical performance, modulus of rupture (55.10 MPa) and compressive strength (119.62 MPa), was 10% replaced by rice husk ash and sintered at 1430 °C. This result was due to the formation of higher strength alumino-silicate phases and stronger bonds between the matrix and the aggregates. In addition, all analyzed materials using rice husk ash showed high density and good resistance to thermal shock, which is desirable for self-flowing refractory castables.

Effect of firing temperature and mineral composition on the mechanical properties of silty clays

Effect of firing temperature and mineral composition on the mechanical properties of silty clays

Fluoride-immobilized co-processing and resource utilization of aluminum-electrolyzed spent cathode carbon in brick-fired kiln

Spent cathode carbon (SCC) is hazardous waste from the electrolytic aluminum industry due to its high levels of soluble fluoride, while brick-fired kiln provides the clay and heating conditions needed to immobilize fluoride. However, SCC reusing is still understudied, meanwhile co-processing and resource utilization of SCC in brick-fired kiln were still not reported in the literatures in addition to a Chinese patent of the authors. Here, the effects of firing temperatures, firing time, clay doses andcalcium doses on the fluoride-immobilized performance of SCC co-processing were explored in a simulated brick-firing kiln, and their mechanisms were analyzed by SEM and XRD. The results indicated that clay-added co-processing in brick-fired kiln was a preferred choice without required additional additives or operations. The leached fluoride met Chinese standards by clay-added co-processing at ≥ 800°C/ ≥ 40g clay/ ≥ 120min. Clay and calcium-added co-processing in brick-fired kiln was another alternative choice with higher fluoride-immobilization rates. The leached fluoride met Chinese standard (GB5085.3-2007) by clay and calcium-added co-processing at ≥ 500°C/ ≥ 30min/ ≥ 5g clay/ ≥ 0.5g CaCO3. SEM and XRD indicated that SiO2 in clay reacted with sodium in SCC and formed vitreous analog (Na1.55Al1.55Si0.45O4) to prevent fluoride ion migration and the newly-formed k-Feldspar (K2O.Al2O3.6SiO2) may adsorb fluoride ions in clay-added co-processing. Soluble fluoride NaF in SCC were converted into water-insoluble cuspidine in clay and calcium-added co-processing, in addition to the crystalline phase conversion in clay-added co-processing. Therefore, the risks of finished bricks to human health and the environment were greatly reduced after clay-added or clay and calcium-added treatments.

Effect of Firing Temperature on Density, Porosity, Impact Strength, and Macro Structure of Crucible Materials Made from Graphite, Kaolin, and Castable Cement

&lt;p class="02abstracttext"&gt;Along with the advancement of modern times and technology, the metal sector plays a critical role in sustaining present growth. The industry's increased competitiveness has hampered the development of numerous small-scale metal casting industries. A crucible made of local graphite can only be used not more than ten melting cycles. This research aims to examine the influence of firing temperature on the density, porosity, impact strength, and macrostructure of graphite, kaolin and castable cement crucible materials. This research applies experiment method at various firing temperature, i.e. 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C. The crucible materials are made of graphite, kaolin, and castable cement. The firing process employs a heating rate of 5 °C/min and a holding time 2 hours. The results show that firing temperature influences the density, porosity, impact strength and macrostructure of crucible. The highest density at 1.86 grams/cm&lt;sup&gt;3 &lt;/sup&gt;was obtained at the firing temperature of 1050 °C. The lowest porosity value of 41% was attained at firing temperature of 1050 °C. However, this value was still below the crucible index criterion. The highest impact strength of 0.003249 Joule/mm&lt;sup&gt;2&lt;/sup&gt; occurred for firing temperature of 1050 °C.&lt;/p&gt;

Effect of firing temperature on humidity self-regulation functionality in a ceramic tile composition

The effect of sintering temperature on the humidity regulating capacity of a ceramic composition based on the mineral gibbsite has been studied. The humidity self-regulation functionality is due to the presence of mesopores in the material.In this work, it has been found that the adsorption-desorption capacity decreases drastically with sintering temperature, preserving a certain regulating activity up to temperatures as high as 1200 °C. The variation of the regulating capacity with temperature is consistent with the transformation of gibbsite from transition alumina to alpha alumina, which leads to a sharp decrease of the mesopore volume which can be determined from mercury porosimetry and especially nitrogen gas adsorption. The adsorption-desorption curves fit well to a pseudo-second order kinetic model and it is found that the equilibrium moisture is proportional to the specific surface area while the sorption rate constant depends on permeability for water diffusion through the porous structure.

