X-RAY INVESTIGATION OF THE EFFECT OF HEAT ON CHINA CLAYS1

X-ray diffraction patterns were obtained from an English china clay and a Georgia sedimentary kaolin, both raw and fired to various temperatures. The chief crystalline constituent of the raw clays was found to be kaolinite. Upon dehydration the kaolinite lattice was destroyed and the clays gave no diffraction pattern. Mullite was formed in both clays at 950°C and the amount increased with increase of firing temperature. In addition to the mullite, free alumina was present in the Georgia clay from 950 to 1100°C and cristobalite at temperatures above 1200°C.     

EFFECT OF ADDING ALUNITE TO REFRACTORY CLAYS

Four grades of alunite ore were mixed with four grades of fire clays, and the mixtures were formed into modulus of rupture test bars. These were fired to cone 13–14. Specimens were tested for bulk density and modulus of rupture after the initial firing and after reheating to 1400°C. and 1600°C. Softening temperatures of the mixtures were also determined. In general, small quantities (not exceeding 20%) of alunites improved fired characteristics, including decreasing the reheat shrinkage and increasing the P.C.E. The purity of both raw materials was the determining factor. Excessive additions of alunite, particularly to the low-grade clays, caused bloating as a result of the evolution of sulfur gases from the alunite.     

PROGRESS REPORT ON INVESTIGATION OF SAGGER CLAYS. V. Preparation of Experimental Sagger Bodies According to Fundamental Properties1

This is a Fifth Progress Report giving the results obtained in a preliminary study of sagger bodies, which is a continuation of an extensive investigation of sagger clays for the purpose of determining the properties of clays and bodies best suited for sagger purposes. The report contains data on the modulus of elasticity, transverse breaking strength, plastic flow, thermal expansion, and resistance to failure due to heat shock of 55 sagger mixes representing 39 different bodies fired at either 1230°C or 1270°C. The 16 bodies prepared in duplicate were tested both after firing at 1230°C and 1270°C. All of these bodies were compounded with two clays whose properties are given in earlier progress reports, and a mixture of graded grog. The grog was graded into sizes so as to result in two types of bodies, those having (1) a coarse and open-grained structure and (2) a dense and fine-grained structure. The data on the fired bodies show that those containing the fine sizes of grog have the higher modulus of elasticity, transverse strength, and in the majority of cases, thermal expansion. Very little difference in total porosity of the two types of bodies is indicated although the rate of absorption shows large differences. The data obtained in this preliminary study indicate that those bodies having (1) a porosity of less than 25% (2) a low modulus of elasticity, (3) as high transverse strength as is compatible with the low modulus of elasticity, and (4) low thermal expansion below 250°C are the most desirable for sagger purposes.     

PROGRESS REPORT ON INVESTIGATION OF FIRECLAY BRICK AND THE CLAYS USED IN THEIR PREPARATION1

This is a progress report of an extensive study of fire clays and fireclay brick. It includes the results of a preliminary study of clays representative of those used in the manufacture of refractories throughout the United States. Chemical analyses and a summary of physical tests are given of both fire clays and the brick manufactured from them. The thermal expansion behavior of the fire clays fired at 1400°C and those of the fire brick “as received” from the manufacturer and also after firing at 1400°C, 1500°C, and 1600°C were studied and the materials classified into groups having characteristic thermal expansions. The moduli of elasticity and rupture were determined at 20°C, 550°C, and 1000°C. The resistance of the brick to spalling in a water-quenching test is expressed in an empirical relation correlating the elasticity, strength, coefficient of expansion, and percentage of grog used in compounding the brick batches. Data are presented on individual bricks made by the same manufacturer showing probable reasons for great differences in the number of quenchings required to cause spalling in the water-dip test.     

SECONDARY EXPANSION OF HIGH-ALUMINA REFRACTORIES*

Several mixtures of raw and calcined diaspore and bauxite with raw and calcined fire clays were prepared and fired at 2700°F. Specimens were refired at higher temperatures, and the linear changes were determined. Bodies composed of high-alumina grog and fire clay expand in the refire, whereas specimens of fire-clay grog that are bonded with ground raw diaspore exhibit shrinkage. The cause of the secondary expansion and shrinkage of these bodies is discussed.     