Effects of firing temperature and Al additive on the microstructures and properties of SiC-CA6 composite refractories

SiC-CA6 composite refractories were successfully prepared by using SiC aggregates, sintered CA6 powders, Al powders and calcined α-Al2O3 as the raw materials. The effects of the firing temperature and Al additive on the physical and mechanical properties, phase evolution and microstructures were studied. The Al additive can significantly affect the properties of the SiC-CA6 composite refractories. With the addition of metallic Al, whiskers with interlocking structures could form in the SiC-CA6 composite refractories during the firing process, which improved the properties of the materials. The SiC-CA6 composites fired at 1450 °C with Al additive had good comprehensive properties with low porosity (14.7%), high strength (15.78 MPa for CMOR and 13.98 MPa for HMOR) and high elastic modulus (54.21 GPa). Compared with the samples without the Al additive, the samples with the Al additive had a lower porosity, and higher density and strength at all firing temperatures. For the samples without Al addition, the decomposition of the phenolic resin could form residual carbon remaining in the samples, which could decrease the oxidation of the SiC in the materials and inhibit the sintering of the samples. With increasing firing temperature, the residual carbon decreased and gradually transformed from a flaky morphology to an amorphous morphology.

Removal of fluorine from solutions using natural brucite sorbent

Wastewater treatment from pollutants, in particular from fluorine ions, is an important and urgent task. This paper presents the results of laboratory studies to remove fluorides from model solutions using a natural sorbent - brucite. Experiments were performed for solutions containing 10, 50 and 100 mgF/l. We studied the effect of firing temperature, reagent consumption and interaction time to the degree of impairment of solutions. The optimal parameters for cleaning wastewater from fluorine ions with an initial concentration of 10 mgF/l to the MPC level are selected.

Preparation of porous halloysite nanotube ceramics with high porosity and low thermal conductivity by foam-gelcasting

In this study, porous halloysite nanotube ceramics were prepared by the foam-gelcasting method using sodium carboxymethyl cellulose (CMC) as a foam stabilising agent, Isobam 104 as a dispersant and triethanolamine lauryl sulphate (TLS) as a foaming agent, respectively. Then, the effects of firing temperature on the phase composition, microstructure, and pore size distribution of porous halloysite nanotube ceramics were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunner-Emmet-Teller (BET), thermogravimetry (TG) and differential scanning calorimetry (DSC). The results showed that the prepared porous ceramics contained both mesopores (10–50 nm) and macropores (100–300 μm). As the firing temperature rose, the linear shrinkage and compressive strength of samples increased, while the porosity decreased. When the porosity was as high as 75.7%, the samples still remained the compressive strength of 1.1 MPa, and had the lowest thermal conductivity, reaching 0.194 W/(m·K) (at a testing temperature of 100 °C).

Effect of firing temperature on the interface between 3Y-TZP zirconia and porcelain.

The purpose of this study is to clarify the porcelain firing temperature conditions that give strongest bonding strength of porcelain to zirconia to manufacture all-ceramic fixed dental prostheses (FDPs) with excellent long-term stability. Opaque porcelain samples (8.0 × 3.0 × 1.2 mm) were placed in the center of zirconia plates (25.0 × 3.0 × 0.5 mm) and fired at temperatures of 950°C, 1,050°C, 1,100°C, and 1,150°C. Schwickerath crack initiation tests, elemental analyses, and morphological changes of the samples were compared. There was no difference in the bonding strength among all the groups of porcelain fired at different temperatures. Elemental analysis of Si and O2 at the interface between the zirconia and porcelain were observed in the 950°C, 1,050°C, and 1,100°C groups. No silicon was found in the 1,150°C group by elemental analysis, and the zirconia plate where the porcelain sample was placed had irregular shape changes. It is suggested that silicon is also involved in chemical bonds due to firing at high temperatures.