Thermomechanical analysis of fired ceramic materials

A thermomechanical analysis of fired clay specimens is carried out and empirical equations are derived for dilatation (shrinkage) as a function of temperature. The experimental results obtained can be used in designing optimum firing technology for preparing ceramics from polymineral clays.Translated from Steklo i Keramika, No. 11, pp. 26–27, November, 1996.     

Effect of the plant waste onto the properties of use and the microstructure of phyllosilicates based ceramics

The present study aimed at using plant waste (Musa Paradisiaca) for manufacturing clay-based ceramics in order to promote lower sintering temperature while preserving the properties of use. Two kaolinic-illitic clays (NZ1 and KO) from Central African Republic were used mixed with 1 to 10 mass% of the plant waste (MP). The clays and the waste exhibited accessory phases: quartz and iron oxides, and K₂O respectively. MP was collected, dried and sieved (<100 μm) previously to its mixture with clays. According to the sintering behavior of KO and NZ1 derived from thermodilatometry, the densification was obtained after firing at 1200°C. Results showed that open porosity decreased from 35% to 17% with increasing temperature in the range 900 to 1200°C for KO and NZ1. This porosity remained in the range 30%-40% while increasing the MP content (firing at 1000°C for 1h.). The optimized MP content was 3 and 5 mass% for KO and NZ1 clay materials respectively. The compressive strength and thermal conductivities were improved compared to clay samples without MP fired at 1200°C. Moreover a significant decrease in the sintering temperature was achieved, leading to energy saving in line with sustainability issues.     

Ceramic behaviour of five Chilean clays which can be used in the manufacture of ceramic tile bodies

This study is focussed on the behaviour of ceramic clays from Chile which has an important local ceramic industry. Five deposits of clays with industrial application have been studied. The clays come from San Vicente de Tagua-Tagua (SVTT), Litueche (L), Las Compañías – Río Elqui (LC), La Herradura – Coquimbo (LH) and Monte Patria – Coquimbo (MP). The chemical and mineralogical compositions of clays were determined by X-ray fluorescence (XRD) and X-ray diffraction (XRD), respectively. Also, the plasticity index (PI) was measured for each sample. The chemical and mineralogical compositions of samples differ considerably. Test samples have been prepared by pressing and firing in the range of 800–1150 °C. Linear contraction (LC), water absorption capacity (WAC) and thermodilatometric analysis (TDA) were done in order to characterize clays after firing. A considerable decrease in the WAC coinciding with the beginning of vitrification, is observed between 1050 and 1100 °C. At 1150 °C the porosity of the tile bodies decreases significantly and the tile bodies became earthenware. All studied clays seem to be easily adaptable to a correct dry pressing ceramic process. In particular, illite–kaolinite-rich samples show the best behaviour. Samples SVTT are suitable for the production of fast firing vitreous pieces. L samples present the highest refractory behaviour.     

SOME CHARACTERISTICS OF ILLINOIS POTTERY CLAY*

Four Illinois pottery clays, representing two extreme types and two intermediate types, are described. Porosity and thermal expansion tests were made on the raw and fired clays. The temperature of the inversion of low to high cristobalite in these clays is much lower than is usually accepted for the lower limit of this inversion because cristobalite is formed at low temperatures. A limited amount of data was obtained on the volume changes of raw clays during firing, the organic matter content, and thermal dissociation.     

Effect of Nepheline Syenite Additives on the Technological Behavior of Ceramics and Porcelain Stoneware Tiles

To study the effect of the Egyptian nepheline syenite raw materials on the sinterability of the ceramic tiles, four batches with concentrations of nepheline syenite (0, 5, 10 and 15%) were prepared. The four batches were ground, mixed, shaped, pressed and fired at temperatures from 1160 °C to 1260 °C. Some technological behaviors of the samples were measured in dry, green state and after firing at 1260 °C; including densfication parameters. Phase composition and microstructural studies refered that mullite was the newly crystallized phase after firing set in a glassy matrix enriched in alkalis. Technological properties of the fired batches revealed that the higher firing temperature and higher nepheline syenite contents produced larger amounts of glassy phase, and consequently bulk density and linear shrinkage increased, while apparent porosity and water adsorption as well as whiteness of fired batches decreased. It is concluded that nepheline syenite could be successfully used as flux in ceramic tiles, while it is not accepted in the production of porcelain bodies because of their coloration.

     

Microwave vs conventional porcelain firing: Macroscopic properties

This paper provides an overview of the macroscopic properties of porcelain tableware fired in a microwave furnace with six magnetrons (each with a nominal power of 900 W) operating at the frequency of 2.45 GHz. The dependence of firing temperature on physical properties such as shrinkage, water absorption, apparent porosity, bulk density, and impact resistance was analyzed. Emphasis is on the differences in the macroscopic properties of microwave and conventionally (gas and electric) fired porcelain. Batches were fired from room temperature up to above the optimum firing temperature (1380°C). Results show similar macroscopic properties for both firing methods, microwave heating required lower firing temperatures (between 1300°C and 1350°C), and shorter processing times (about 70 minutes). The main differences between microwave and electric firing methods occur in a temperature band of 300°C above the porcelain eutectic temperature (close to 1000°C).     

Moessbauer Study of the Effect of Calcium Content on Iron Oxide Transformations in Fired Clays

Sixteen clays were studied by Moessbauer measurements in their unfired and fired forms. Thirteen of these clays were collected near ancient Greek sites and the remaining three in England and the United States. They were fired at 700°, 900°, and 1080°C to investigate the physicochemical transformations of the clay components in this temperature range. The results show distinctly different responses of the clays, according to their CaO content. Firing of the calcareous clays (>5 % CaO content) at 700°C produces small oxide particles (mainly hematite). This phase remains stable on further firing up to 1080°C, where it decreases slightly. Contrary to this behavior, the noncalcareous clays display a continuous increase of the amount and size of the oxide particles when fired up to 1080°C. This difference is attributed to stable calcium aluminosilicate phases which are formed in the calcareous clays as opposed to the continuous vitrification which occurs for the noncalcareous clays by firing up to 1080°C. The variation of the quadrupole splitting of the central doublet with firing temperature also differs for the two types of clays. The quadrupole splitting values among the clays span a range of 1.1 to 1.6 mm/s for a firing temperature of 700°C. Above this temperature, a distinct separation of the two groups occurs with the quadrupole splitting of calcareous clays lying higher.     

Gravimetric and spectroscopic studies of the chemical combination of moisture by as-fired and reheated terracotta

The application of microbalance measurements to investigate the chemical recombination of moisture with fired clay ceramics is demonstrated. The kinetics of mass gain at constant temperature and relative humidity (RH) are studied for terracotta fired at temperatures between 800 °C and 1200 °C. The experimental results show that mass gain proceeds in two stages. The second stage mass gain is taken as a measure of the capacity for long-term chemical combination with moisture based on the (time)^1/4 law. A maximum rate of mass gain is obtained for terracotta fired at 1000 °C. It is shown that reheating a sample of fired terracotta at any temperature between 500 °C and the original firing temperature will return the material to its as-fired state. This is supported by Raman spectroscopy. Exceeding the original firing temperature during reheating alters the subsequent rate of reaction to moisture, suggesting a method for determining the original firing temperature of ceramic artefacts.     

Stable Isotopes of Carbon and Oxygen as a Possible New Tool for Estimating Firing Temperatures of Ancient Pottery

The ratios of the stable isotopes of carbon (¹³C/¹²C) and oxygen (¹⁸O/¹⁶O) in calcite in clays that contain carbonate and that were fired at temperatures between 500 and 700 °C displayed a good correlation with the firing temperature for both isotopes. The isotopic composition indicated that the original carbonate was either completely decomposed or that it exchanged with environmental CO₂ to obliterate the original isotopic signature. The isotopic effects seem to be kinetically controlled. The isotope thermometer is used to compare refiring temperatures of pottery which was fired under controlled conditions. The differences between calculated and actual temperatures did not exceed 132 °C and in most cases was about 50 °C or less. Isotopic analysis of pottery from Tel Miqne-Ekron and Deir el Balah, dated to the 13th–12th centuries BC, gave an average ancient firing temperature of about 600 °C. If shown to be of general validity, then perhaps this technique can be used also for estimating preparation temperatures of ancient mortar, especially when preserved in dry climates where later changes would be minimized.     

Mineralogy and physico-chemical properties of alluvial clays from Sanaga valley (Center,Cameroon): Suitability for ceramic application

Alluvial clays from four localities along the Sanaga River (Center Cameroon) were studied by physico-chemical, mineralogical and technological characterization in order to assess their suitability as ceramic raw materials. The chemical compositions indicated that SiO₂ (65–70%) and Al₂O₃ (12–15%) are major elements while Fe₂O₃ is less (4–7%). Kaolinite, quartz and feldspar are the main minerals. Particle size distribution and chemical composition are indicative of “plastic red clays” belonging to heavy sandy clays group. Their medium to high plasticity is suitable for fired earth and fine ceramics products. Pressed samples were fired at temperatures ranging between 900 and 1100 °C for coarse ceramic products. Linear shrinkage, flexural strength and water absorption indicated that the clays from one site (Mbandjock) are good for brick making. Clays from the three other localities present poor technological properties (higher shrinkage and cracks), they need degreasers before use as ceramic raw materials. Although water absorption and flexural strength parameters are good for all the studied samples, firing shrinkage needs to be improved.     

Reuse of ceramic sludge in the production of vitrified clay pipes

The use of various wastes in the manufacture of different ceramic products has proved to be beneficial from both economic and environmental points of view. In the present paper, the simultaneous reuse of ceramic sludge waste (from 0% to 6.2%) and cullet (from 0% to 10%) in the production of vitrified clay pipes has been investigated. Samples were pressed at 15?MPa, dried and subsequently fired at three different temperatures (1050, 1150 and 1250?°C) and soaking times (1, 2 and 3?h). A two-level factorial design was applied to study the effect of the various parameters on the properties of fired bodies. It was concluded that an economic recipe can be prepared involving the use of 6.2% sludge (dry basis) and 5% cullet with the balance clay +?grog followed by firing the dried body at 1050?°C for one hour only. The produced fired specimens yielded bending strength exceeding the minimum requirement for any size of soil embedded pipes and for 8″ and larger concrete cradled pipes. The suggested composition makes use of an environmentally polluting waste (sludge) and broken glass waste (cullet) and involves firing for one single hour at a temperature about 200?°C lower than commonly used.     

A METHOD FOR DETERMINING THE LOSS OF WEIGHT OF CLAYS DURING FIRING1

A simple apparatus has been devised for the comparison of the loss of weight of clays during firing by the direct weighing of the sample while suspended in the furnace. Preliminary results indicate that while the total loss of weight of the different clays varies considerably, the losses in any temperature range expressed in per cent of the total weight lost at 1800°F show a much closer relationship. The comparative ease of control will make it possible to examine the clays under a wide range of conditions.     

THERMAL EXPANSION OF SILICA MORTARS AFTER FIRING AT 950°, 1200°, and 1500°C1

The thermal expansion between 20°C and 950°C of silica mortars after being fired at 950°, 1200°, and 1500°C was compared with the expansion of the materials before firing. The mortars showed marked difference in expansion behavior after being fired to 1500°C, indicating that there was developed a large amount of tridymite The thermal expansion of the fired samples is less than that of the unfired samples.     

Fabrication of MgO-CaZrO3 refractory composites from Egyptian dolomite as a clinker to rotary cement kiln lining

Dolomite is used as basic lining for rotary cement kiln due its high refractoriness, corrosion resistance against basic environments and high coating performance, but its poor hydration resistance limits its use. In this work, Egyptian dolomite was converted into outstanding refractory magnesia-calcium zirconate composite (M-CZ) by the addition of 37.8–47.8?wt% zirconia. The mixtures were milled, uni-axially formed and fired at a temperature of 1400–1550?°C for 2?h. Thermal analysis, phase composition, microstructure, densification parameters, and other technological properties of the fired specimens were investigated. It was found that sintered M-CZ composite with bulk density (3.95?g/cm³), cold crushing strength (170?MPa), and high coating ability can be obtained by firing Egyptian dolomite with 37.8?wt% zirconia at temperature 1500?°C with the formation of belite as a secondary product. This composite can be nominated to be produced on a large industrial scale.     

EFFECTS OF VARIOUS CLAYS USED IN MILL ADDITIONS ON PROPERTIES OF A TITANIA-OPACIFIED ENAMEL*

In the course of investigations made in connection with the development of titania-opacified enamels, it was observed that color, reflectance, and acid resistance were influenced by the clay used in the mill additions. In a subsequent search for a clay which would produce the best over-all enamel properties, a study of the effects of twenty clays of different types was made under standardized conditions of milling, spraying, and firing. Coatings sprayed at 30 gm. per sq. ft. were examined for bisque strength, brushing behavior, and tearing. After firing for 3 minutes at 1540°F., and at 1640°F., the reflectance, color, gloss, and acid resistance of the enamels were observed. It has been concluded that the success or failure of an enamel is determined to an important degree by the clay used in the mill addition.